evaluating the influence of aseismic ridge subduction and accretion(?) on detrital modes of forearc...

Ž .Lithos 46 1999 17–42

Evaluating the influence of aseismic ridge subduction andž /accretion ? on detrital modes of forearc sandstone: an example

from the Kronotsky Peninsula in the Kamchatka Forearc

Kathleen M. Marsaglia a,), Paul Mann b, Ronda J. Hyatt c, Hilary C. Olson b

a UniÕersity of California, San Diego, Scripps Institution of Oceanography, Geosciences Research DiÕision, La Jolla, CA 92093-0220, USAb Institute for Geophysics, UniÕersity of Texas at Austin, 4412 Spicewood Springs Road, Building 600, Austin, TX 78759-8500, USA

c Department of Geological and EnÕironmental Sciences, UniÕersity of Texas at El Paso, El Paso, TX 79968, USA

Received 1 February 1998; accepted 19 June 1998

Abstract

The Kronotsky Peninsula, in the forearc region of the Kamchatka magmatic arc, lies on trend with the Emperor Seamountchain situated on the currently subducting Pacific tectonic plate. Detrital modes of volcaniclastic sandstone interbedded with

Ž .mafic Eocene ? basement rocks and within the overlying sedimentary sequence provide insight into the late CenozoicŽ .geologic history of this area. Eocene ? and basal Miocene sandstones are primarily composed of variably altered mafic

volcanic debris. Their detrital modes are similar to those of Emperor Seamount sandstones and Hawaiian beach sands.Although aspects of the stratigraphy and volcaniclastic sand composition are consistent with a seamount setting, there is nophysical evidence for an accretion event, and the suggested Eocene age for this unit makes an Emperor Seamount originunlikely. A seamount origin cannot be ruled out for older Kronotsky basement complexes, however. A Miocene lull inKronotsky volcanism was followed by rapid basin subsidence and influx of arc-derived turbidites from the west. Detritalmodes of these sandstones are typical of a moderately evolved continental or micro-continental arc. An anomalously highproportion of sedimentary lithic fragments is the only possible compositional fingerprint attributable to seamount or ridgesubduction. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Forearc; Seamount; Sandstone; Petrology; Tectonics

1. Introduction

1.1. Geologic setting and stratigraphy

From west to east, the geology of the KamchatkaPeninsula records a complex history of subduction,magmatism and terrane accretion since the Creta-

) Corresponding author. Present address: Westport TechnologyCenter International, 6700 Portwest Drive, Houston, TX 77024,USA. Fax: q1-713-864-9357; E-mail: [email protected]

Žceous e.g., Watson and Fujita, 1985; Zinkevich and.Tsukanov, 1993; Geist et al., 1994 . It is an excellent

location to investigate the effects of seamount sub-duction and possible accretion in that the PacificPlate, including the Hawaiian–Emperor Seamountchain, is actively subducting beneath the Eurasian

ŽPlate along the Kurile–Kamchatka Trench Figs. 1.and 2 .

The Kronotsky Peninsula, a southeastern-trendingspur of the Kamchatka Peninsula, lies on trend withthe subducting Emperor Seamount chain. The Upper

0024-4937r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0024-4937 98 00054-1

( )K.M. Marsaglia et al.rLithos 46 1999 17–4218

( )K.M. Marsaglia et al.rLithos 46 1999 17–42 19

Ž . Ž .Fig. 2. Map of the Kamchatka Peninsula showing extent of middle to Late Cretaceous exotic ? terranes light gray , pre-CretaceousŽ . Ž . Ž .metamorphic rocks black , and major thrust belts. Modified from Geist et al. 1994 after Zonenshain et al. 1990 and Shul’diner et al.

Ž .1979 .

Ž .Cretaceous to Eocene ? basement rocks of thispeninsula, from oldest to youngest the Cape Ka-menisty, Kubovsk, and Kozlovsk Groups, are collec-

Žtively referred to as the Kronotsky terrane Figs.3–5; Watson and Fujita, 1985; Zinkevich and

.Tsukanov, 1993 . This terrane consists of basaltflows that exhibit columnar jointing, interbeddedwith extremely indurated volcaniclastic sandstonesr

Žtuffs, volcanic breccias and conglomerates Geist et.al., 1994 and references therein . Geochemical ana-

lyses of the basalt indicate a strong seamount signa-ture, with compositions intermediate between arc and

Žseamount Rotman and Markovskiy, 1968;.Kepezhinskas et al., 1995 . These forearc basement

rocks may have formed in situ, or may be accretedŽWatson and Fujita, 1985; Geist et al., 1994; Pecher-

.sky et al., 1997a,b .The Upper Cretaceous to Eocene basement rocks

are disconformably overlain by Miocene to lowerŽPliocene volcanic-sedimentary sequences Tyushev

Fig. 1. Geosat gravity map of the northwest Pacific Ocean showing the location of the Kamchatka Peninsula, Kronoty Peninsula, AleutianRidge, Kamchatka and Aleutian trenches, Hawaiian-Emperor Seamount chain, Orbrutchev Rise, and Deep Sea Drilling and Ocean Drilling

Ž . Ž . Ž .Program sites open circles mentioned in the text. Shading corresponds to relative gravity highs darker vs. gravity lows lighter . GravityŽ . Ž .data from Sandwell and Smith 1995 . White box outlines study area Fig. 5 .


Fig. 3. Simplified geologic map of central Kamchatka PeninsulaŽ .after Zinkevich and Tsukanov 1993 . From east to west: KTs

Kronotsky Terrane; TBsTyushevka Basin; VOsVetlovskiy andOzernoy–Valaginskiy Terranes; EK sEast Kamchatka VolcanicBelt; CGsCentral Kamchatka Graben; CCsCenozoic Clastic

Ž .Sequences also includes TB and CG ; CK sCentral KamchatkaVolcanic Belt; MCsMetamorphic Complexes. See text for de-scription of units.

. ŽFormation of the Tyushevka basin Figs. 3–5;.Zinkevich and Tsukanov, 1993 . The basal

Ž . Ž .Miocene ? succession Hot Springs Member , con-sists of 50–100 m of shallow-marine conglomerateand sandstone deposited unconformably on basaltic

Ž .basement Lagoe et al., 1995 . Large clasts withinthe unit appear to be basement-derived and sand-stone beds locally contain impure diatomite, woodfragments, and shallow-marine macrofossils. Thiscoarse-grained shallow-water unit passes upward

Ž . Ž .through a thin -50 m transitional ? sequence intoŽ . Ža thick )500 m deep-water succession Olga.Member of thinly bedded, fine-grained turbidites cut

Ž .by submarine channels Lagoe et al., 1995 . Theminimum age for this unit is uppermost Miocene to

Ž .lowermost Pliocene 5.3–4.7 Ma .Although this basin has been locally compressed

and uplifted on the Kronotsky Peninsula, seismicdata suggest that it extends offshore, to the north andsouth along strike, in a more or less undeformedstate. The Eocene to lower Pliocene sequence was

Ž .uplifted, folded, and eroded Fig. 5 , possibly inŽassociation with seamount subduction Lagoe et al.,

.1995 . Very coarse-grained, fluvial sandstone over-lies the angular unconformity developed on top ofthe folded Miocene–Pliocene forearc sedimentary

Fig. 4. Schematic stratigraphic column for the southern KronotskyPeninsula. See text for description of units.

Ž .sequence Lagoe et al., 1995 . Cobble clasts of tur-bidite sandstone are present along the basal contact,and the sedimentary sequence is capped by arc-de-rived lava flows. For more details of the basinstratigraphy, structure, magmatism and tectonic evo-

Ž .lution, see Kepezhinskas et al. 1995 and Lagoe etŽ .al. 1995 .

1.2. Origin of Kronotsky basement rocks

Previous workers have suggested that the UpperCretaceous to Eocene basement rocks of the Kronot-sky Peninsula may have formed in a magmatic-arc or

Ž .Fig. 5. Detailed geologic map of the Kronotsky Peninsula outlining the two main areas Kronoki and Rakatinskay where samples werecollected. Representative structural sections for these two sample areas are also given.


Table 1Counted and recalculated parameters

Counted parameters

Qp: polycrystalline quartzQm: monocrystalline quartzP: plagioclase feldsparŽ .P ArD : altered or dissolved plagioclase

K: potassium feldsparŽ .Unst Feld: unstained Na? feldspar

Ž .Lvo: other volcanic lithic includes microcrystalline aggregatesLvv: vitric volcanic lithic

brglsbrown glassclglscolorless glassblglsblack glassArDsaltered or dissolved glass

Lvf: volcanic lithic with felsitic textureLvml: volcanic lithic with microlitic texture

brglsbrown glassclglscolorless glassblglsblack glassArDsaltered or dissolved glass

Lvl: Volcanic lithic with lathwork texturebrglsbrown glassclglscolorless glassblglsblack glassArDsaltered or dissolved glass

Lmv: metavolcanic lithicLmm: polycrystalline mica lithicLmt: quartz–mica tectonite lithicLma: quartz–feldspar–mica aggregate lithicPhyl: phyllite lithic

Ž .Lsi: siltstone or sandstone matrixrcement sedimentary lithicLsa: argillite shale lithicLsc: sedimentary carbonate lithicLsch: sedimentary chert or cherty argillite lithic

Ž .Glau: glauconite celadonite?Bio Sil: siliceous microfossilBio Car: calcareous microfossil or bioclastCarb: carbonate mineralsBiot: biotiteOp D: opaque dense mineralsNonOp D: nonopaque dense mineralsOtherrunknown: other miscellaneous and unidentified grainsTotal: total points counted

Other interstitial categoriesFrwk Grains: framework grainsMatr Silt: silt-sized matrixMatr Clay: clay-sized matrixInter Por: interparticle porosityCaly Cmt: interparticle clay mineral cement

Ž .Red Zeol Cmt: interparticle zeolite cement with dark red calcium stainŽ .Pink Zeol Cmt: interparticle zeolite cement with light pink calcium stain

Op Cmt: authigenic opaque mineral cement


Ž .Table 1 continued

Ž .QsQmqQp FsPqP ArD qKqUnst FeldsLsLmqLvqLs LvsLvoqLvvqLvfqLvmiqLvlLmsLmvqLmmqLmtqLmaqPhyl LssLsaqLscqLsiqLsch

Recalculated parameters

Ž .QFL %Qs100)Qr QqFqL LmLvLs%Lms100)LmrLŽ .QFL %Fs100)Fr QqFqL LmLvLs%Lvs100)LvrLŽ .QFL %Ls100)Lr QqFqL LmLvLs%Lss100)LsrL

Ž Ž ..QmKP%Qms100)Qmr QmqPqP ArDŽ Ž ..QmKP%Ps100)Pr QmqPqP ArDŽ Ž ..QmKP%Ks100)Kr QmqPqP ArD

w Ž . Ž . Ž .x w Ž . Ž . Ž .Total colorless glasssLvclLvbrLvbl%Lvcls100) Lvv clgl qLvml clgl qLvl clgl r Lvv clgl qLvml clgl qLvl clglŽ . Ž . Ž . Ž . Ž . Ž .xqLvv brgl qLvml brgl qLvl brgl qLvv blgl qLvml blgl qLvl blgl

w Ž . Ž . Ž .x w Ž . Ž . Ž .Total brown glasssLvclLvbrLvbl%Lvbrs100) Lvv brgl qLvml brgl qLvl brgl r Lvv clgl qLvml clgl qLvl clglŽ . Ž . Ž . Ž . Ž . Ž .xqLvv brgl qLvml brgl qLvl brgl qLvv blgl qLvml blgl qLvl blgl

w Ž . Ž . Ž .x w Ž . Ž . Ž .Total black glasssLvclLvbrLvbl%Lvbls100) Lvv blgl qLvml blgl qLvl blgl r Lvv clgl qLvml clgl qLvl clglŽ . Ž . Ž . Ž . Ž . Ž .xqLvv brgl qLvml brgl qLvl brgl qLvv blgl qLvml blgl qLvl blgl

w Ž . Ž . Ž . Ž . Ž . xLvfLvmlLvl%Lvfs100)Lvfr Lvml blgl qLvl blgl qLvml clgl qLvl clgl qLvml brgl qLvfw Ž . Ž . Ž . Ž Ž . Ž . Ž . Ž . Ž . xLvfLvmlLvl%Lvmls100) Lvml blgl qLvml clgl qLvml brgl r Lvml blgl qLvl blgl qLvml clgl qLvl clgl qLvml brgl qLvfw Ž . Ž . Ž . Ž Ž . Ž . Ž . Ž . Ž . xLvfLvmlLvl%Lvmls100) Lvl blgl qLvl clgl qLvl brgl r Lvml blgl qLvl blgl qLvml clgl qLvl clgl qLvml brgl qLvf

Ž . Ž .Fr%Qs100) Q r TotalŽ . Ž .Fr%Fs100) F r Total

Ž . Ž .Fr%Volcs100) Lv r TotalŽ . Ž .Fr%Mets100) Lm r TotalŽ . Ž .Fr%Seds100) Ls r TotalŽ . Ž .Fr%Micas100) Biot r Total

Ž . Ž .Fr%Ds100) OpDqNonOpD r Totalw Ž . Ž . Ž .x Ž .Fr%Lvbls100) Lvv blgl qLvml blgl qLvl blgl r Total

Percent cement typeŽ . Ž . Ž%Clay clay mineral s100) Clay Cmt r Frwk GrainsqMatr SiltqMatr Clayq Inter PorqClay CmtqRed Zeol Cmt

.qPink Zeol CmtqOp CmtŽ . Ž . Ž%Red Zeol calciumy rich zeolite s100) Red Zeol Cmt r Frwk GrainsqMatr SiltqMatr Clayq Inter PorqClay Cmt

.qRed Zeol CmtqPink Zeol CmtqOp CmtŽ . Ž . Ž%Pink Zeol calciumypoor zeolite s100) Pink Zeol Cmt r Frwk GrainsqMatr SiltqMatr Clayq Inter PorqClay Cmt

.qRed Zeol CmtqPink Zeol CmtqOp CmtŽ . Ž . Ž%Op opaque mineral s100) Op Cmt r Frwk GrainsqMatr SiltqMatr Clayq Inter PorqClay CmtqRed Zeol Cmt

.qPink Zeol CmtqOp CmtŽ . Ž . Ž%Tot Matrix total detrital silt and clay s100) Matr SiltqMatr Clay r Frwk GrainsqMatr SiltqMatr Clayq Inter Por

.qClay CmtqRed Zeol CmtqPink Zeol CmtqOp CmtŽ . Ž .%Tot Cement clay mineral, zeolite and opaque mineral cement s100) Clay CmtqRed Zeol CmtqPink Zeol CmtqOp Cmt

Ž .r Frwk GrainsqMatr SiltqMatr Clayq Inter PorqClay CmtqRed Zeol CmtqPink Zeol CmtqOp CmtŽ . Ž . Ž%Tot Por total interstitial porosity s100) Inter Por r Frwk GrainsqMatr SiltqMatr Clayq Inter PorqClay Cmt

.qRed Zeol CmtqPink Zeol CmtqOp CmtŽ . Ž .%Tot CqP total cement and interstitial porosity s100) Inter PorqClay CmtqRed Zeol CmtqPink Zeol CmtqOp Cmt

Ž .r Frwk GrainsqMatr SiltqMatr Clayq Inter PorqClay CmtqRed Zeol CmtqPink Zeol CmtqOp CmtŽ . Ž .IGV intergranular volume s100) Matr SiltqMatr Clayq Inter PorqClay CmtqRed Zeol CmtqPink Zeol CmtqOp Cmt

Ž .r Frwk GrainsqMatr SiltqMatr Clayq Inter PorqClay CmtqRed Zeol CmtqPink Zeol CmtqOp Cmt

oceanic-ridgerseamount setting. Some workers in-terpret the basaltic basement complex as the productof nascent arc magmatism in that it has a composi-

Žtion between island–arc and oceanic tholeiite e.g.,

Rotman and Markovskiy, 1968; Raznitsin et al.,.1985; Kepezhinskas et al., 1995 . Geochemical anal-

Ž .yses of Eocene ? to Recent volcanic rocks from theKronotsky region show a decreasing seamount-like


Ž .signature with time Kepezhinskas et al., 1995 . TheŽ .Eocene ? basalts are tholeiitic or alkaline, whereas

the Pliocene–Pleistocene volcanic rocks of the Rail-road Ridge Formation are mostly calc-alkaline an-

Ž .desites. Kepezhinskas et al. 1995 interpreted thistrend as indicating that the geochemistry of thesubducting, seamount-bearing slab was most influen-tial when subduction first initiated along this margin,but they do not specify the nature of the crust thatthis arc developed upon, continental crust vs. ac-

Ž .creted oceanic terranes. Geist et al. 1994 suggestedthat the Kronotsky basement complex represents anaccreted intraoceanic-arc terrane that was emplacedin the Eocene and uplifted in the Miocene as a resultof reinitiation of subduction. Their basis for thisinterpretation includes paleomagnetic data whichsuggest that these rocks are far travelled, originating

Ž .at a latitude of 41"6N Bazhenov et al., 1992 .Ž .Zinkevich and Tsukanov 1993 proposed that the

suture zone lies below the Tyushevka basin. Alter-Ž .nately, Avdeiko 1980 suggested that the basement

rocks of the Kronotsky Peninsula may be an exten-sion of the Obrutchev Rise, the northernmost exten-sion of the Hawaiian–Emperor Ridge. This rise con-sists of mafic rocks estimated to be of Late Creta-ceous age based on the age of sediment recovered atDeep Sea Drilling Project Site 192 on the Meiji

Ž .Guyot 70–73 Ma; Worsley, 1973 and radiometricages of mafic basement drilled at Ocean Drilling

ŽProgram Site 883 on the Detroit Seamount 81 Ma;.Keller et al., 1995 . If the Kronotsky forearc base-

ment is accreted, seamount-bearing oceanic crust,then the seamount-like geochemistry of the Eocenebasement rocks would be primary, rather than anartifact of seamount assimilation during slab subduc-tion. If the accreted seamount was part of the Em-peror Seamount chain, then it should be older than;75 Ma given the progressive age trend observed

Žfor Hawaiian and Emperor Seamounts Keller et al.,.1995 . The present alignment of the Emperor

Seamount chain with the Kronotsky Peninsula couldŽ .be fortuitous in that Watson and Fujita 1985 sug-

gest that the Kronotsky Seamount may have beenassociated with Tertiary hot-spot activity on theKula–Pacific ridge axis. Terrane accretion or forearcextension and magmatism may have been related toor facilitated by the change in Pacific Plate motion at

Ž .43 Ma Engebretson et al., 1985 .

1.3. Purpose of study

The purpose of this study was to determine detri-tal modes of volcaniclastic sandstone within thebasement and cover complexes in order to clarify theorigin of basement and illustrate the effects of aseis-mic ridge subduction on subsequent forearc evolu-tion. First, we address the possibility that the Kronot-sky basement rocks constitute an accreted Hawai-ian–Emperor Seamount. The geology of the Hawai-ian–Emperor Seamount chain is known from deep-sea drilling and by analogy with the modern Hawai-ian Islands. We use sand detrital modes from Eocene

Žto Paleocene seamount cover sequences Deep Sea.Drilling Program Sites 430–433; this study and

Ž .modern Hawaiian beach sands Marsaglia, 1993 ,along with stratigraphic arguments, to evaluate thelikelihood of seamount accretion in the Kronotskyregion. We then compare and contrast sand detritalmodes for the Tyushevka basin fill with those ofsandrsandstone produced during triple-junction mi-gration along other modern active margins. Our re-sults provide an actualistic model that is applicableto other forearc sequences affected by ridge subduc-tion andror accretion.

2. Methods

Representative sandstone samples were collectedfrom each of the major units exposed on the Kronot-sky Peninsula in conjunction with a larger strati-graphic study. The samples were taken along sec-tions measured in the Rakatinskay and Kronoki studyareas pictured in Fig. 5. For comparison, sand andsandstone samples were collected from Deep SeaDrilling cores recovered at Sites 430, 431, 432 and433 located on Emperor Seamounts in the northwest

Ž .Pacific Ocean Fig. 1 . The stratigraphic sectioncored at Site 433 is most complete and consists ofapproximately 350 m of tholeiitic basalt flow unitsor lobes, followed by 30 m of alkalic basalt flowunits, overlain by a sedimentary section that consists

Ž .from base to top of reefal carbonate sand 160 m ,Ž .calcareous ooze 20 m , and mixed siliceous–

Ž .calcareous ooze 30 m . Sandy intervals were sam-pled from between flows and within the reefal car-


Tab

le2

Raw

data


Tab

le3

Rec

alcu

late

dpa

ram

eter

s

Sam

ple

QF

LQ

mK

PL

mL

vLs

Lvc

lLvb

rLvb

lL

vfL

vmlL

vlF

r%F

r%F

r%F

r%F

r%F

r%F

r%F

r%F

r%nu

mbe

r%

Q%

F%

L%

Qm

%K

%P

%L

m%

Lv

%L

s%

Lvc

l%

Lvb

r%

Lvb

l%

Lvf

%L

vml

%L

vlQ

FV

olc

Met

Sed

Mic

aO

pDN

onD

Lvb

l

NH

-76

0.0

32.6

67.4

0.0

0.0

100.

03.

384

.212

.52.

346

.651

.10.

051

.148

.90.

026

.746

.51.

86.

90.

01.

414

.720

.7

NH

-25

6.6

22.0

71.4

18.9

2.7

78.4

5.6

63.6

30.8

14.3

68.1

17.6

0.0

85.4

14.6

4.7

20.0

41.3

3.7

20.0

0.3

0.7

6.7

7.0

NH

-51a

7.6

15.3

77.1

31.1

4.9

63.9

4.2

58.0

37.7

17.6

69.2

13.2

6.1

72.7

21.2

6.4

14.0

41.1

3.0

26.8

2.3

1.7

3.7

4.0

NH

-55a

4.6

21.1

74.3

14.3

10.0

75.7

3.3

65.4

31.3

38.9

52.6

8.4

7.0

86.0

7.0

3.3

20.0

46.0

2.3

22.0

0.7

0.3

4.3

2.7

NH

-68a

5.3

13.7

81.1

18.8

10.4

70.8

0.0

71.0

29.0

21.3

66.7

12.1

0.0

85.7

14.3

3.0

13.0

54.7

0.0

22.3

0.0

2.3

1.0

5.7

NH

-106

0.3

9.5

90.2

3.4

0.0

96.6

0.0

100.

00.

06.

973

.719

.30.

087

.112

.90.

39.

388

.70.

00.

00.

00.

71.

016

.7N

H-1

377.

121

.671

.421

.62.

775

.77.

350

.042

.72.

861

.136

.15.

266

.228

.65.

319

.332

.04.

727

.30.

71.

07.

08.

7N

H-1

56a

7.6

15.1

77.3

27.9

4.9

67.2

3.1

66.2

30.7

31.7

58.7

9.5

8.3

79.8

11.9

5.7

14.7

49.7

2.3

23.0

1.0

0.0

0.0

4.0

NH

-194

3.7

21.0

75.3

9.5

1.6

88.9

1.5

80.9

17.6

1.4

64.8

33.8

3.4

65.3

31.4

2.0

19.0

55.0

1.0

12.0

0.0

2.0

7.0

16.3

Ave

rage

5.4

17.4

77.3

18.2

4.7

77.2

3.1

69.4

27.5

16.9

64.4

18.8

3.8

78.5

17.7

3.8

16.2

51.1

2.1

19.2

0.6

1.1

3.8

8.1

SD

2.3

4.3

5.7

8.5

3.5

10.2

2.4

14.3

12.4

12.6

6.2

10.0

3.2

8.6

8.0

1.9

3.7

15.9

1.6

8.5

0.7

0.8

2.7

5.1

NH

-236

5.2

16.8

78.0

15.8

0.0

84.2

2.2

91.0

6.7

32.8

46.4

20.8

0.0

81.1

18.9

3.0

16.0

67.7

1.7

5.0

0.0

0.3

3.3

13.3

NH

-264

1.1

28.5

70.4

1.3

1.3

97.5

0.0

97.9

2.1

6.1

83.4

10.5

0.0

74.1

25.9

0.3

26.3

63.7

0.0

1.3

0.0

0.7

2.0

6.3

NH

-21b

0.0

4.8

95.2

0.0

36.4

63.6

0.0

98.1

1.9

0.0

93.3

6.7

0.9

67.6

31.5

0.0

4.7

91.0

0.0

1.7

0.0

1.7

0.9

4.3

NH

-27

0.0

19.8

80.2

0.0

0.0

100.

00.

094

.35.

70.

083

.017

.00.

075

.224

.80.

014

.354

.70.

03.

30.

01.

016

.75.

7N

H-1

760.

039

.061

.00.

00.

010

0.0

0.0

97.2

2.8

32.4

64.7

2.9

0.0

90.0

10.0

0.0

9.7

14.7

0.0

0.4

0.0

0.8

0.0

0.4

NH

-228

1.1

62.9

36.0

1.8

0.0

98.2

0.0

96.9

3.1

20.7

50.0

29.3

2.1

85.4

12.5

0.7

37.3

20.7

0.0

0.7

1.7

1.7

9.7

5.7

Ave

rage

0.3

31.6

68.1

0.5

9.1

90.5

0.0

96.6

3.4

13.3

72.8

14.0

0.8

79.6

19.7

0.2

16.5

45.3

0.0

1.5

0.4

1.3

6.8

4.0

SD

0.5

21.7

22.1

0.8

15.8

15.5

0.0

1.4

1.4

13.9

16.7

10.2

0.9

8.7

8.8

0.3

12.5

30.5

0.0

1.1

0.7

0.4

6.8

2.2

NH

-20

0.0

10.9

89.1

0.0

0.0

100.

00.

899

.20.

00.

760

.339

.00.

087

.212

.80.

010

.786

.30.

70.

00.

00.

31.

319

.0N

H-3

70.

06.

693

.40.

00.

010

0.0

0.0

100.

00.

00.

557

.342

.20.

077

.622

.40.

06.

389

.30.

00.

00.

00.

34.

030

.7N

H-1

390.

07.

792

.30.

00.

010

0.0

0.0

100.

00.

00.

087

.312

.70.

075

.724

.30.

07.

792

.00.

00.

00.

00.

00.

39.

7N

H-1

420.

07.

492

.60.

04.

895

.20.

010

0.0

0.0

0.0

56.9

43.1

0.0

78.3

21.7

0.0

7.0

87.7

0.0

0.0

0.0

0.0

1.3

15.7

NH

-150

b2.

05.

093

.025

.020

.055

.00.

081

.918

.172

.822

.54.

70.

097

.62.

41.

75.

075

.70.

016

.70.

00.

00.

73.

3A

vera

ge0.

47.

592

.15.

05.

090

.00.

296

.23.

614

.856

.928

.30.

083

.316

.70.

37.

386

.20.

13.

30.

00.

11.

515

.7S

D0.

81.

91.

510

.07.

717

.60.

37.

27.

229

.020

.616

.30.

08.

28.

20.

71.

95.

60.

36.

70.

00.

11.

39.

2


430A

-2-c

c0.

07.

192

.90.

00.

010

0.0

0.0

100.

00.

00.

00.

010

0.0

0.0

90.0

10.0

0.0

1.3

17.3

0.0

0.0

0.0

0.0

0.0

5.3

430A

-3-c

c0.

00.

010

0.0

0.0

94.3

5.7

0.0

35.0

65.0

0.0

100.

00.

00.

00.

025

.80.

01.

60.

00.

00.

010

.243

0A-4

-10.

00.

010

0.0

0.0

22.9

77.1

0.0

100.

00.

00.

084

.615

.40.

00.

012

.30.

041

.30.

00.

01.

10.

043

0A-4

-20.

09.

990

.10.

00.

010

0.0

0.0

85.9

14.1

0.0

39.4

60.6

0.0

97.1

2.9

0.0

8.0

63.2

0.0

10.3

0.0

0.0

4.6

11.5

Con

tam

?43

2-1-

42.

95.

192

.035

.70.

064

.30.

096

.93.

19.

675

.015

.40.

057

.842

.21.

63.

051

.30.

01.

60.

00.

01.

65.

343

2A-1

-10.

07.

492

.60.

00.

010

0.0

0.0

100.

00.

00.

048

.351

.70.

069

.230

.80.

05.

771

.80.

00.

00.

00.

04.

68.

643

2A-2

-10.

020

.979

.10.

00.

010

0.0

0.0

100.

00.

00.

035

.964

.10.

070

.429

.60.

013

.049

.30.

00.

00.

00.

06.

711

.2C

onta

m?

433A

-1-1

9.9

15.5

74.7

39.0

0.0

61.0

0.0

32.8

67.2

32.0

0.0

68.0

0.0

55.2

44.8

7.6

11.9

18.8

0.0

38.6

0.0

0.0

1.0

11.2

433A

-16-

10.

00.

010

0.0

0.0

25.0

75.0

0.0

0.0

100.

00.

00.

00.

30.

01.

00.

00.

00.

00.

343

3B-1

-10.

00.

010

0.0

0.0

0.0

100.

00.

00.

00.

00.

04.

20.

00.

00.

00.

0C

onta

m?

433B

-3-1

7.1

0.0

92.9

100.

00.

00.

00.

00.

010

0.0

0.3

0.0

0.0

0.0

4.5

0.0

0.0

0.0

0.0

433C

-1-1

0.0

0.0

100.

00.

010

0.0

0.0

0.0

0.0

100.

00.

010

0.0

0.0

0.0

0.0

9.1

0.0

0.0

0.0

0.0

2.3

9.1

433C

-3-1

0.0

5.0

95.0

0.0

0.0

100.

00.

084

.215

.80.

00.

010

0.0

0.0

9.1

90.9

0.0

1.0

16.0

0.0

3.0

0.0

0.0

0.0

4.0

Con

tam

?43

3C-3

-20.

04.

995

.10.

00.

010

0.0

0.0

94.8

5.2

6.3

0.0

93.8

0.0

35.3

64.7

0.0

1.7

30.8

0.0

1.7

0.0

0.0

0.3

5.0

Con

tam

?43

3C-3

-32.

06.

092

.00.

00.

010

0.0

0.0

95.7

4.3

0.0

0.0

100.

00.

07.

792

.30.

01.

014

.80.

00.

70.

00.

00.

04.

7T

otal

Ave

rage

1.4

5.4

93.0

17.4

082

.50

68.8

31.1

3.6

25.6

70.6

064

.735

.30.

633.

1125

.40

7.2

00

1.4

5.7

Tot

alS

tand

ard

3.0

6.2

7.52

32.9

032

.90

39.6

39.6

9.0

33.5

33.4

032

.933

1.9

4.4

23.2

013

.60

02.

14.

3de

viat

ion

Con

tam

?A

vera

ge4.

36.

389

.334

.90

65.0

064

.035

.911

.918

.769

.30

3961

1.9

3.5

23.1

09.

40

0.5

5.2

Con

tam

?S

tand

ard

4.0

5.6

8.2

40.9

040

.90

45.0

45.0

13.9

37.5

38.5

023

.123

.23.

24.

819

.20

16.4

00

0.7

3.9

devi

atio

nU

ncon

tam

Ave

rage

05.

094

.90

010

0.0

071

.228

.70

28.7

71.2

077

.522

.50

2.9

26.5

06.

10

01.

96.

0U

ncon

tam

Sta

ndar

d0

6.7

6.7

00

51.7

039

.139

.10

33.4

33.4

030

.230

.20

4.5

25.8

012

.80

02.

54.

7de

viat

ion


Tab

le4

Raw

poin

tco

unt

data

and

reca

lcul

ated

para

met

ers

for

inte

rsti

tial

cate

gori

es

Sam

ple

Frw

kM

atr

Mat

rIn

ter

Cla

yR

edP

ink

Op

Per

cent

cem

ent

type

%T

ot%

Tot

%T

ot%

Tot

num

ber

Gra

ins

Sil

tC

lay

Por

Cm

tZ

eol

Zeo

lcm

t%

Cla

y%

Red

%P

ink

%O

pM

atri

xC

emen

tP

orC

qP

IGV

Cm

tC

mt

NH

-76

NH

-25

300

470

9234

00

07.

20

00

9.94

7.19

19.5

26.6

36.6

NH

-51a

300

1816

7320

00

04.

70

00

7.96

4.68

17.1

21.8

29.7

NH

-55a

NH

-68a

NH

-106

NH

-137

NH

-156

aN

H-1

94av

erag

e6.

00.

00.

00.

08.

95.

918

.324

.233

.2N

H-2

36N

H-2

64

NH

21-B

233

00

07

7713

02.

123

.33.

90

029

.40

29.4

29.4

NH

-27

300

40

482

8711

316

.717

.72.

20.

60.

8137

.30.

8138

.138

.9N

H-1

76N

H-2

2830

013

00

20

760

0.5

019

.40

3.32

19.9

019

.923

.3av

erag

e6.

413

.78.

50.

21.

428

.90.

329

.130

.5N

H-2

0N

H-3

730

00

00

6844

00

16.5

10.7

00

027

.20

27.2

27.2

NH

-139

NH

-142

NH

-150

b

DS

DP

Frw

kM

atr

Car

bIn

ter

Cla

yZ

eol

Op

Intr

aP

erce

ntce

men

tty

pe%

Tot

%T

ot%

Tot

%T

otS

ite-

cG

rain

sS

iltr

Cla

Cm

tP

orC

mt

Cm

tC

mt

cmt

%C

lay

%C

art

%Z

eol

%O

pM

atri

xC

emen

tP

orC

qP

IGV

430A

-2-c

c75

3611

180

01

10

7.8

00.

725

.51.

412

.814

.246

.843

0A-3

-cc

128

9555

190

40

10

18.3

1.3

031

.61.

66.

38

57.5

430A

-4-1

179

972

290

20

90

24.7

0.7

03.

03.

69.

913

.638

.543

0A-4

-217

463

531

50

09

1.7

17.9

00

21.3

4.5

0.3

4.9

41.2

432-

1-4

304

432A

-1-1

174

8024

10

00

210

8.6

00

28.7

7.0

0.3

7.4

37.6

432A

-2-1

223

016

1042

00

5114

.45.

50

00

27.2

3.4

30.6

23.4

433A

-1-1

303

433A

-16-

129

343

3B-1

-128

743

3B-3

-129

043

3C-1

-188

063

240

00

80

360

00

4.37

13.7

18.1

49.7

433C

-3-1

300

433C

-3-2

299

433C

-3-3

298


bonate unit at this site. The remaining samples wereŽ .taken from thinner 50–60 m sedimentary sections

overlying basaltic basement rocks at Sites 430 and432, and a few sandy cores recovered at Site 431.

The unconsolidated samples were first sieved forŽ .the sand fraction 0.0625–2 mm , and the consoli-

dated samples were vacuum-impregnated with blue-dyed epoxy to better define porosity relationships.Thin sections produced from these samples werethen stained for potassium and calcium feldspar us-ing the method outlined by Marsaglia and TazakiŽ .1992 . This method results in the etching and stain-ing of calcium- and potassium-bearing zeolites aswell. Modal compositions were determined for 20Kronotsky and 15 Emperor samples using the

ŽGazzi–Dickinson point-count method Dickinson,.1970; Ingersoll et al., 1984 . Sand-sized mineral

components or phenocrysts in lithic fragments weretotalled with monomineralic grains, minimizing com-positional effects of grain-size variation among sam-ples. Grid spacings were larger than the maximumgrain size and maximized coverage of each thin

Ž .section. Up to 300 grains total in Table 1 wereŽcounted per thin section see Van der Plas and Tobi

Ž .1965 for discussion of reliability of point-count.data . Additionally, interstitial components, such as

porosity, cement and matrix, for selected sampleswere recorded and tabulated separately from thegrain counts.

Counted grains were placed into 37 categoriesŽ .Table 1 that include textural subdivisions of vol-

Ž .canic lithic types based on Dickinson 1970 ,Ž .Marsaglia 1991, 1992 . Interstitial categories and

recalculated parameters are also defined in Table 1.Raw point-count data are presented in Table 2, recal-culated parameters in Table 3, and interstitial dataand percentages in Table 4.

3. Petrographic descriptions: Kronotsky samples

3.1. Grain types

Diverse monomineralic and lithic components areŽ .present in Kronotsky sandstones Fig. 6 . The most

abundant monomineralic component is plagioclasefeldspar, followed by quartz and nonopaque denseminerals. The nonopaque dense minerals are most

commonly pyroxene with lesser amphibole and rareŽ .epidote and olivine sample NH-76 . Minor amounts

Ž .of potassium feldspar, albite unstained feldspar ,biotite, muscovite and opaque dense minerals arealso present. The plagioclase feldspar and nonopaquedense minerals range from fresh to extremely altered.Monocrystalline quartz is more common than poly-

Ž .crystalline quartz chert .Volcanic lithic fragments are the dominant grain

type found in Kronotsky sandstones. These includeŽ .colorless, brown and black tachylitic opaque-rich

Ž .glassy fragments as well as minor felsitic Lvf andŽ .holocrystalline fragments Lvo . The colorless glassy

fractions primarily exhibit holohyaline vitric textureŽ .Lvv , whereas the brown and black volcanic lithic

Ž .fractions are more enriched in microlitic Lvml andŽ . Ž .lathwork Lvl types. The microlites silt sized and

Ž .laths sand sized in these fragments are most com-monly plagioclase. Brown and colorless volcanicfragments exhibit various degrees of vesicularity,ranging to pumicerscoria. Brown glassy volcaniclithic fragments are most abundant except for oneEocene sample, where colorless glassy fragmentsdominate. In the latter, the shards tend to be wellsorted, exhibit bubble and bubble-wall shapes, and

Ž .take a light potassium stain yellow .Sedimentary lithic fragments are common in sev-

eral samples. They include variably silty argilliteŽ . Ž .Lsa , where the silt fraction can be volcanic vitric

Ž .to terrigenous quartzose with minor diatom frag-ments. Fragments within which the silt percentage is

Ž .greater than 50% are designated as siltstone Lsi . Insome cases, argillaceous sedimentary lithics appearsimilar to glass fragments altered to clay minerals;care was taken to differentiate the two types oflithics based on the presence of silt or microlites,diatom fragments, microporosity, and internal tex-ture. Sample NH-76 contains a large fragment ofquartzose sandstone with metamorphic lithic frag-ments. Rare pyroclastic lithics, including fragmentsof welded tuff, are placed in the ‘other’ category.

Ž .Sedimentary or ‘dirty’ chert fragments Lsch , someof which exhibit ghosts of radiolaria and silica-filledfractures, are a minor component.

Metamorphic lithic fragments are the least abun-dant lithic type found in Kronotsky sandstone. Wherepresent, they predominantly consist of fine quartzose

Ž . Ž .schist Lmt and polycrystalline mica Lmm .


Polymineralic fragments of feldspar, mica and quartzŽ . Ž .Lma are common in one sample NH-137 . Theseare likely gneissic or coarse schist fragments, but insome cases, may be fragments of felsic igneousintrusive rocks. A few fragments of igneous intrusive

Ž .rock e.g., Fig. 5C were recognized. Metamorphicfragments in some samples appear to be metavol-canic lithics.

Other minor constituents are biotite, muscovite,phosphate, bioclasts and glauconite. A few samples


contain a minor percentage of calcareous foraminiferaŽ .and shell fragments ? , or siliceous sponge spicules

and diatoms. Glauconite is abundant to dominant intwo Miocene samples. Relict circular meshworksindicate that, in many instances, the glauconite hasreplaced or enveloped diatoms.

3.2. Diagenesis

The high proportion of labile volcanic material inthese sandstones results in diverse grain alteration,

Ž .grain dissolution, and cementation Fig. 6 . The ma-jor authigenic phases are clay minerals and zeolites.Grains are coated by clay in the youngest fluvial

Ž .deposits NH-76 , but these coatings could representŽinfiltrated clay cutans clay coatings associated with

.exposure and soil development rather than cement.The underlying, Mio–Pliocene turbidite sandstonesare characterized by thin clay mineral rim cements,overprinted by fine acicular zeolite cement in oneinstance. In the turbidite sandstones, grains are rarelyreplaced by zeolite. In the older Miocene and Eocenesections, the clay rim cements are overprinted bypore-fill zeolite, and grains are commonly replacedby clay minerals and zeolites. The interparticle zeo-lite cements show some zonation, with the earliestphase being more calcium rich, as indicated by a

Ž .darker red Ca stain. Volcanic and monomineralicgrains are commonly replaced by a calcium- andpotassium-bearing zeolite in one Eocene sample, andby authigenic potassium feldspar or potassium zeo-lite in several Miocene and Eocene samples. The

Ž .color green to brown and morphology of the clayalteration products and cements indicate that they arelikely smectite or chlorite. In glauconite-bearing

samples, some of the glauconite fragments couldrepresent altered volcanic glass.

Plagioclase feldspar and nonopaque dense miner-Ž .als e.g., pyroxene are locally dissolved in the

Mio–Pliocene turbidites. In some basal Miocene andEocene sandstones, most of the plagioclase andnonopaque dense minerals have been dissolved andreplaced by calcium-bearing zeolites that take a lightpink stain. Brown glassy fragments are particularlysusceptible to alteration and dissolution. Many frag-ments have been partly to wholly altered to clayminerals without first being dissolved. There is acontinuum of alteration, so that the brown-glass cate-gory may include brown glass partly altered to clayminerals. Other fragments have partly to whollydissolved, forming secondary porosity, and in somecases, particularly in the Miocene and Eocene sam-ples, have been later infilled with zeolite cement.The infilling zeolite cement is commonly calcium-bearing, as indicated by it’s pale to dark pink stain.Colorless glass alteration includes dissolution, devit-rification and silicification. A few completely dis-solved grains of unknown mineral or lithic affinitywere placed in the ‘other’ category.

Secondary porosity due to dissolution ofmonomineralic grains and volcanic glass is not tabu-lated separately in Table 2, but may total up to a fewpercent of the rock’s volume. Primary residual poros-ity ranges up to 20% in the Mio–Pliocene turbidites,but has been reduced to less than 1% in the older

Ž .basal Miocene and Eocene samples Table 4 . Themore porous samples have 5–7% clay rim cement,whereas the more diagenetically altered samples haveas much as 17% clay mineral cement and up to 27%zeolite cement. The range of intergranular volumesfor both the younger and older sandstones is similar,both ranging up to 37–38%.

Ž .Fig. 6. Photomicrographs of Kronotsky sandstones. All in plane-polarized light. A Group 2 sample NH-194. Silty argillite fragment onŽ .right and fractured radiolarian-bearing, siliceous argillite on left. Unstained part of slide. Scale bar is 0.10 mm. B Group 2 sample NH-51a.

General view of microporous sandstone. Colorless stretched pumice fragment in lower left and various dark silty argillite and altered vitricŽ . Žlithic fragments. Unstained part of slide. Scale bar is 0.1 mm. C Group 4 sample NH-228. Large fragment of intrusive igneous rock quartz

. Ž .and slightly altered plagioclase . Scale bar is 0.05 mm. D Group 4 sample NH-27. Predominantly black vitric and microlitic grains. Whiterectangular grain on left is potassium feldspar surrounded by dark clay rim and pore-filling Ca-zeolite cements. Stained part of slide. Scale

Ž .bar is 0.05 mm. E Group 5 sample NH-37. General view of monomineralic feldspar and pyroxene grains and black vitric and microliticŽ .volcanic lithic fragments. Dark clay rims and pore-filling Ca-zeolite cements are present. Unstained part of slide. Scale bar is 0.05 mm. F

Group 5 sample NH-150b. Colorless glass shards exhibiting bubble-wall and cuspate morphologies. Unstained part of slide. Scale bar is0.10 mm.


4. Petrographic descriptions: Emperor Seamountsamples

4.1. Grain types

The DSDP samples from the Emperor Seamountsare predominantly composed of calcareous bioclastic

Ž .and variably altered volcaniclastic debris Table 2 .Bioclasts include whole and fragmented red algae,bryozoans, echinoderms, foraminifers, mollusks,worm tubes and sponge spicules. Monomineraliccrystals of plagioclase and nonopaque dense miner-

Ž .als e.g., pyroxene , are present both as individualgrains and phenocrysts in volcanic lithic fragments.The volcanic lithic fragments are dominantly brown,altered brown, and black tachylitic varieties with

Ž . Ž .vitric Lvv to microcrystalline Lvml and Lvl tex-tures. Carbonate and siltstone sedimentary lithicfragments are also present. Unconsolidated samplesexhibit a wider array of monomineralic grain andlithic fragment types, including monocrystallinequartz, colorless glass and siliceous sedimentary lithicfragments.

4.2. Diagenesis

Lithified samples show moderate diagenetic alter-ation, including grain alteration, grain dissolutionand multi-phased cementation. Bioclasts, feldspar andglassy volcanic lithic fragments are locally wholly topartly dissolved, creating minor secondary porosity.Glass alteration includes palagonitization, as well asalteration to clay minerals. Carbonate cement is pre-sent in all the lithified samples and is the dominantauthigenic phase. Minor zeolite and clay cements arepresent in a few samples. A more detailed discussionof the diagenetic history of these samples can be

Ž .found in the paper of Hyatt and Marsaglia 1995 .

5. Sandstone detrital modes

For the purposes of this study, the Kronotskysamples were grouped from oldest to youngest as

Ž . Ž . Žfollows Table 1 : Group 5sEocene ? sandstones.interbedded with basalt flows ; Group 4sMiocene

Žbasal Miocene sandstones unconformably overlying.Eocene basaltic basement; Hot Springs Member ;

ŽGroup 3s transitional Miocene sandstones transi-tional from the basal unit to the fine-grained turbidite

. Žunit ; Group 2sMio–Pliocene fine-grained tur-.bidite unit; Olga Member ; and Group 1spost-early

ŽPliocene sandstones overlying angular unconformitydeveloped on top of folded Mio–Pliocene unit; Rail-

.way Ridge Formation . Sandstone detrital modes areplotted by these groups in Figs. 7–12.

In general, the Kronotsky sandstones areŽ .feldspatholithic Figs. 7 and 8 and predominantly

Ž .composed of plagioclase feldspar Fig. 9 and brownŽ .glassy volcanic lithic fragments Figs. 10 and 11 .

However, there is some significant variation amonggroups. In particular, Group 2 sandstones are en-riched in quartz grains, sedimentary and to a lesserextent metamorphic lithic fragments, and felsitic vol-canic lithic fragments with respect to the othergroups. The Groups 1 and 4 samples have similarcompositions except that the Group 4 sandstones

Ž .exhibit a greater range of feldspar Figs. 7 and 8 andŽ .colorless volcanic lithic Fig. 11 proportions. Group

3, the transitional group, contains one sample that issimilar to Group 2 in composition and one samplesimilar to Group 4. The Group 5 sandstones areessentially composed of only brown to black glassyvolcanic lithics and plagioclase, with the exception

Ž .of one unique sample NH-150b . The latter sampleis predominantly composed of colorless glassy frag-ments with some quartz, potassium feldspar, andsedimentary lithic fragments.

Fig. 7. Standard QFL plot of Kronotsky samples and key tosample groupings. See Table 1 for definition of parameters.


Fig. 8. Mean values for Kronotsky Groups 2, 4 and 5 and forŽ .uncontaminated Emperor samples open star plotted on a QFL

ternary diagram truncated at QFL %Qs53%. Range of detritalŽ .modes for feldspatholithic Hawaiian beach sands Marsaglia, 1993

is indicated by arrowed line along base of diagram. Fields areŽ .after Dickinson et al. 1983 in capital letters and Marsaglia and

Ž .Ingersoll 1992 in lower case letters: BUsbasement uplift,ROs recycled orogen, DA sdissected arc, TA s transitional arc,UA sundissected arc, cascontinental arc, ias intraoceanic arc.See Fig. 7 and Table 1 for key and explanations.

As outlined below, the Emperor Seamount sam-ples can be divided into two groups, one with andone without downhole contamination. On average,the non-bioclastic fraction of the uncontaminatedsand and sandstone samples is predominantly com-

posed of volcanic lithic fragments and minor plagio-Ž .clase grains Figs. 7–10 . The volcanic lithic fraction

consists of subequal amounts of brown and blackŽ .glassy volcanic lithic fragments Fig. 11 , exhibitingŽ .microlitic and lathwork textures Fig. 12 .

Sandstone detrital modes provide some insightinto the tectonic history of the Kronotsky Peninsula.

Ž .Dickinson and Suczek 1979 and Dickinson et al.Ž .1983 defined compositional fields representing var-ious tectonic settings, including undissected, transi-tional and dissected arcs, on a QFL ternary diagramby plotting sample means for large data sets from the

Ž .literature. Marsaglia and Ingersoll 1992 collected amore extensive data set for Cenozoic, deep marine,arc-related sand and sandstone from primarily cir-cum-Pacific DSDP sites and redefined the QFLundissected arc field, dividing it into intraoceanicand continental-arc subfields. They also defined in-traoceanic and continental-arc fields on QmKP, Lm-LvLs, and LvfLvmlLvl ternary plots using this samedata set. It should be noted that Marsaglia andIngersoll’s intraoceanic-arc field overlaps their conti-nental-arc field, implying that sample means that fallin the intraoceanic-arc field are equivocal. Althoughsand and sandstone associated with intraoceanic vol-canic centers were not included in the study of

Fig. 9. Standard QmKP ternary plots. See Fig. 7 and Table 1 for key and explanations. All Hawaiian data plot at P apex.


Fig. 10. Standard LmLvLs ternary plots. See Fig. 7 and Table 1for key and explanations. All Hawaiian data plot at Lv apex.

Ž .Dickinson et al. 1983 , detrital modes for modernŽbeach sand from the Hawaiian Islands Marsaglia,

.1993 and for Cenozoic volcaniclastic sandstones

Fig. 11. Plot of colorless, brown and black glassy volcanicproportions. See Fig. 7 and Table 1 for key and explanations.Altered volcanic glass in uncontaminated Emperor samples waslikely brown. The addition of the altered fraction to the brownglassy fragment total results in a compositional shift as indicatedby arrowed line extending from open star symbol.

Fig. 12. LvfLvmlLvl ternary plot. See Fig. 7 and Table 1 for keyand explanations.

Žrecovered from Hawaiian–Emperor Seamounts this.study , including pyroclastic and epiclastic debris,

overlap those of intraoceanic arcs. The main differ-ences between sand with a mafic vs. an intermediateprovenance are that the mafic sand contains no quartzgrains, colorless glass fragments, or felsitic volcaniclithic fragments and is enriched in tachylitic glassyvolcanic debris, holocrystalline volcanic lithic frag-

Ž .ments Lvo , and volcanic lithic fragments exhibitingŽ .lathwork texture. See Marsaglia 1993 and Nesbitt

Ž .and Wilson 1992 for discussions of the effects ofweathering on detrital modes of basaltic sand.

When mean values for Kronotsky sandstones areplotted on the compositional diagrams described

Ž .above Figs. 8–10 and 12 they are consistent with amagmatic-arc source. Group 2 sandstones have asignificant amount of quartz and sedimentary lithics,and thus their sample means generally plot in thecontinental-arc field, whereas sandstones in Group 5tend to plot in the intraoceanic-arc fields on thesediagrams. The Group 4 sandstones exhibit lithic andmonomineralic populations that plot in the intrao-ceanic-arc fields, but have higher than expectedfeldspar proportions on a QFL plot. The overall lowquartz content of the Groups 4 and 5 samples is


more characteristic of intraoceanic-arc provenance.However, the similarity of Group 5 sand detritalmodes with those of the Hawaiian–Emperor

Ž .Seamounts and islands Figs. 7–12 suggests thepossibility of an alternative tectonicrmagmatic set-ting as discussed below.

6. Discussion

6.1. ProÕenance of sand at DSDP sites 430–433

We were quite surprised to find felsic componentssuch as monocrystalline quartz, colorless glass,quartz-bearing sedimentary fragments, and sedimen-tary chert in some of the unconsolidated samplestaken from Emperor Seamount cores, in part because

Žof their reported Eocene to Paleocene ages Jackson.et al., 1980 . It is possible that this material is

Ž .autochthonous intrabasinal , but we consider theŽ .material to be allochthonous extrabasinal down-hole

contamination of Pliocene–Pleistocene ice-rafted de-Ž .bris for the following reasons: 1 the presence of

Žice-rafted debris in drill cores at higher Pliocene–.Pleistocene stratigraphic levels at Emperor drill sites

Ž . Ž .e.g., Jackson et al., 1980 ; 2 the recovery of sandysections was very poor, there were drilling problemsat Sites 430–433 and some holes were abandoned

Ž . Ž .because of caving Jackson et al., 1980 ; 3 felsicvolcanic, metamorphic and sedimentary lithic frag-ments, and monocrystalline quartz grains are notpresent in the lithified samples that we examined

Ž . Ž .from these Sites 430–433 Table 2 ; and 4 indropstone suites recovered in Pliocene–Pleistocenesections at sites drilled on the northernmost Emperor

Ž .Seamounts e.g., Site 883 in Fig. 1 , McKelvey et al.Ž .1995 document the presence of arc-derived vol-canic and volcaniclastic clasts, as well as metamor-phic clasts, rare plutonic rocks, and clastic sedimen-tary rocks that include clasts of argillaceous chert.They propose that the likely sources of this debriswere ice bergs that travelled from Kamchatka Penin-sula and the eastern Bering Sea. This suite of clasttypes is similar to the lithic fragments found in thesandy fractions of many of the unconsolidated sam-ples recovered at Sites 430–433 and also to pebbles

Ž .recovered at Site 431 Jackson et al., 1980 .Because of the likely presence of extrabasinal

materials in some samples, we have designated four

quartz- and colorless-glass-bearing samples as ‘sus-pect’ and only use the remaining eleven samples as

Žcomparators for Kronotsky basement rocks Tables.1–4 . The monomineralic and lithic populations in

the uncontaminated samples from Sites 430–433 areconsistent with a mixed mafic-volcanic andshallow-marine carbonate source. The mean compo-sition of these samples falls within the range of

Ž .compositions documented by Marsaglia 1993 forŽ .Hawaiian beach sand Figs. 7–9, 11 and 12 , except

Žin terms of volcanic to sedimentary lithic ratios Fig..10 . Some Emperor samples are enriched in calcare-

ous and argillaceous lithic fragments. These could bereworked fragments of beach, lagoonal or reef rock,or possibly be ‘pseudo-lithics’ representing calcare-

Žous interbeds disaggregated during drilling the low.recovery at these sites supports this . A mean value,

rather than a range of values, is plotted on thevarious ternary diagrams because of the relativelylow proportion of volcaniclastic debris, as opposedto bioclastic debris, in these samples. The high de-gree of alteration of consolidated samples is alsoimportant and must be considered in provenancedeterminations. For example, tallying altered glassyfragments with unaltered brown glassy fragmentsproduces a significant shift in mean composition onFig. 11. Such a shift is important, because MarsagliaŽ .1993 has pointed out that the proportion of black tobrown vitric components likely reflects the mode ofproduction of the volcaniclastic debris, as discussedbelow.

6.2. ProÕenance of mafic basement rocks and( )Miocene coÕer sequence Groups 5 and 4

If the Kronotsky basement rocks originated aspart of an oceanic plateau or ridge, then sedimentaryfacies and basement lithologies should be similar tothose of seamounts and islands along the Hawaiian–

ŽEmperor chain to the east e.g., Jackson et al., 1980;.Floyd, 1991; Marsaglia, 1993 , or ancient equiva-

lents accreted and now exposed at plate marginsŽ .e.g., MacPherson, 1983 . Seamounts are character-ized by basement complexes composed of pillowedtholeiitic to alkalic basalt with a carapace of hyalo-

Žclastites, overlain by subaerial basalt flows Floyd,.1991 with intercalations of lapilli tuff and sand, and

local soil horizons. If emergent at the time magma-


tism ceased, then the volcanic edifice would beeroded and eventually subside, producing an uncon-formity overlain by relatively conformable shallow-marine to deep-marine sediments. The thickness ofthe sedimentary cover sequence would be variable,from hundreds of meters in lagoonal regions to a fewtens of meters or less on volcanic promontories,dependent in part on the degree of reef developmentprior to subsidence below the photic zone. Factorsthat would affect pelagic sediment accumulation af-ter subsidence below the photic zone include water–mass productivity, water depth, and degree of fine-sediment winnowing by currents. Thus, the Creta-ceous–Miocene stratigraphy observed on the Kronot-sky Peninsula, namely a basal section of interbeddedbasaltic volcanic flows and volcaniclastic units, un-conformably overlain by a shallow-marine epiclasticdeposits with minor carbonate bioclastic debris, inturn overlain by a thin pelagic sequence, could beattributed to a seamount or volcanic plateau setting.

In addition to stratigraphy and basement age andgeochemistry, the detrital modes of Groups 4 and 5volcaniclastic sandstones are key to discerning theorigin of Kronotsky basement and Miocene coversequence. Most Groups 4 and 5 sandstones exhibitcompositions similar to Hawaiian–Emperor volcani-clastic sand and sandstone, particularly in terms of

Ž .volcanic lithic proportions Figs. 7–12 ; they aredominated by brown glassy components exhibitingmicrolitic textures, and contain a significant amountof black tachylitic glass. Black tachylitic glass is aproduct of mafic volcanism, with the color a functionof slow cooling rates and the nucleation of opaque

Žmicrolites see discussion in the paper of MarsagliaŽ ..1993 . Sideromelane or brown translucent maficglass is produced by more rapid quenching wherebasaltic lava comes in contact with water. Thus, therelatively higher proportion of brown glass to blackglass in the Kronotsky samples, as compared to mostHawaiian samples, may be a function of cooling rate;Hawaiian samples are primarily composed of epi-clastic debris derived from subaerial basalt flows thatcooled relatively slowly, whereas the Kronotsky sandmay be hydroclastic debris produced during rapidquenching of basalt erupted in a submarine setting.

Both the basement complex and the Miocenecover sequence locally contain felsic vitric compo-

Ž .nents. Sample NH-150b Group 5 is a felsic vitric

Ž .tuff, whereas sample NH-228 Group 4 contains afew percent colorless-vitric component. The color-

Žless glass in the former is potassium rich takes a.light potassium stain , appears to be fresh and com-

pletely unaltered, and the latter sample is unusual inŽ .that the dominant )60% components are plagio-

clase feldspar and glauconite. This minor felsic com-ponent rules out neither an intraplate mafic volcanicsource, nor an intraoceanic arc source. Intraoceanicarcs, when in an extensional tectonic regime, canproduce large quantities of felsic pyroclastics, as in

Ž .the Izu–Bonin arc Taylor et al., 1990 . Likewise,minor, late-stage felsic eruptions are also known

Žfrom intraoceanic, hot-spot volcanoes e.g.,.MacPherson, 1983; Hall, 1987 . It should be noted,

however, that turbidites derived from active intrao-Žceanic magmatic arcs e.g., Aleutians, Marianas,

.Tonga consistently contain a significant colorlessŽglassy component Marsaglia and Devaney, 1995;

.Marsaglia et al., 1995a . Thus, the lack of admixedcolorless glass in the Kronotsky basement samplesfurther supports an intraoceanic plateaurseamountprovenance.

One might expect disruption and discordance inassociation with the tectonic emplacement of aseamount, but the disconformable nature of the con-

Ž .tacts between the Eocene ? basement and basalMiocene epiclastic sequences, and apparently con-formable contacts within the Miocene and Pliocenebasin fill implies that the basement rocks and associ-ated cover sequence were not significantly deformedprior to erosion and burial. The absence of angular

Ž .unconformity ies within the sequence might supportthe argument that the basement was not tectonicallyemplaced, but consists of in-situ forearc basementrocks. This hypothesis is additionally supported bythe presence of trace amounts of quartzose igneousintrusive fragments in the Hot Springs Member;these fragments were likely derived from westernsources because no felsic igneous intrusions havebeen described within the Kronotsky basement ter-ranes. However, we have only examined samplesfrom the uppermost basement unit. And indeed, anangular unconformity was reported by Shapiro and

Ž .Seliverstov 1975 between the Cretaceous Ka-menisty Cape Group and overlying Paleogene se-quences. This leaves open the possibility of an olderaccretion event.


Nevertheless, the Kronotsky volcanic rocks lieanomalously close to the present trench. If theyrepresent a nascent arc developed along the marginduring the Eocene, then there must have been signifi-cant tectonic erosion of the forearc prior to thedevelopment of the Sredinny arc in the Oligocene.Other possible explanations for forearc magmatisminclude forearc rifting, as hypothesized for the Izu–

Ž .Bonin forearc Taylor et al., 1990 , or spreading-ridgesubduction, as proposed for the Taitao ophiolite in

Žsouthern Chile Behrmann et al., 1992; Forsythe and.Prior, 1992 ; however, we consider these scenarios

less likely.

(6.3. ProÕenance of transition zone sandstones Group)3

As discussed above, the two samples from thetransition zone are not transitional in compositionbetween groups, but exhibit end-member composi-

Ž .tions one Group 4 and one Group 2 . This issignificant in that it suggests that there was an abruptchange in sand provenance during the Miocene inassociation with basin subsidence.

(6.4. ProÕenance of Mio–Pliocene turbidites Group)2

Sandstone in the Mio–Pliocene turbidite sectionhas a more diverse monomineralic and lithic popula-tion in comparison to the underlying units. Thesesamples contain traces of metamorphic lithics andlocally, a significant proportion of sedimentary lithicsŽ .Fig. 10 . Their volcanic lithic populations include

Ž .some felsitic fragments Fig. 12 and a significantŽ .fraction of colorless glassy fragments Fig. 11 . The

diversity of grain types suggests a wide assortmentof volcanic, sedimentary and metamorphic sourcerocks. Limited paleocurrent measurements from thissequence indicate west-to-east transport.

6.4.1. Volcanic source terranesŽThere are two main volcanic belts central Kam-

.chatka and east Kamchatka that may have suppliedŽ .pyroclastic debris to the Tyushevka Basin Fig. 3 .

Sedimentation in this basin occurred at about theŽ .same time ;5 Ma that the locus of arc magmatism

shifted 150 km eastward from the central Kamchatka

Ž .volcanic belt Sredinny Range to the east Kam-chatka volcanic belt. This shift was accompanied byregional extension and formation of the central Kam-chatka graben, an intraarc basin that separates thetwo volcanic chains and that has been characterized

Žby basaltic magmatism during the Quaternary Erlich,1968; Shanster and Shapiro, 1988; Zinkevich and

.Tsukanov, 1993; Cao et al., 1995 . The central Kam-chatka volcanic belt contains large ignimbrite fieldsand a thick sequence of andesite, dacite and rhyoliteinterbedded with clastic units, capped by plateaubasalts, whereas volcanic sequences of the east Kam-chatka volcanic belt are composed of basalt, an-desite–basalt, and andesite, with rare dacite and

Ž .ingimbrite Parfenov et al., 1993 . Ash layers recov-Žered at ODP Site 882 on the Obruchev Rise Detroit

.Seamount indicate that somewhat continuous inter-mediate to felsic volcanic activity may have occurredalong the Kamchatka Peninsula during the late

Ž .Miocene and Pliocene Cao et al., 1995 .Unaltered volcaniclastic debris found in the Olga

Member turbidite samples suggests a mixed prove-Žnance with contributions from mafic black glassy

. Žfragments , intermediate to mafic brown glassy

. Žfragments and felsic colorless glassy and felsitic

. Ž .fragments eruptions Fig. 11 . The dominance ofbrown and black glassy fragments in Samples NH-194 and NH-137 indicate that these had predomi-nantly mafic to intermediate volcanic sources, but asignificant percentage of colorless glass in a fewsamples suggests a felsic source. Thus, the volcani-clastic lithic population could have been derivedfrom both the more felsic central Kamchatka beltandror the more mafic east Kamchatka volcanicbelt. More detailed glass and phenocryst chemistrywould be needed to better constrain the volcanicprovenance.

The presence of a minor proportion of alteredŽ .pre-diagenesis and devitrified volcanic debris sug-gests some recycling of the older volcanic terranesthat lie directly inboard of the Kronotsky Peninsula.

Ž .The Ozernoy–Valaginskig Valaginskiy and Vet-lovskiy terranes consist of Cretaceous to lowerEocene assemblages of basalt, andesite, tuff, serpen-tinite melange and volcaniclastic and siliceous sedi-mentary rocks that are thought to represent facies ofprimitive magmatic-arc and accretionary complexesŽRotman et al., 1973; Parfenov et al., 1993; Zinke-


.vich and Tsukanov, 1993 . These rock units couldhave been exposed at the time of Olga Memberdeposition and could have been a source of volcani-

Ž .clastic debris see discussion below .

6.4.2. Sedimentary source terranesSedimentary lithic fragments are also more abun-

dant in the transitional Miocene samples and theŽ .Plio–Pleistocene samples Groups 1–3 . They are

absent from Groups 4 and 5, except for one sampleŽ .NH-150b . The presence of vitric silt- and siliceousmicrofossil-bearing mudstone fragments and sedi-

Ž .mentary dirty chert fragments is consistent with aforearc source. This source may have been the Vala-ginskiy terrane discussed above as a possible source

Ž .of recycled volcanic debris. Rotman et al. 1973reported that this terrane contains Cretaceous–Paleogene volcaniclastic sandstones, black argillitesand cherty sediments. These Cretaceous–Paleogenerocks are separated from the overlying Miocene rocksby an angular unconformity and hiatus. Alterna-tively, these sedimentary lithic fragments could havebeen produced in response to uplift and erosion ofthe forearc fill, or the formation and downcutting ofsubmarine channels through the basin.

6.4.3. Metamorphic source terranesMetamorphic rock fragments are present in one

Ž .Miocene transition sample NH-236 and Plio–Ž .Pleistocene samples Groups 1 and 2 . These meta-

morphic components consist predominantly ofquartz–mica tectonite with traces of polycrystallinemica and quartz–feldspar–mica aggregate fragments,consistent with having been derived from quartzosemetasedimentary, schist and gneissic rocks. Outcropsof pre-Cretaceous basement rocks are concentratedin the central Kamchatka and Ganal’ metamorphicterranes of south-central Kamchatka. Rocks in theseterranes include granite, gneiss and amphibolite togreenschist-facies metasedimentary and metavol-

Žcanic rocks, including quartzite Fig. 2; Shul’diner etal., 1979; Watson and Fujita, 1985; Fedorchuk et al.,1993; Parfenov et al., 1993; Zinkevich and Tsukanov,

.1993 .Although quartz, quartzose siltstone and meta-

morphic fragments found in Kronotsky samples couldhave been derived directly from basement outcrops

along the southern axis of the Kamchatka Peninsula,alternatively they may have been recycled from up-lifted and eroded Cretaceous to Paleocene clastic

Ž . Žunits described by Shapiro et al. 1992 unfor-tunately direct comparison of their data set with ours

Ž .is questionable because Shapiro et al. 1992 usedpolished sections that were not stained for feldsparrecognition, they counted only 150 points per sec-tion, grouped nonferrous minerals in the lithic cate-gory, and provide no detailed information on lithicproportions in that they were not able to differentiate

.altered volcanic from sedimentary lithic fragments .Ž .Shapiro et al. 1992 did not include samples from

the Kronotsky Peninsula in their study, but discussedthe various western source terranes that may haveprovided sand to the Kronotsky area. They dividedtheir sandstones into two provenance groups by re-gion, one on the western margin of the KamchatkaPeninsula with a transitional- to dissected-arc prove-

Žnance average compositional range of: QFL %Q.20–35, QFL %F 20–35, and QFL %L 35–50 , and

the other to the east with an undissected intrao-ceanic- to continental-arc provenance. The westernsequence consists of turbidite sandstone and in-terbedded bimodal volcanic and volcaniclastics. Thelithic population in these sandstones is predomi-nantly volcanic, but includes some metamorphic andshallow-intrusive rock fragments that suggest a con-tinental source. Their eastern province exhibits atemporal trend from quartz poor, feldspatholithicŽQFL %Qs0; undissected intraoceanicrcontinental

. Žarc to more quartzofeldspathic QFL %Qs20;.undissected continental arc sandstone compositions

with time. The source of continental detritus in theseŽsandstones was either the Asian margin Shapiro et

.al., 1992 , or perhaps continental basement rocks ofŽthe Kamchatka Peninsula the Okhotsk microconti-

.nent of Geist et al., 1994 . Sand may have been, inpart, recycled from uplifted sedimentary terranes.Sandstone detrital modes were determined by Shapiro

Ž . Ž .et al. 1992 for Lower Paleocene to Eocene? andUpper Cretaceous turbidite sandstones across theKamchatka Peninsula.

( )6.5. ProÕenance of Railway Ridge sample Group 1

Pliocene through Quaternary magmatism acrossthe Kamchatka Peninsula was dominantly basaltic


Ž .Parfenov et al., 1993 . The youngest sample exam-ined in this study comes from the base of the Rail-way Ridge Formation, directly above the uncon-formable contact with the folded Olga Member. Ithas a few percent sedimentary lithic fragments, but isoverwhelmingly composed of volcaniclastic debris,with lesser plagioclase, olivine and pyroxene. Thevolcanic component consists of roughly equal pro-

Žportions of brown and black glassy fragments Fig..11 ; these fragments exhibit roughly equal propor-

Ž .tions of microlitic and lathwork textures Fig. 12 .The sample contains no quartz or potassium feldsparand only a few percent metamorphic and colorless

Ž .glass fragments Table 3 . The volcanic andmonomineralic components in this sample are verysimilar to those in modern Hawaiian beach sandsŽ .Figs. 7–12 , consistent with derivation from a sub-aerial basaltic volcanic center.

6.6. Forearc signature of aseismic ridge subduction?

Triple-junction interaction along subduction zonescan produce forearc uplift and erosion, and result inthe recycling of forearc sedimentary sequences, suchas trench fill, accretionary prism and forearc basin

Ž .fill Pavlis et al., 1995 . The stratigraphic signaturesof these events in the forearc include regional uncon-

Žformities across deformed forearc sequences Pavlis.et al., 1995 and a significant recycled sedimentary

component in sediment shed from uplifted regionsŽMarsaglia and Ingersoll, 1992; Marsaglia et al.,

.1992, 1995b . The loci of uplifts and adjacent basinsmay migrate along a margin with triple-junctionmigration. Similar effects can be produced by aseis-

Ž .mic ridge subduction Pavlis et al., 1995 .There is evidence for both uplift and sedimentary

recycling in the Kronotsky forearc region, possiblyassociated with aseismic ridge subduction. Uplift

Ž .events include forearc uplift in the Miocene ? , anddeformation and uplift of forearc-basin fill in the

Ž . Ž .Pliocene ? Lagoe et al., 1995 . In addition, thesedimentary-lithic component in the Mio–Pliocenebasin fill suggests uplift and erosion of sedimentaryunits in the source region, somewhere along thebasin margin. Temporal shifts in the locus of forearcuplift across the margin could be a function ofsubduction angle of the seamount chain or irregular

Ž .topography isolated seamounts along the ridge.

In the case of a ridge–trench–trench triple junc-tion, such as the Chile Triple junction, spreading-ridge subduction can result in anomalously near-

Ž .trench magmatism e.g., Taitao Ophiolite , tectonicerosion of the accretionary prism, and an influx ofhydrothermal fluids into the overlying accretionary

Žwedge Forsythe and Nelson, 1985; Behrmann et al.,.1992; Forsythe and Prior, 1992 . Kronotsky Mio–

Pliocene turbidite sands exhibit classic indicators ofearly marine diagenesis, including dissolution of vol-canic glass and plagioclase and early clay-rim ce-mentation. There appear to be no diagenetic over-prints associated with their deformation and upliftabove sea-level. The Kronotsky forearc strata are

Ž .considered by Levin 1995 to have good reservoirpotential in that the sandstones have permeabilitiesof up to 11 mD and porosities up to 21%, anddisplay oil shows in wells drilled along the marginsof the basin. Porosity estimates of 17–20% forMio–Pliocene sandstones based on point-count datacollected in this study are in good agreement withthe measured porosities of 20–21% reported by LevinŽ .1995 . We observed a progressive increase in diage-netic intensity down through the sequence, however,that almost obliterates porosity in the older units.Geothermal gradient in the Kronotsky forearc basinis nonlinear, with recorded temperatures of 808C at 1km, 1208C at 2 km, 1808C at 3 km, and 2308C at 4

Ž .km Levin, 1995 . The differences in degree andstyle of sandstone alteration in the point-countedsamples could be attributed to variations in originalcomposition and maximum depth of burial.

7. Summary and conclusions

The interbedded basalt flows and volcaniclasticsediments of the Kozlovsk Group and overlyingshallow-marine to deep-marine cover sequence mir-rors that found on Emperor Seamounts. In addition,sandstone detrital modes for the volcaniclastic unitsinterbedded with the flows and overlying shallow-marine facies are consistent with a seamount origin.However, stratigraphic considerations, such as thedisconformable contact between the Kozlovsk Groupand overlying cover sequences, the lack ofaccretion-related structural disruption, and the likelypost-Cretaceous age of the Kozlovsk Group suggest


that the Kozlovsk Group is in situ and was notaccreted to the Kamchatkan forearc. We cannot ruleout an accretion origin for older basement terraneson the Kronotsky Peninsula, because the seamountgeochemical signature of the Kozlovsk basalts couldbe a product of eruption through an accretedseamount terrane. Perhaps the Kozlovsk Group rep-resents early subduction-related magmatism of anascent arc developed on an accreted terrane. Insupport of this model, sandstone detrital modes forthe Kozlovsk Group and overlying shallow-marinecover sequence overlap those of poorly evolved ornascent intraoceanic magmatic arcs.

Cessation of volcanism on the Kronotsky Penin-Žsula was followed by rapid subsidence thermal sub-

.sidence or subduction erosion? and influx of arc-de-rived sediment from the west. The moderate sedi-mentary lithic component in these sandstones indi-cates uplift and erosion of forearc sedimentary se-quences. Such sedimentary lithic input is a finger-print of forearc disruption due to triple-junction in-teraction, but in the Olga example, it is likely linkedto subduction of an aseismic ridge or seamount.Post-depositional deformation, uplift and erosion ofthe Tyushevka Basin succession could also be tied toridge or seamount subduction.

Acknowledgements

This study would not have been possible withoutthe field assistance of N. Harun, N. Tsukanov and A.Pachalov. We also thank M. Bazhenov, N. Lev-ashova and M. Shapiro for assistance in the field, J.Barron for paleontological data, and D. Abbott, R.Ingersoll, M. Shapiro and J. Winterer for theirthoughtful reviews. Financial support was providedby NSF-EAR9404963 to Mann and M. Lagoe. Thisis UTIG contribution a1388.

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evaluating the influence of aseismic ridge subduction and accretion(?) on detrital modes of forearc...

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