40Ar/39Ar thermochronologic constraints on deformation, metamorphism and cooling/exhumation of a Mesozoic accretionary wedge, Otago Schist, New Zealand

Download 40Ar/39Ar thermochronologic constraints on deformation, metamorphism and cooling/exhumation of a Mesozoic accretionary wedge, Otago Schist, New Zealand

Post on 29-Oct-2016

212 views

Category:

Documents

0 download

Embed Size (px)

TRANSCRIPT

  • the high-grade metamorphic core between 109 and 100 Ma.

    Tectonophysics 385 (2004) 181210low-grade south flank (hanging wall to the Remarkables Shear Zone or Caples Terrane) range from 144 to 156 Ma (n=5).

    Most of these samples show complex age spectra caused by mixing between radiogenic argon released from neocrystalline

    metamorphic mica and lesser detrital mica. Several of the hanging wall samples with ages of 144147 Ma show no

    evidence for detrital contamination in thin section or in the form of the age spectra. Apparent ages from the high-grade

    metamorphic core (garnetbiotitealbite zone) range from 131 to 106 Ma (n=13) with a strong grouping 113109 Ma

    (n=7) in the immediate footwall to the major Remarkables Shear Zone. Most of the age spectra from within the core of

    the schist belt yield complex age spectra that we interpret to be the result of prolonged residence within the argon partial

    retention interval for white mica (f430330 jC). Samples with apparent ages of about 110109 Ma tend to giveconcordant plateaux suggesting more rapid cooling. The youngest and most disturbed age spectra come from within the

    Alpine chlorite overprint zone where samples with strong development of crenulation cleavage gave ages 85107 and

    f101 Ma, due to partial resetting during retrogression. The bounding Remarkables Shear zone shows resetting effects due(hanging wall to the Hyde-Macraes and Rise and Shine ShMajor shear zones separating the low-grade and high-grade parts of the schist define regions of separate and distinct

    apparent age groupings that underwent different thermo-tectonic histories. Apparent ages on the low-grade north flank

    ear Zones) range from 145 to 159 Ma (n=8), whereas on the(shear zone deformation). This was followed either by very

    form of extensional (tectonic) exhumation and cooling of40Ar/39Ar thermochronologic constraints on deformation,

    metamorphism and cooling/exhumation of a Mesozoic accretionary

    wedge, Otago Schist, New Zealand$

    D.R. Graya,*, D.A. Fosterb

    aVIEPS School of Earth Sciences, University of Melbourne, Melbourne, Vic. 3010, AustraliabDepartment of Geological Sciences, University of Florida, Gainesville, FL 32611-2120, USA

    Received 27 October 2003; accepted 6 May 2004

    Available online 2 July 2004

    Abstract

    Structural thickening of the Torlesse accretionary wedge via juxtaposition of arc-derived greywackes (Caples Terrane)

    and quartzo-feldspathic greywackes (Torlesse Terrane) at f120 Ma formed a belt of schist (Otago Schist) with distinctmica fabrics defining (i) schistosity, (ii) transposition layering and (iii) crenulation cleavage. Thirty-five 40Ar/39Ar step-

    heating experiments on these micas and whole rock micaceous fabrics from the Otago Schist have shown that the main

    metamorphism and deformation occurred between f160 and 140 Ma (recorded in the low grade flanks) through 120 Magradual cooling or no cooling until about 110 Ma, with some

    www.elsevier.com/locate/tectoto dynamic recrystallization with apparent ages of 127122 Ma, whereas overprinting shear zones within the core of the

    schist show apparent ages of 112109 and f106 Ma. These data when linked with extensional exhumation of high-grade

    0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

    doi:10.1016/j.tecto.2004.05.001

    $ Supplementary data associated with this article can be found, in the online version, at doi: 10.1016/j.tecto.2004.05.001.

    * Corresponding author. Tel.: +61-3-8344-6931; fax: +61-3-8344-7761.

    E-mail addresses: drgray@unimelb.edu.au (D.R. Gray), dfoster@geology.ufl.edu (D.A. Foster).

  • ast G

    ceous

    al exhumation.

    Controversy still exists over how the Otago Schist

    D.R. Gray, D.A. Foster / Tectonophysics 385 (2004) 181210182al., 1976; Bradshaw, 1989; Mortimer, 1993a,b). It

    represents a period of continuous sedimentation and

    belt formed, the relative components of crustal thick-

    ening and thinning, the rate and nature of cooling andCretaceous accretionary wedge that developed along

    the Mesozoic margin of East Gondwana (Coombs etwhole rocks from selected fabric elements, and then

    linking the overall timing of fabric development with

    tectonic parameters external to the schist belt.

    The Otago Schist belt provides a well-exposed

    example of the exhumed deeper levels of a Jurassic

    1985). Recently, recognition of extensional shear

    zones (Deckert et al., 2002; Forster and Lister, 2003)

    has led to models of core complex-related exhuma-

    tion (Forster and Lister, 2003), although Mortimer

    (1993a,b, p. 242) had previously argued for extension-rocks in other parts of New Zealand indicate that the E

    f11090 Ma period.D 2004 Elsevier B.V. All rights reserved.

    Keywords: Convergent margin tectonism; Accretionary wedge; Creta

    history; Otago Schist; New Zealand

    1. Introduction

    Within the deeper level of accretionary sediment

    wedges, the processes of structural evolution and

    exhumation of metamorphosed and transposed inter-

    bedded sandstone and mudstone sequences are only

    partly understood. Structures deep within these wedges

    are characterized by intense schistosity development,

    large-scale thrust-nappes and crosscutting shear zones

    (e.g. Hara et al., 1990; Kusky et al., 1997; Aoya and

    Wallis, 2003). Establishing which structures and fab-

    rics relate to thickening of the schist wedge, that is, by

    underplating in a simple wedge model, versus those

    structures that form by vertical thinning and/or related

    extensional collapse with exhumation is problematic.

    Various hypotheses for the structural metamorphic

    evolution of wedges have testable predictions. For

    example, models of underplating combined with ver-

    tical shortening suggest fabrics dominated by coaxial

    strain and symmetrical flattening, uniform gradual

    cooling and exhumation via erosion (e.g. Feehan and

    Brandon, 1999; Ring and Brandon, 1999); whereas

    models of shortening via nappe development at depth

    followed by extensional exhumation and erosion sug-

    gest non-coaxial deformation overprinted by exten-

    sional shear zones and nonuniform cooling histories. In

    this paper, we address some of these issues for the

    Otago Schist belt of New Zealand (Fig. 1) using

    mesoscopic structural overprinting relationships,40Ar/39Ar step heating experiments on micas andaccretionary prism accretion (Rakaia and Pahau Ter-ondwana margin underwent significant extension in the

    tectonics; 40Ar/39Ar geochronology; Tectonic exhumation; Cooling

    ranes; Bradshaw, 1989) in the hanging wall to a long-

    lived subduction system that bordered the then Gond-

    wana continent. Consisting largely of monotonous

    quartzo-feldspathic schist with minor intercalated mi-

    caceous schist, greenschist and metachert, the Otago

    Schist includes variously metamorphosed rocks of

    Torlesse (quartzo-feldspathic greywackes), Caples

    (volcaniclastic greywackes) and Aspiring (chert-mafic

    volcanics) affinities. These have undergone a mutual

    syn- to post-amalgamation regional metamorphism to

    become part of the Otago Schist (Mortimer, 1993a,b,

    2000; Graham and Mortimer, 1992).

    Exposed as a region of domed flat-lying foliation

    (Fig. 1b) within intensely deformed quartzo-feldspath-

    ic greywacke the Otago Schist crops out as an f150km wide, elongate, NW-trending, metamorphic belt

    (Fig. 2) cored by garnetbiotitealbite greenschist

    facies schist coincident with a medial antiform (Mor-

    timer, 1993a,b, 2000). Mineral parageneses indicate

    PT conditions of 450 jC and 810 kbar (Mortimer,2000) suggesting burial to depths off2030 km.

    Previous KAr and ArAr geochronology applied

    to whole rock and partial mineral separates from the

    Otago Schist argue for a Jurassic age for schist

    metamorphism and a Cretaceous age for uplift and

    final closure of the isotopic systems (see Harper and

    Landis, 1967; Adams et al., 1985; Adams and Gabites,

    1985). Stratigraphic and provenance data indicate that

    exhumation of the Otago Schist was complete by

    the late Early Cretaceous (f105 Ma, Adams et al.,the process of exhumation. New thermochronologic

  • D.R. Gray, D.A. Foster / Tectonophysics 385 (2004) 181210 183and geochronologic data presented in this paper further

    constrain the timing of regional deformation and

    metamorphism, as well as the cooling history and

    exhumation of the schist belt. These data show that

    much of the deformation is significantly younger than

    previously considered and that apparent cooling ages

    from the high-grade schist core coincide with the

    timing of overall extension in other parts of the New

    Zealand landmass.

    Fig. 1. (a) Geologic map of New Zealand showing the key tectonic elemen

    and Torlesse terrane components. The Otago Schist deformation/metamor

    defining a NW-trending schist belt. (Map modified from tectonostratigrap

    showing the crustal architecture of the southern part of the South Island (b2. Geological background

    2.1. Geological makeup

    The islands of New Zealand (Fig. 1a) consist of

    distinct terranes (Coombs et al., 1976; Bishop et al.,

    1985; Frost and Coombs, 1989) that represent dif-

    ferent parts of the arc-trench-subduction system that

    was active along the Gondwana margin from Triassic

    ts, the main terranes and provinces, the distribution of plutonic rocks

    phism overprints the Caples terrane and part of the Rakaia terrane

    hic terrane map of IGNS Qmap Series map). (b) Geological profile

    ased on seismic reflection profile from Mortimer et al. 2003, Fig.8).

  • D.R. Gray, D.A. Foster / Tectonophysics 385 (2004) 181210184Nevis Bluffto Late Cretaceous times (e.g. Bradshaw, 1989;

    Mortimer, 2004). These terranes include:

    Median Batholith (former Median Tectonic Zone):

    Cordilleran-style composite batholith consisting

    of TriassicEarly Cretaceous subduction-related I-

    type plutonic, volcanic and sedimentary rocks that

    intrude and separate Permian of the Eastern

    Province Brook Street Terrane from the lower to

    mid-Palaeozoic Gondwana margin assemblages of

    the Western Province (Kimbrough et al., 1994;

    Muir et al., 1988; Mortimer et al., 1999).

    Brook Street terrane: Permian arc sequence of

    layered ultramafic-gabbro sequences, diorites and

    volcaniclastic sediments (roots of an intra-oceanic

    arc).

    Fig. 2. Map of the Otago Schist showing the CaplesTorlesse boundary

    Cretaceous fold-nappes and Cenozoic folds, the areal distribution of the

    locations (based on maps in Mortimer, 1993a,b and Fig. 24 of Turnbull, 20

    zone; RemN: Remarkables fold-nappe; BenN: Bendigo fold-nappe; ManNMurihiku terrane: Triassic to Jurassic volcanogenic

    sandstone, siltstone and tuff (probable forearc basin

    sequence).

    Dun Mountain-Matai terrane: Permian ophiolite

    melange, and volcanogenic sediments.

    Caples terrane: Permian to Triassic/Jurassic?

    volcaniclastic marine flysch (trench slope or trench

    floor deposit).

    Rakaia/Older Torlesse terrane: Permian to Middle

    and Late Triassic turbiditic quartzo-feldspathic

    sandstone and argillites (submarine fan sequence).

    2.2. Crustal structure

    The fault-bounded ophiolite and melange of the

    Dun Mountain-Matai terrane defines a steeply N-

    , the general schist structure, including axial surface traces of both

    north and south flanks to the schist core, and the ArAr sample

    00). RSSZ: Rise and Shine shear zone; HMSZ: Hyde-Macraes shear

    : Manorburn fold-nappe; BrN: Brighton fold-nappe.

  • D.R. Gray, D.A. Foster / Tectonophysics 385 (2004) 181210 185dipping interface between the gently folded, inter-

    leaved arc forearc sequences (Median Batholith,

    Brook Street and Murihiku terranes) to the south

    and the deformed Torlesse composite-terrane subma-

    rine fan sediments to the north (Fig. 1b). The Otago

    Schist occupies a domal culmination in the immediate

    hanging wall to the Livingstone Fault and has lower

    metamorphic grade units (Caplessouth flank and

    Rakaianorth flank) structurally overlying the high-

    grade schist core. Deep crustal seismic profiling (see

    Fig. 8 of Mortimer et al., 2003) indicates the schist

    core is centered on the medial domal antiform defined

    by a broad warp in flat-lying schistosity, with a

    maximum subsurface width of f220 and f20 kmof structural relief (Fig. 1b). Northwards beyond the

    Waihemo Fault, there is a transition into the tectoni-

    cally imbricated and weakly metamorphosed Perm-

    ianTriassic greywacke sequence of the Rakaia

    (Older Torlesse) terrane.

    South of the Livingstone fault the crustal section

    is composed of a f1015-km-thick succession ofMurihiku forearc sediments overlying a f10-km-thick arc sequence of Brook Street volcanics,

    intruded to the south by the Median Batholith that

    constitutes the largely JurassicCretaceous arc. To

    the north of the Dun Mountain ophiolite slice, the

    crustal section is composed of structurally thickened

    (f20 km thickness), Permian to Triassic/Jurassicsediments of the Rakaia terrane with an overlying

    f10-km-thick wedge of trench sediments (Caplesterrane) immediately adjacent to the Livingstone

    fault.

    2.3. Otago Schist structure

    Structure of the Otago Schist is dominated by

    schistosity and transposed layering at the mesoscale

    (Fig. 3), whereas shear zones and apparent recum-

    bent isoclinal hinges generally in a transposition

    layering occur at the macro- or regional scale

    (Means, 1963, 1966; Wood, 1963). Flat-lying schis-

    tosity of the Otago Schist core consists of L-S

    tectonite with a dominant foliation and marked

    stretching and/or rodding lineation (Mortimer,

    1993a,b). The schist displays variable morphology

    due to variations in strain, transposition, metamor-

    phic differentiation and metamorphic grade (Bishop,1972; Mortimer, 1993a,b, 2003; Norris and Bishop,1990). Important boundaries between structural

    domains within the Otago Schist are defined by

    shear zones (Norris and Craw, 1987; Craw, 1998;

    Mortimer, 2000; Deckert et al., 2002). Major bound-

    aries include (1) the CaplesTorlesse boundary (Fig.

    2), which is defined by a high strain zone typified by

    the Remarkables Shear Zone (Cox, 1991); (2) the

    northern boundary with the high grade core is

    marked by the Hyde-Macraes Shear Zone in eastern

    Otago and the Rise and Shine Shear Zone in central

    Otago (Deckert et al., 2002) (see HMSZ and RSSZ,

    Figs. 2 and 3) further northwest, the boundary is

    marked by a shear zone separating the biotite

    garnetalbite assemblages in multiply transposed

    psammitic schist of the Torlesse terrane from pelitic

    schist intercalated with metavolcanics containing rare

    biotite of the Aspiring association (see Fig. 4a of

    Craw, 1998).

    Other shear zones, characterized by markedly

    higher strain than the surrounding schist, appear as

    narrow, crosscutting, phyllonite zones (cf. Forster

    and Lister, 2003, Fig. 7d). These truncate zones of

    steep enveloping surface (e.g. Niger Nappe, Fig. 4a)

    that have been interpreted as macro-folds (Means,

    1963, 1966; Turnbull, 1981) in the core of the schist

    (see Cretaceous fold axial surface traces, Fig. 2).

    These folds, or half-folds (after Mortimer, 1993a,b,

    p. 242) show a visible strain gradient increase from

    upper limb to hinge to lower limb (e.g. Remarkables

    Nappe, Fig. 5; Brighton nappe, Fig. 6). The Man-

    orburn structure of Means (1963, 1966) (see ManN,

    Fig. 2) shows a change from transposition l...

Recommended

View more >