structural geology lectures series 3.pdf

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128

STRUCTURAL GEOLOGY

Lectures 35-36

Models for Ductile Failure(Power law creep)

Ductile deformation occurs if the rock under stress does not loose its strength by

means of a brittle failure. This behavior is illustrated using stress-strain curves from rock deformation experiments (Fig. 35-1). Each test is run at constant strain rate which means

that in a triaxial test the piston is advanced into the cylindrical rock sample at a constant

rate. The initial behavior of the rock is elastic for which a linear stress-strain curve is

shown. Brittle failure causes a complete lose of strength. Ductile flow shows that thestrength is maintained during continuos straining of the sample.

(Fig. 35-1)

Percent ductility is a measure of the amount of strain that a rock undergoes before losing strength. Ductility varies with lithology. The strongest and most brittle of

the rocks is a quartzite or silica cemented sandstone. In contrast, halite is very weak and

will undergo large amounts of ductile flow without brittle failure. Figure 35-2 shows avariety of rocks and their relative ductilities as a function of depth of burial within the

earth. Starting with the most brittle there is silica cemented sandstone, dolomite, calcite-

cemented sandstone, shale, limestone, and halite.

Initially constant strain-rate tests were most convenient for laboratory

experiments. However, conditions within the crust of the earth closely resembleconstant stress tests such as that shown in Figure 35-3. This is so because the differential

stress within the crust does not change rapidly with time.The most interesting characteristic of constant stress tests is that steady state

creep is achieved. This is a state where the rock exhibits no change of strain with time.

Steady state creep occurs during the linear portion of the strain-time curve shown inFigure 35-3.



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(Fig. 35-2)

(Fig. 35-3)

Various mechanisms of ductile flow were introduced during the previous lecture.Each of these mechanisms can be dominate during the creep of rocks. The dominate

mechanism depends on the temperature and differential stress affecting the rock. Figure

35-4 is a plot of the temperature of deformation verses stress. The temperature T is

mormalized to the melting temperature (Tm ) by the ratio T/Tm. The stress of

deformation is normalized by the shear modulus of the rock (µ).

Various creep mechanisms include the following:

Nabarro-Herring Creep - bulk diffusion of point vacancies down a stressgradient. Recall that a point vacancy is a single missing atom.

Anelastic Creep - below a critical shearing stress for large dislocation

movement mechanisms as Coble Creep take place.



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Low-Temperature Creep - includes multiplication and glide of

dislocations. Stresses have to be reasonably high to cause thistype of creep.

High-Temperature Creep - at higher temperatures edge dislocations can

climb and screw dislocations can cross slip.

(Fig. 35-4)

An equation for Nabarro-Herring Creep gives the strain rate (é) in terms of stress

σ

é = (αDVaσ)/kTL2.

α is a geometric factor; L is the diameter of the grain; D is the diffusion coefficient; Va is the atomic volume; T is the temperature; σ is stress; and k is the Boltzman number.

Steady creep flow of rock materials can also be modeled using the Weertman Equation

é = Aexp(-Qc/RT) f(σ)

where T = temperature, Qc = creep activation energy, and R = gas constant. This

equation can be evaluated using a plot of log σ versus -log (é) by rearranging the aboveequation

log(é/A) =Q

c/RT + φlogσ

where φ is the slope of the lines given in Fig. 35-5 and Qc is determined as the slope of

the plot of log é versus 1/T at constant stress. Experiments show that creep rate at high

temperature is a strong function of stress. Figure show the functional relationship

between é and σ in terms of three power law equations. Power law creep is non linear

flow



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é = ασn

whereas Newtonian viscosity is linear flow

é = ασ

Figure 35-7 shows a deformation mechanisms map for calcite (limestone). Given a stress

and temperature the diagram in Fig. 35-7 shows which of six mechanisms are favored.

These mechanisms include cataclasis, pressure solution, dislocation glide, dislocationclimb, Coble creep, and Nabarro-Herring creep

(Fig. 35-5)

.



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(Fig. 35-6)

(Fig. 35-7)



133

STRUCTURAL GEOLOGY

Lectures 39

Paleozoic Geology of the Cordillera(Rocky Mountain Geology)

Late Proterozoic and Paleozoic Stratigraphy of the Western U.S. reflect the first three of

the four stages of the Wilson Cycle including

1.) The Rift Stage:

Proterozoic Basins largely reflect the configuration of western North America as a

consequence of the breakup of Rodinia. Some of the more important stratigraphicunits include:

-- The Belt Supergroup of Montana and Alberta overlain by the WindermereSeries

-- The Uinta Mountain Supergroup of northeastern Utah

-- The Grand Canyon Supergroup of northern Arizona

2.) The Drift Stage:

Initial sedimentary blanket accompanying the drift stage is a basal Cambrian quartzitewhich is found within the Cordillera as well as the Appalachian Mountains. This

basal quartzite is given different names depending on location:

-- Flathead Sandstone (Montana)

-- Prospect Mountain Quartzite (Central Nevada)-- Tintic Quartzite (West-central Utah)

-- Tapeats Sandstone (Arizona)

-- Lodore Sandstone (Northeastern Utah)

The continental platform was then dominated by carbonates through the Carboniferous.

The area is the Paleozoic Miogeocline (Figure 39-1). Some of the more famous

carbonate units include:

Cambrian:

-- Bonanza King (Southwestern Nevada)

-- Mauv (Grand Canyon)

-- Snowy Range (Montana)

Ordovician



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-- Big Horn Dolomite (Montana-Wyoming)

-- Ely Springs Dolostone (Westcentral Utah)

Devonian

-- Jefferson-Three Forks (Montana-Wyoming)-- Temple Butte Limestone (Grand Canyon)

Mississippian

-- Madison (Montana-Wyoming)

-- Madison (Westcentral Utah)-- Redwall (Grand Canyon)

Paleozoic Tectonics of Western US - Extensional tectonism with the formation and

modification of shelf basins and offshore region, seems to have characterized

most of the late Paleozoic.

Figure 39-1 - Paleogeographic and tectonic map of late Paleozoic time showingCordilleran elements as well as intracratonic Ancestral Rocky Mountains and



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Ouachita orogenic belt (adapted from Burchfiel et al., 1992). The eastern edge of

the Havallah Basin in central Nevada is the location of the Golconda Thrust of theSonoma Orogeny. The foredeep of the Antler Orogeny is shown as EFAF.

3.) The Convergence Stage: During the later Paleozoic, the western edge of the NorthAmerican Plate became a backarc basin with island arcs forming somewhere off

shore. Convergence was marked by clastic wedges starting in Mississippian time

with the advent of the Antler Orogeny.

Compressional tectonic events along the shelf edge were restricted to two short

intervals about 10 to 20

. duration in the earliest Mississippian and in the Late Permian to earliest Triassic.

Antler Orogeny - Earliest Mississippian - lower Paleozoic chert-shale sequences and

associated mafic volcanic rocks of the Roberts Mountains allochthon werecomplexly deformed and thrust eastward over the edge of the continental shelf.

Strata of the Roberts Mountains allochthon represent continental slope, rise, and

basinal settings inferred to have been deposited west of the coeval, early

Paleozoic continental shelf.

1.) Rocks that now form the allochthon most likely represent the deposits

of a series of extensional basins developed intermittently along theedge of the continental margin in early Paleozoic time. Basins are

underlain by rifted continental crust.

2.) Pelagic and hemipelagic sediments and alkalic basalt of the Slaven

Chert constitute the youngest and structurally lowest units of the

Roberts Mountains allochthon, implying that an extensional or transtensional tectonic setting characterized the region immediately

offshore the continental margin in the Late Devonian.

3.) Deformation appears to be more penetrative and is associated with

lower greenschist-facies metamorphism westward indicating

deformation within a tectonic setting characterized by a relativelyelevated, rather than depressed geotherm, but no detailed work has

been done to address this question.

Rocks of the Roberts Mountains allochthon are overlain by upper Paleozoic

shallow-marine sequences.

Havallah Basin: shortening ended abruptly in early Late Mississippian time with onset of extension.



136

Antler foredeep basin accumulated in gradually shoaling waters.

Havallah basin and other offshore basins formed as a series of rift basins by

continued episodic extension into the late Paleozoic. Island arcs were

separated from rift basins.

Ancestral Rockies formed in the Pennsylvanian: This series of uplifts and adjacent deep

basins are most pronounced in Colorado and Utah. The timing for the Ancestral

Rockies corresponds not with west-coast tectonics but rather the formation of theOuachita Mountains of Oklahoma. This foreland fold-thrust belt formed as a

consequence of South American converging upon the Gulf Coast region of the

USA.

Some of the more famous basins of this time include the Paradox basin of

southeastern Utah and the Eagle basin of central Colorado.

Oquirrh Basin : 300 m of shallow sedimentary rocks of Atokan time. (OB inFigure 39-1).

The complex transition to extensional faulting in the Ancestral Rockies reflects

tectonic activity along the western margin , which may have acted as a

“free-face” tectonic boundary at this time.

Limestone turbidites flooded offshore basins like the Havallah.

Sonoma Orogeny: By Late Permian time, the McCloud island arc moved closer to the

continental margin by shortening or subducting the depositional basement of theHavallah sequence, resulting in imbricate thrust faulting.

Golconda allochthon - deep-marine sedimentary rocks and associated volcanic rocks of

late Paleozoic age, the Havallah sequence, were emplaced onto the outer shelf above the Golconda thrust during the Late Permian to Early Triassic Sonoma

orogenic event.

Rocks of the allochthon range from latest Devonian to Late Permian age.

Evidence for age of thrusting:

Triassic to Jurassic marine sequences were deposited both east of and

depositionally above the Golconda allochthon.

Autochthonous Permian strata of the overlap sequence are conformably

overlain by fossiliferous lowest Triassic, hemipelagic sedimentary

rocks that , in turn, are overlain by Lower Triassic submarine-fandeposits derived from the encroaching allochthon.



137

STRUCTURAL GEOLOGY

Lectures 40

Mesozoic Geology of the Cordillera(Rocky Mountain Geology exclusive of Laramide Deformation)

Early and Middle Triassic (245-230 Ma) - accretion of a major Paleozoic island-arc

terrane in northwest Nevada-northern California (the McCloud belt). This was

the same period as the termination of the Sonoma Orogeny.__ On the stable

continent deposition is marked by the Moenkopi Formation - Fluvial near itseastern limits but represents an intertidal environment over most of the Colorado

Plateau. Has marine limestone and gypsum near its western margins.

Late Triassic (230-208 Ma) - Blue Mountains arc (a belt of highly disrupted oceanic

rocks in Oregon). Klamath Mountains (Permian-Triassic subduction complex).During this period the Chinle Formation accumulated on the stable continentalinterior. The Chinle is characterized by fluvial channel-fill sandstones and

conglomerates form the basal members. Throughout most of the formation rock

types vary from mudstone to siltstone to sandstone with bentonitic clays and

volcanic ashes. Some channel fill deposits are known as the Shinarump Member.On top of the Chinle we find eolian dunes of the Wingate sandstone encroaching

from the north.__

Figure 40-1: Stratigraphic section of the Jurassic System along the Arizona-Utah state

line (adapted from Peterson, 1988).



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The Jurassic is characterized by three major rock units. From base to top these are the

Glen Canyon Group, the San Rafael Group and the Morrison Formation (Figure40-1).

Early Jurassic - (208-187 Ma) Aztec-Navajo-Nugget dune fields.__These dune

fields covered an area equivalent to the modern Saharan dune fields of Africa.

Middle Jurassic - (197-163 Ma) Carmel seaway.__This was a precursor to theCretaceous interior seaway.

Late Jurassic - (162-144 Ma). Morrison Formation may record the first evidenceof a broad western orogenic highland flanked to the east by a regional

foreland basin. Possible this detritus was shed from the sheets of the

Sevier belt.__

Jurassic northward migration of the North American plate resulted in a latitudinal path where the dry trade-wind belt was traversed during the Early and

Middle Jurassic deposition of the Navajo-Aztec-Nugget dune fields. Themore cooler westerly belt was then encountered during the deposition of

the Upper Jurassic Morrison Formation.__

Nevadan orogeny -basis for the collided exotic-arc model for the western Sierra-Klamath

belt. Manifested by the tight folding and slaty cleavage development on the upper

Oxfordian Mariposa Formation. A major Nevadan structure includes theFoothills Fault system and the major east-dipping thrust faults that bound the

Josephine ophiolite in the western Klamaths. (approximate age = 162 Ma).__

Back-arc tectonic elements along the Cordillera_- Areas east of a narrow Triassic to

Middle Jurassic magmatic arc that developed along the western edge of the

continental United States had been subject to no early Mesozoic tectonism. TheMiddle to Late Jurassic archipelago included a western region of small, locally

ocean-floored, intra-arc basins that acted as a tectonic buffer zone between Pacific

ocean plates to the west and the North American continent to the east. DuringCallovian/Oxfordian collapse of the offshore arc region, coupling between the

subducting oceanic plate(s) and western North America is believed to have

increased as the intra-arc buffer zone was eliminated. Such a region of strongcoupling is believed to have occurred from the central Sierra Nevada northward

through the Blue Mountains of Oregon and into central Canada. With the onset of

opening of the North Atlantic in Middle Jurassic time (ca 175 Ma), convergencerates along the western continental margin increased and remained relatively high

for most of Mesozoic time._

- Regional setting of back-arc convergence: Weak coupling between subducting oceanic plates and the north American Plate led to little or no back-arc deformation.

During late Mesozoic time, increased convergence rates produced increased



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coupling between oceanic plates and the North American Plate, possibly due to

the active overriding of the offshore subducting plates by the west-movingcontinental plate. Events such as the development of Nevadan and pre-Nevadan

west-directed thrusts in the Klamath Mountains, the closure of the small intra-arc

ocean basins (e.g., Galice), east-directed thrusting in the northern Sierra

(Taylorsville thrust), and the beginning of closure of the Star Peak/Luning basinin the late Middle Jurassic to early Late Jurassic time are viewed as expressions of

the onset of strong coupling along the western margin of the Cordillera. This

coupling was accompanied by an east west broadening of arc magmatism.Deformation began in the eastern Sevier belt only after deformation in the

Luning-Fencemaker belt had sufficiently thickened the crust beneath the Star

Peak/Luning basin to permit shortening stresses to be tranmsmitted farther east.

- Late Jurassic through Cretaceous foreland fold-thrust belts: Starting in the Middle

Jurassic time tectonism started moving eastward to eventually extend 1000 km.

Tectonism north of Las Vegas divided into three deformation belts (Figure 40-

2):_

- Luning-Fencemaker belt: Thrust system moved basinal sediments onto shelf sedimentsto the east in western Nevada._

- Eureka belt: Cretaceous Newark Canyon Group formed within an active fold-thrust beltin Central Nevada._



140

Figure 40-2: Paleogeographic and tectonic map of the Middle Jurassic to Late Cretaceous

time exclusive of Laramide basement block uplifts (adapted from Burchfiel et al.,

1992). Dark areas are largely plutons and volcanic areas of this period.

- Sevier belt; Late Jurassic to Late Cretaceous. A foredeep developed along its easternmargin and received up to 6 km of debris eroded from the rising mountains.

Thrusting is thin skinned with changes in their style along strike (Figure 40-2)._

- Montana: bedding anisotropy of sedimentary strata of the Middle Proterozoic

Belt Group controlled thrusting_

- Idaho: thrusts carry crystalline basement rocks where they cross a paleogeographic high (Salmon or Lemhi arch) that marked the southern

uplands for the Belt basin._

- South of Snake River Plane: thrusts develop within the eastern portion of the

Late Proterozoic and Paleozoic miogeocline. Each more westerly thrust

fault carried a thicker miogeoclinal section._



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- southeastern California the thrust belt turns away from the miogeoclinal hinge

and crosses into crational strata and their underlying Precambrian basement._Sevier thrusts ultimately involve crystalline basement rocks._

150 Ma - inception of the subduction-related tectonic elements with magmatism

switching off in the Sierra Nevada batholith and jumping to more restricted centers in the Rocky Mountains.__Subduction along western margin of the US

continental margin gave rise to the magmatic arc (Sierran batholithic belt);

forearc-basin deposits (The Great Valley Group); and subduction complex(Franciscan Complex).__

80 Ma - The mid-Late Cretaceous also witnessed the waning of thin-skinned deformationin the Cordilleran fold and thrust belt and the inception of Laramide deformation.



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STRUCTURAL GEOLOGY

Lecture 41

Laramide Geology of the Cordillera(Rocky Mountain Geology exclusive of Laramide Deformation)

The term, Laramide, is reserved to that period of time in which basement-block upliftsoccurred within the Wyoming Province of the Rocky Mountains. Actually, basement

involved faulting during the Larmide extended throughout the Cordillera from southern

Montana to northern Arizona and from the Black Hills of South Dakota to the Sevier

Orogenic Belt to the west. East of the Sevier Belt are two major tectonic provinces that predate the development of the Basin and Range: The Colorado Plateau and the

Laramide Belt (Figure 41-1). The Colorado Plateau is, itself, a major uplifted block of

basement rock but several smaller high angle Laramide faults are found within the plateau. Laramide structures within the Colorado Plateau include the East Kaibab

Monocline (KU), the San Rafael Swell (SRU), Circle Cliffs (CCU), Monument Uplift(MU), Defiance Uplift (DU), Uncompahgre Uplift (UnU), and White River Uplift(WHU).



143

Figure 41-1. Sketch map showing the Rocky Mountain foreland province with major

structures and sedimentary basins (adapted from Miller et al., 1992).

It is important to draw a distinction between the detachment faulting of the sedimentary

cover rock during the formation of the Cordilleran Thrust Belt known as the Sevier

Orogeny and the basement-involved faulting of the Laramide. Both tectonic stylesinvolve low angle faults that may cut updip as they approach the surface. However, the

depth and possibly the extent of the Laramide faults may be much greater. The major

basement block uplifts of the Laramide Province north of the Colorado Plateau includeseveral mountain ranges of note, particularly with the vicinity of Wyoming (Figure 41-2).

From north to south these ranges are the Beartooth Mountains (BTU), the Big Horn

Mountains (BiU), the Owl Creek Mountains (OCU), the Wind River Mountains (WRU),the Laramie Range (LU), the Uinta Mountains (UU), and the Front Range (FRU).

Basins between the uplifts also play a prominent role in Laramide geology. Major basins

include the Powder River Basin (PRB), the Big Horn Basin (BHB), the Green River

Basin (GRB), the Unita Basin (UB), the Piceance Basin (PB), the Denver Basin (DB),and the San Juan Basin (SJB).

Figure 41-2. Basement fault map, central Rocky Mountain foreland (adapted from Stone,1993).



144

The complexity of the Laramide structures does not end at the large scale. When a single

basin is viewed in detail, geologists have found that the margins of these basins are broken into a complex set of faults leading to smaller scale anticlinal structures. A

notable example is the Big Horn Basin (Figure 41-3). Well known structures bounding

the Big Horn Basin include Elk Basin Anticline to the north, Rattlesnake Mountain to the

west and the Sheep Mountain Anticline on the northeast side of the basin. The Pryor Mountains are found just east of Elk Basin and Owl Creek Mountains are located to the

south of the Big Horn Basin.

Figure 41-3. Basement fault map of the Big Horn Basin (Adapted from Stone, 1993)

The cause and timing of the Laramide tectonism is still under debate. The earliest

Laramide structures include the Moxa Arch and the Teton uplifts of western Wyoming

which date from 80 Ma, well back into the Cretaceous. These structures are synchronouswith the later thrusting along the Sevier Belt. Hence, there is little reason to think of the

foreland detachments of the Sevier Belt and the basement faulting of the Laramide as

being different in time. They were overlapping events.

In terms of kinematics the earliest basement thrusting occurred on north-south striking

faults dating from the Late Cretaceous (80 Ma) to the Early Paleocene (58 Ma). Later east-west striking faults were active during the Late Paleocene (53 Ma) to Late Eocene

(38 Ma). The change in direction of thrusting first toward the east and later toward the

northeast and to the north was probably tied to plate tectonics. From the late Cretaceousto the Paleocene the North Atlantic Ocean was opening and the motion of North America

was to the west (hence, eastward thrusting). The net movement of the North American



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craton was to the south-southwest during the middle Paleocene through the Eocene as the

Arctic Ocean opened. This net southward motion cause the rotation of the ColoradoPlateau during the Eocene and is believed to have caused the major uplift of the Uinta

Mountains as shown in Figure 41-4.

Figure 41-4. Clockwise rotation of the Colorado Plateau during the Laramide should

have caused major uplift of the Uinta Mountains in early Laramide (adapted from Gries,1983).

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