Allen J. McGrew

Thermobarometric Constraints on the Tectonothermal Evolution of the East Humboldt Range Metamorphic Core Complex, Nevada

The East Humboldt Range of Nevada provides a record of deep-crustal tectonic processes in a classic Cordilleran metamorphic core complex located in the hinterland of the Late Cretaceous to early Tertiary Sevier orogenic belt. New constraints reported here on the metamorphic history of this terrane suggest an overall clockwise pressure-temperature-time ( P-T-t ) path that began with deep tectonic burial and metamorphism at kyanite + staurolite + garnet grade before Late Cretaceous time (possibly in Late Jurassic time?). Subsequently, higher-temperature Late Cretaceous peak metamorphism overprinted this event, resulting in widespread partial melting and leucogranite injection contemporaneous with emplacement of a large-scale recumbent fold (the Winchell Lake nappe). A new 207Pb*/206Pb* date of 84.8 ± 2.8 Ma on syntectonic leucogranite from the hinge zone of this fold constrains the age of this major phase of tectonism. Metamorphism at this time probably reached the second sillimanite isograd, at least at deep structural levels, with peak P-T conditions of 800 °C and >9 kbar. High-grade conditions persisted during extensional tectonic denudation throughout much of Tertiary time. In conjunction with previously published work, the petrologic and thermobarometric results reported here for the northern East Humboldt Range delineate a steeply decompressional P-T trend that extends from ∼9 kbar and 800 °C to 5 kbar and 630 °C. In the light of decompressional reaction textures, microstructural evidence, and previously published thermochronometric results, we interpret this trend as a P-T-t path for Late Cretaceous to Oligocene time. At least 2 kbar of this decompression (equivalent to at least 7 km of denudation) occurred in Late Cretaceous to early Tertiary time. This interpretation supports the idea that tectonic exhumation of deep-crustal rocks in northeastern Nevada began during or immediately after the closing stages of the Sevier orogeny. Finally, the steepness of the proposed P-T-t path implies a thermal evolution from a colder to a much hotter geotherm, a circumstance that probably requires plastic thinning of the lower plate in addition to brittle attenuation and removal of the upper plate during Tertiary extension.

40Ar/39Ar thermochronologic constraints on the tectonothermal evolution of the Northern East Humboldt range metamorphic core complex, Nevada

Abstract The northern East Humboldt Range (NEHR) of northeastern Nevada exposes a suite of complexly deformed migmatitic, upper amphibolite-facies rocks in the footwall of the Ruby Mountains-East Humboldt Range (RM-EHR) detachment fault. New 40Ar/39Ar data on hornblende, muscovite, biotite, and potassium feldspar help constrain the kinematic and thermal evolution of this terrain during Tertiary extensional exhumation. Hornblende samples from relatively high structural levels yield discordant age spectra that suggest initial cooling during early Tertiary time (63–49 Ma). When coupled with petrological constraints indicating a strongly decompressional P-T-t path above 550°C, the hornblende data suggest that exhumation of the RM-EHR may have initiated in early Tertiary time, approximately coincident with the initial phases of unroofing in the Wood Hills immediately to the east and with the end of thrusting in the late Mesozoic to early Tertiary Sevier orogenic belt of eastern Nevada and western Utah. This temporal coincidence suggests that gravitational collapse of tectonically thickened crust in the internal zone of the Sevier belt could have driven the initial phases of unroofing. Thermal history during the final stage of exhumation of the NEHR is constrained by discordant hornblende cooling ages of 36-29 Ma from deep structural levels and biotite, muscovite, and potassium feldspar cooling ages of 27-21 Ma from a range of structural levels. Comparison of muscovite, biotite, and potassium feldspar cooling ages with previously published fission-track cooling ages implies very rapid cooling rates at temperatures below the closure temperature for muscovite (270°–350°C), but time gaps of > 7 m.y. between hornblende and mica cooling ages suggest that cooling at higher temperatures was more gradual. In addition, comparison of 40 Ar 39 Ar mica cooling ages with previously published fission-track apatite cooling ages suggests pronounced thermal gradients between the NEHR and adjacent areas during latest Oligocene to earliest Miocene time. Such thermal gradients could be readily explained if the RM-EHR detachment fault dipped > 30° between the 300°C and 100°C isotherms. Finally, 40 Ar 39 Ar biotite cooling ages increase southward through the East Humboldt Range, compatible with northward extrapolation of a previously recognized pattern of WNW-younging biotite cooling ages from the Ruby Mountains. A simple model involving the propagation of footwall uplift in the direction of tectonic transport beneath an initially listric normal fault can explain the principle features of the Oligocene to Miocene thermochronologic data set for the RM-EHR.

Garnet plasticity in the lower continental crust: implications for deformation mechanisms based on microstructures and SEM-electron channeling pattern analysis

Abstract Elongated garnets which preserved deformational microstructures (boudinage, pinch and swell structures) occur in granulite facies quartzites in the Highland Complex of Sri Lanka. In spite of these microstructures and in contrast with previous reports on garnet plasticity, there are no or only few signs of intracrystalline deformation like subgrains or lattice distortion. This can be explained by annealing and slow static cooling from high temperatures. SEM-electron channeling pattern analysis reveals that the garnets have a significant crystallographic preferred orientation. Aspect ratio/grain-size analysis shows that the deformation mechanism is grain-size sensitive. These features indicate a diffusion assisted dislocation glide mechanism with dominant 1/2 {110} slip system. A comparison of the deformation behavior between garnet, quartz and feldspar shows, that differences in flow strength are low under the high-grade conditions (850±50°C). Garnet is still the phase with the highest flow strength, but (dry) quartz is slightly stronger than feldspar.

Allochthonous Archean basement in the northern East Humboldt Range, Nevada

Archean basement rocks occur in the core of a fold nappe in the northern East Humboldt Range, Nevada. The Archean rocks consist primarily of plutonic orthogneiss and metasedimentary paragneiss. Both units contain numerous amphibolitic bodies interpreted as mafic intrusions. U-Pb zircon studies indicate that the orthogneiss unit is at least 2520 ±110 Ma and therefore is an example of an Archean granitoid. These data extend the known distribution of Precambrian basement rocks into northeastern Nevada and provide new constraints on the delineation of the Archean Wyoming province. The basement rocks are surrounded by upper amphibolite facies, metasedimentary rocks correlated with late Precambrian to Devonian(?) rocks of the eastern Great Basin miogeoclinal sequence. The contact between the Archean basement rocks and the metasedimentary mio geoclinal sequence is interpreted as a premetamorphic and prefolding, low-angle fault. The age and prefolding geometry of this low-angle fault are currently poorly constrained, but it may record important Mesozoic crustal shortening in the hinterland of the Sevier orogenic belt. The age of nappe formation is not yet fully established, but at least some isoclinal folds in the area were formed in the Tertiary and are kinematically linked to a major extensional deformation.

Transition from Contraction to Extension in the Northeastern Basin and Range: New Evidence from the Copper Mountains, Nevada

New mapping, structural analysis, and 40Ar/39Ar dating reveal an unusually well‐constrained history of Late Eocene extension in the Copper Mountains of the northern Basin and Range province. In this area, the northeast‐trending Copper Creek normal fault juxtaposes a distinctive sequence of metacarbonate and granitoid rocks against a footwall of Upper Precambrian to Lower Cambrian quartzite and phyllite. Correlation of the hanging wall with footwall rocks to the northwest provides an approximate piercing point that requires 8–12 km displacement in an ESE direction. This displaced fault slice is itself bounded above by another normal fault (the Meadow Fork Fault), which brings down a hanging wall of dacitic to rhyolitic tuff that grades conformably upward into conglomerate. These relationships record the formation of a fault‐bounded basin between 41.3 and 37.4 Ma. The results are consistent with a regional pattern in which volcanism and extension swept southward from British Columbia to southern Nevada from Early Eocene to Late Oligocene time. Because the southward sweep of volcanism is thought to track the steepening and foundering of the downgoing oceanic plate, these results suggest that the crucial mechanisms for the onset of regional extension were probably changes in plate boundary conditions coupled with convective removal of mantle lithosphere and associated regional magmatism and lithospheric weakening. Paleobotanical data indicate that surface elevations in northeastern Nevada were not significantly different than at present, suggesting that gravitational instability of overthickened continental crust was not the dominant force driving the onset of crustal thinning in mid‐Tertiary time.

A Crustal Cross-Section for a Terrain of Superimposed Shortening and Extension: Ruby Mountains-East Humboldt Range Metamorphic Core Complex, Nevada

Geologic mapping coupled with geochronological, thermobarometric, and seismic reflection studies provide the data for constructing a crustal cross-section through a Tertiary extensional orogen in the eastern Great Basin, western U.S. Cordillera. Oligocene-Miocene sedimentary and volcanic rocks were deposited during brittle, upper-level crustal extension dominated by high-angle normal faults, rotation of the strata and the faults themselves, and the progressive evolution of a low-angle detachment fault system. Together with nonmetamorphosed to very low-grade rocks of the Cordilleran miogeocline, the synextensional strata comprise an upper crustal suprastructure that was attenuated during Tertiary crustal extension. Structurally below the suprastructure and commonly separated from it by a regional detachment fault is a transitional metasedimentary/granitoid zone which preserves a principally Mesozoic magmatic, metamorphic, and deformational history. In turn, this zone grades downward abruptly into a 1.5- to 2.0-km-thick, upper amphibolite facies Tertiary shear zone that forms the top of a mylonitic to nonmylonitic migmatitic infrastructure of variable age. This infrastructural zone clearly records a complex Mesozoic history, but is in part characterized by a Tertiary magmatic-metamorphic-deformational history that appears to increase in intensity with depth.

The origin and evolution of the southern Snake Range Decollement, east central Nevada

Regional and local stratigraphic, metamorphic, and structural constraints permit reconstruction of the southern Snake Range extensional deformational system in east central Nevada. The dominant structure of the range, the southern Snake Range decollement (SSRD), operated during Oligocene and Miocene extensional deformation to exhume a footwall of multiply deformed metasedimentary and plutonic rocks. Intrusion of three plutons (∼160 Ma, 79.1 ± 0.5 Ma, and 36 ± 1 Ma, respectively) and development of two cleavages preceded the onset of extensional deformation. Plastic deformation of lower plate metasedimentary rocks accompanied the early phases of regional extension and produced bedding-parallel grain shape foliations and WNW trending stretching lineations. These fabrics parallel the SSRD even in low-strain domains, suggesting that a significant component of pure shear strain probably accompanied noncoaxial deformation associated with motion on the SSRD, consistent with other lines of evidence. Meanwhile, hanging wall rocks were greatly extended by at least two generations of tilt block-style normal faults soling into the SSRD, with the earlier faults antithetic to the SSRD and the later faults dipping in the same direction as the SSRD. A retrodeformed regional cross-section sequence illustrates plausible alternative schemes for reconstructing the southern Snake Range extensional system. In one scheme, the SSRD forms as a crustal scale stretching shear zone separating an upper plate that extends on steeply inclined normal faults from a lower plate that stretches by penetrative flow. In the other, lower plate deformation incorporates a component of coaxial stretching, but the SSRD also functions as a conventional shear zone accommodating through-going displacement between opposing plates. In either case, as tectonic unroofing proceeds, differential isostatic unloading induces the SSRD to rotate to steeper dips as it migrates into the frictional sliding regime, thus enabling it to remain active as a brittle normal fault until it finally rotates to its present shallow inclination. In either scenario, cross-section constraints suggest that total extension accommodated by the SSRD was probably between 8 km and 24 km.

One-dimensional kinematic model of preferred orientation development

Abstract The one-dimensional model to analyse the kinematics of crystallographic preferred orientation of Ribe (1989) is presented and developed further. It is argued that this approach can be applied to rotational deformations where the predominant deformation mechanism is grain boundary sliding. Two contrasting situations are distinguished. The first is where lattice rotations of opposing sense occur and there are orientations for which the rotation rate is zero. In this case a continually intensifying preferred orientation at an orientation with zero rotation rate will result. The second situation is where the rotation of the lattice is in the same sense for all orientations. Initially maxima develop in the orientation of greatest negative divergence in the lattice rotation rate function. A steady-state preferred orientation profile is possible which is the normalised inverse of the function describing lattice rotation rate vs. orientation and the maxima are at the orientations for which the lattice rotation rate is a minimum. The intensity of the preferred orientation is a function of the ratio of the greatest to least lattice rotation rates. The results are applied to a natural mylonite preferred orientation which consists of a c axis maximum in the mylonitic foliation perpendicular to the stretching lineation. It is argued that the crystal lattices rotate about a stably oriented c axis and the profile through the orientation distribution describing the probability of finding particular orientations differing by a rotation about c is inverted to give an estimate of the lattice rotation rate profile. It is found that the lattice rotates slowest when the second-order prism direction a is aligned parallel to the foliation normal and fastest when a is aligned sub-parallel to the stretching lineation.

SHRIMP-RG U-Pb isotopic systematics of zircon from the Angel Lake orthogneiss, East Humboldt Range, Nevada: Is this really Archean crust?: COMMENT

Premo et al. (2008) present new SHRIMP-RG U-Pb zircon data from the orthogneiss of Angel Lake in the East Humboldt Range, Nevada, reinterpreting the age and origin of a rock body that previously had been interpreted as delimiting the southwestern extent of the Archean Wyoming province based on integrated field study and U-Pb ID-TIMS zircon analysis (Lush et al., 1988). Using zircons from the same sample originally collected by A.W. Snoke and J.E. Wright (RM-9), Premo et al. (2008) document Late Cretaceous ages of 91–72 Ma for the clear, oscillatory-zoned rims of many zircons from this sample, conclusively demonstrating the presence of Late Cretaceous melt in this migmatitic gneiss. Furthermore, they reinterpret the core grains as detrital zircons inherited from Meso- to Neoproterozoic metasedimentary source rocks. The potential detrital grains chiefly yield Late Archean apparent ages but also include a variety of Proterozoic apparent ages.

Rhomb-dominated crystallographic preferred orientations in incipiently deformed quartz sandstones: A potential paleostress indicator for quartz-rich rocks

We describe quartz crystallographic preferred orientations (CPOs) from incipiently deformed quartz sandstones characterized by low-intensity but unambiguous alignment of the poles to positive {r} and/or negative {z} rhombs. These distinctive CPOs appear at minimal strains and in grains with scarcely modified original detrital boundaries. We consider the hypothesis that these patterns reflect Dauphiné twinning (a 180° misorientation about the c-axis) that preferentially affects grains oriented with the elastically stiffer z-rhombs at high angle to the maximum principal stress direction. Twinning facilitates elastic deformation by aligning the more compliant r-rhombs at high angle to the greatest principal stress. Crystallographic maps show that about two-thirds of all grains (by area) are twinned, and untwinned grains are oriented with an r-rhomb perpendicular to the inferred shortening direction. We document this pattern from low-grade quartzite from three locations: the Eureka Quartzite of northeastern Nevada (USA); the Mesón Group of northwestern Argentina; and the Antietam Formation of the Blue Ridge of central Virginia (USA). The widespread presence of these CPOs in minimally deformed quartz rocks suggests that they may be useful in defining paleostress trajectories.