Kevin H. Mahan

Sheeted intrusion of the synkinematic McDoogle pluton, Sierra Nevada, California

Field, microstructural, and geochronologic evidence indicates that the Late Cretaceous McDoogle pluton, located near the eastern margin of the Sierra Nevada batholith, was emplaced as a subvertically sheeted complex into a steep reverse-sense shear zone. Evidence of internal subvertical sheeting includes abundant concordant wall-rock inclusions and screens that separate the pluton from adjacent Jurassic plutons, preservation of a ghost tectonostratigraphy in the distribution of the inclusions, and rare interphase contacts. The solid-state tectonic fabric in the wall rocks and the magmatic, submagmatic, and weak solid-state fabrics in the McDoogle pluton all are concordant and record northeast-southwest horizontal shortening, subvertical extension, and a significant component of northeast-side-up simple shear. Intrusive contacts generally are concordant with the fabrics but, where discordances occur, the wall-rock fabric invariably is truncated by the contact. However, late synkinematic emplacement of the McDoogle pluton is indicated by synintrusive boudinage of apophyses of the pluton and by the overall concordance of pluton fabrics including magmatic lineation. Zircon U-Pb isotope dates were obtained from the quartz monzodiorite central phase of the McDoogle pluton (94 ± 4 Ma), the mafic granodiorite border phase (94.8 ± 0.6 Ma), and an older hornblende granodiorite included in the central phase (97.6 ± 0.4 Ma). Granodiorite orthogneiss in the wall rock yielded a U-Pb zircon date of 164 +2.1/–6.8 Ma, which we interpret to be the crystallization age and which is indistinguishable from a new age of 164.5 ± 2.3 Ma for the Twin Lakes pluton. A U-Pb titanite date of 94.4 ± 0.9 Ma from the 164 Ma orthogneiss sample may reflect thermal effects of the intruding McDoogle pluton, synkinematic growth of titanite in the shear zone, or perhaps both. The age of the Sawmill Lake shear zone that hosts the McDoogle pluton is bracketed between 148 Ma, the age of the Independence dike swarm, and 92 Ma, the age of the Lamarck pluton that crosscuts both the McDoogle pluton and the shear-zone fabrics. Regional evidence suggests a shear-zone age younger than 110 Ma, but older deformation is possible. The predominance of horizontal shortening over transcurrent motion and an age older than 90 Ma exclude a direct relationship to the dextral Sierra Crest shear system, which has been proposed to pass near the study area. The location, age constraints, and kinematics of the shear zone are consistent with both the later stages of movement in the East Sierran thrust system and isostatic sinking of the arc magmatic complex into its substrate.

Reconstruction of a large deep-crustal terrane: Implications for the Snowbird tectonic zone and early growth of Laurentia

The ∼2800-km-long Snowbird tectonic zone is a well-recognized but still enigmatic feature in the western Canadian Shield. It has been interpreted as a Paleoproterozoic continental suture or an Archean strike-slip fault system, but here we suggest that the distinctive geometry of the central Snowbird tectonic zone is primarily due to the interaction of crosscutting Paleoproterozoic intracontinental thrust and strike-slip shear zones having a length of hundreds of kilometers. First, a major zone of thrust-sense shearing, coeval with early continent-continent collision between the Superior and western Churchill provinces, accommodated uplift of a large exposure of granulite facies lower continental crust. Younger strike-slip shear zones, perhaps analogous to Asian fault systems behind the Himalayan orogen, offset the thrust zone. Thus, the current geometry and distribution of deep-crustal rocks in this region represent a relatively late stage in the tectonic evolution of the western Churchill province rather than an accretionary one. Earlier structures oriented at a high angle to the Snowbird tectonic zone may record the fundamental accretionary history in this part of Laurentia.

Continental uplift through crustal hydration

Isostatic surface uplift of large continental regions lacking deformation remains largely unexplained. Evidence from the eastern parts of the Cordilleran orogen in the western United States suggests that increased buoyancy in the lower crust supports the elevations of the High Plains and Wyoming craton. We suggest that hydration of the lower crust associated with the Laramide orogeny produced surface uplift by replacing dense mineral phases such as garnet with less dense phases such as amphibole and mica. Seismic and petrologic evidence from Wyoming and Montana is consistent with such changes. Comparable hydration in the Colorado Plateau is dated to the early Tertiary. Beyond establishing a newly recognized mechanism for broad continental uplift, such hydration suggests that interactions of subduction-derived fluids and the lithosphere can be more profound than previously envisioned.

An Exhumation History of Continents over Billion-Year Time Scales

Continental Thermocouple The patchy presence of billions-of-years-old continental crust indicates a complex coupling between the buoyant forces keeping the lithosphere floating on the mantle and the persistent erosional forces gradually wearing the crust away. Measuring long-term rates of exhumation—the creation of new rock surfaces due to erosion—can reveal how the crust is thermally coupled to the underlying mantle, but techniques to do so have often only been able to resolve a limited temperature range across narrow slices of geologic time. Blackburn et al. (p. 73) used uranium-lead thermochronology, which is sensitive to the much higher temperatures representative of lower crustal depths, to construct a long-term quantitative model of exhumation and erosion for North America. Thermochronology indicates a balance between low erosion rates and slow thermal cooling in old continental crust. The continental lithosphere contains the oldest and most stable structures on Earth, where fragments of ancient material have eluded destruction by tectonic and surface processes operating over billions of years. Although present-day erosion of these remnants is slow, a record of how they have uplifted, eroded, and cooled over Earth’s history can provide insight into the physical properties of the continents and the forces operating to exhume them over geologic time. We constructed a continuous record of ancient lithosphere cooling with the use of uranium-lead (U-Pb) thermochronology on volcanically exhumed lower crustal fragments. Combining these measurements with thermal and Pb-diffusion models constrains the range of possible erosion histories. Measured U-Pb data are consistent with extremely low erosion rates persisting over time scales approaching the age of the continents themselves.

Deep crustal xenoliths from central Montana, USA: Implications for the timing and mechanisms of high-velocity lower crust formation

Integration of petrologic, chronologic and petrophysical xenolith data with geophysical observations can offer fundamental insights into understanding the evolution of continental crust. We present the results of a deep crustal xenolith study from the northern Rocky Mountain region of the western U.S., where seismic experiments reveal an anomalously thick (10–30 km), high seismic velocity (compressional body wave, Vp > 7.0 km/s) lower crustal layer, herein referred to as the 7.x layer. Xenoliths exhumed by Eocene minettes from the Bearpaw Mountains of central Montana, within the Great Falls tectonic zone, include mafic and intermediate garnet granulites, mafic hornblende eclogite, and felsic granulites. Calculated pressures of 0.6–1.5 GPa are consistent with derivation from 23–54 km depths. Samples record diverse and commonly polymetamorphic pressure-temperature histories including prograde burial and episodes of decompression. Samples with barometrically determined depths consistent with residence within the seismically defined 7.x layer have calculated bulk P-wave velocities of 6.9–7.8 km/s, indicating heterogeneity in the layer. Shallower samples have markedly slower velocities consistent with seismic models. New monazite total U-Th-Pb data and a variety of additional published geochronology indicate a prolonged and episodic metamorphic history, beginning with protolith ages as old as Archean and followed by metamorphic and deep crustal fluid-flow events ca. 2.1 Ga, 1.8–1.7 Ga, and 1.5–1.3 Ga. We suggest that the 7.x layer in this region owes its character to a variety of processes, including magmatic underplating and intraplating, associated with multiple tectonic events from the Neoarchean to the Mesoproterozoic.

Crustal segmentation, composite looping pressure-temperature paths, and magma-enhanced metamorphic field gradients: Upper Granite Gorge, Grand Canyon, USA

The Paleoproterozoic orogen of the southwestern United States is characterized by a segmented, block-type architecture consisting of tens of kilometer-scale blocks of relatively homogeneous deformation and metamorphism bounded by subvertical highstrain zones. New fi eld, microstructural, and petrologic observations combined with previously published structural and geochronological data are most consistent with a tectonometamorphic history characterized by a clockwise, looping pressure-temperature (P-T) path involving: (1) initial deposition of volcanogenic and turbiditic supracrustal rocks at ca. 1.75‐1.74 Ga, (2) passage from 750 °C). Notable variations along the transect are also primarily thermal in nature and include differences in the temperature of the prograde history (i.e., early andalusite versus kyanite), equilibrium pressures recorded at peak temperatures, and intensity of late-stage thermal spikes due to local dike emplacement. High-precision ΔPT “relative” thermobarometry confi rms lateral temperature variations on the order of 100‐250 °C with little to no variation in pressure. The Upper Granite Gorge thus represents a subhorizontal section of lowermost middle continental crust (~0.7 GPa). Results imply that the entire ~70-km-long transect decompressed from ~0.7 to ~0.3‐ 0.4 GPa levels as one large coherent block in the Paleoproterozoic. The transect represents a 100% exposed fi eld laboratory for understanding the heterogeneity and rheologic behavior of lowermost middle continental crust during orogenesis. Hot blocks achieved partial melting conditions during penetrative subvertical fabric development. Although these blocks were weak, large-scale horizontal channel fl ow was apparently inhibited by colder, stronger blocks that reinforced and helped preserve the block-type architecture. Development of dramatic lateral thermal gradients and discontinuities without breaks in crustal level is attributed to: (1) spatially heterogeneous advective heat fl ow delivered by dense granitic pegmatite dike complexes and (2) local transcurrent displacements along block-bounding high-strain zones over an ~15‐20 m.y. time interval. Exhumation of the transect from 25 to 12 km depths is interpreted to refl ect erosion synchronous with penetrative development of steeply dipping NE-striking foliations and steeply plunging stretching lineations, consistent with an orogen-scale strain fi eld involving NW-SE subhorizontal shortening and subvertical extension during crustal thickening.

Transpressive uplift and exhumation of continental lower crust revealed by synkinematic monazite reactions

Exposures of continental lower crust provide fundamental constraints on the thermal-mechanical behavior of continental lithosphere during orogeny. The applicability of fi results, however, requires knowledge of whether these data pertain to deformation during lowercrustal residence or during uplift and exhumation of deep crust. Dating synkinematic monazite-producing reactions provides one way to evaluate deformation styles in the deep crust. We report on the implications of monazite reaction dating for the timing of fabric formation and movement along three crustal-scale shear zones in northern Saskatchewan, western Canadian Shield. The structures accommodated dextral transpressive strain during oblique- and thrust-sense displacement that was coeval with uplift and exhumation of >20,000 km 2 of continental lower crust (>1.0 GPa) to middle-crustal levels (<0.5 GPa). In situ Th-U‐total Pb monazite data reveal that monazite rims in all three shear zones grew synkinematically at 1849 ± 6 Ma (2σ, mean square of weighted deviates = 0.8). The style of deformation involved localized strain concurrent with segmentation and translation of rheologically strong blocks of deep crust along mutually interacting shear zones during transpression.

Low-temperature thermochronologic constraints on the kinematic history and spatial extent of the Eastern California shear zone

The Stateline fault system is a 200-km-long zone of active right-lateral shear along the California-Nevada border, United States. Recent identification of 30 ± 4 km of dextral offset since 13.1 Ma on the southern segment of the fault requires significant displacement to extend farther south than has been commonly considered in the past. However, major structures exposed where the fault projects to the south reveal predominantly dip-slip extensional faulting, suggesting that displacement is transferred into substantial northwest-oriented extension in eastern Ivanpah Valley. New (U-Th)/He apatite data from Proterozoic orthogneiss in the southern McCullough Range and northern New York Mountains support this model by recording dates as young as 5 ± 1 Ma in the structurally deepest parts of the footwalls to the range-bounding normal faults. This age is distinctly younger than both the ages of regional extension in surrounding areas and the youngest (U-Th)/He apatite dates reported from the immediately adjacent Colorado River extensional corridor. Late Miocene–Pliocene extension in Ivanpah Valley, contemporaneous with that elsewhere in the Eastern California shear zone, provides an independent line of support that the eastern margin of the Eastern California shear zone extends to the California-Nevada border. If this age marks the onset of deformation on the State-line system, then long-term slip rates on the southern segment may be as high as 5 mm/yr, significantly higher than the present-day estimate of 0.9 mm/yr derived from geodetic observations across the northern segment of this fault system.

Foreland-directed propagation of high-grade tectonism in the deep roots of a Paleoproterozoic collisional orogen, SW Montana, USA

The study of deeply exhumed ancient collisional belts offers important constraints on geologic processes and properties complementary to inaccessible portions of the crustal column in active orogens. The ca. 1.8−1.7 Ga Big Sky orogeny in southwest Montana is a major convergent belt associated with the Proterozoic amalgamation of Laurentia. New structural, petrologic, and geochronologic data from the Northern Madison Range, crossing the NE-SW trend of the belt, record key information about the internal dynamics of the orogen. At least two phases of Big Sky−related deformation are preserved, both nearly coeval with peak metamorphic conditions of ∼0.9−0.8 GPa and >700 °C. Metamorphic zircon grains from a deformed mafic dike yield a weighted mean ion probe U-Pb date of 1737 ± 28 Ma (2σ). Monazite grains from a metapelite yield electron microprobe U-Th total-Pb dates of ca. 1750−1705 Ma, spanning prograde, peak, and retrograde intervals. Exposed Proterozoic paleodepths range from deeper levels (∼45−40 km; 1.2 GPa) in the northwestern end of the range to shallower levels (∼30−25 km) in the central-southeast area. The age of high-grade tectonism appears to become younger southeastward away from the core of the orogen, from ca. 1810−1780 Ma in the Highland Mountains, to ca. 1780−1750 Ma in the Ruby Range, Tobacco Root Mountains, and northwesternmost Northern Madison Range, and 1750−1720 Ma in the central Northern Madison Range. These spatial and temporal patterns of lateral growth and propagation of the orogen are similar to those observed in other collisional orogenic systems, and they may reflect multiple collision phases, protracted collision, and/or postcollisional collapse.

Characteristics of deep crustal seismic anisotropy from a compilation of rock elasticity tensors and their expression in receiver functions

Rocks in the continental crust are long-lived and have the potential to record a wide span of tectonic history in rock fabric. Mapping rock fabric in situ at depth requires the application of seismic methods. Below depths of microcrack closure seismic anisotropy presumably reflects the shape and crystallographic preferred orientations (CPOs) influenced by deformation processes. Interpretation of seismic observables relevant for anisotropy requires assumptions on the symmetry and orientation of the bulk elastic tensor. We compare commonly made assumptions against a compilation of 95 bulk elastic tensors from laboratory measurements, including electron backscatter diffraction (EBSD) and ultrasound, on crustal rocks. The majority of samples developed fabric at pressures corresponding to mid-lower crustal depths. Tensor symmetry is a function of mineral modal composition, with mica-rich samples trending towards hexagonal symmetry, amphibole-rich samples trending towards an increased orthorhombic symmetry component, and quartz-feldspar-rich samples showing a larger component of lower symmetries. 77% of samples have a best-fit hexagonal tensor with slow-axis symmetry, as opposed to mantle deformation fabric that usually has fast-axis symmetry. The best-fit hexagonal approximation for crustal tensors is not elliptical, but deviates systematically from elliptical symmetry with increasing anisotropy, an observation that affects the magnitude and orientation of anisotropy inferred from receiver function and surface wave observations. We present empirical linear relationships between anisotropy and ellipticity for crustal rocks. The maximum out-of-plane conversion amplitudes in receiver functions scale linearly with degree of anisotropy for non-elliptical symmetry. The elliptical assumption results in a bias of up to 1.4 times true anisotropy.