Mountains Metamorphic Core Complex Research Articles

Jurassic–Cenozoic tectonics of the Pequop Mountains, NE Nevada, in the North American Cordillera hinterland

Abstract The Ruby Mountains–East Humboldt Range–Wood Hills–Pequop Mountains (REWP) metamorphic core complex, northeast Nevada, exposes a record of Mesozoic contraction and Cenozoic extension in the hinterland of the North American Cordillera. The timing, magnitude, and style of crustal thickening and succeeding crustal thinning have long been debated. The Pequop Mountains, comprising Neoproterozoic through Triassic strata, are the least deformed part of this composite metamorphic core complex, compared to the migmatitic and mylonitized ranges to the west, and provide the clearest field relationships for the Mesozoic–Cenozoic tectonic evolution. New field, structural, geochronologic, and thermochronological observations based on 1:24,000-scale geologic mapping of the northern Pequop Mountains provide insights into the multi-stage tectonic history of the REWP. Polyphase cooling and reheating of the middle-upper crust was tracked over the range of <100 °C to 450 °C via novel 40Ar/39Ar multi-diffusion domain modeling of muscovite and K-feldspar and apatite fission-track dating. Important new observations and interpretations include: (1) crosscutting field relationships show that most of the contractional deformation in this region occurred just prior to, or during, the Middle-Late Jurassic Elko orogeny (ca. 170–157 Ma), with negligible Cretaceous shortening; (2) temperature-depth data rule out deep burial of Paleozoic stratigraphy, thus refuting models that incorporate large cryptic overthrust sheets; (3) Jurassic, Cretaceous, and Eocene intrusions and associated thermal pulses metamorphosed the lower Paleozoic–Proterozoic rocks, and various thermochronometers record conductive cooling near original stratigraphic depths; (4) east-draining paleovalleys with ∼1–1.5 km relief incised the region before ca. 41 Ma and were filled by 41–39.5 Ma volcanic rocks; and (5) low-angle normal faulting initiated after the Eocene, possibly as early as the late Oligocene, although basin-generating extension from high-angle normal faulting began in the middle Miocene. Observed Jurassic shortening is coeval with structures in the Luning-Fencemaker thrust belt to the west, and other strain documented across central-east Nevada and Utah, suggesting ∼100 km Middle-Late Jurassic shortening across the Sierra Nevada retroarc. This phase of deformation correlates with terrane accretion in the Sierran forearc, increased North American–Farallon convergence rates, and enhanced Jurassic Sierran arc magmatism. Although spatially variable, the Cordilleran hinterland and the high plateau that developed across it (i.e., the hypothesized Nevadaplano) involved a dynamic pulsed evolution with significant phases of both Middle-Late Jurassic and Late Cretaceous contractional deformation. Collapse long postdated all of this contraction. This complex geologic history set the stage for the Carlin-type gold deposit at Long Canyon, located along the eastern flank of the Pequop Mountains, and may provide important clues for future exploration.

Geologic field evidence for non-lithostatic overpressure recorded in the North American Cordillera hinterland, northeast Nevada

There is a long-standing discrepancy for numerous North American Cordillera metamorphic core complexes between geobarometric pressures recorded in the exhumed rocks and their apparent burial depths based on palinspastic reconstructions from geologic field data. In particular, metamorphic core complexes in eastern Nevada are comprised of well-documented ~12–15 km thick Neoproterozoic–Paleozoic stratigraphy of Laurentia's western passive margin, which allows for critical characterization of field relationships. In this contribution we focus on the Ruby Mountain–East Humboldt Range–Wood Hills–Pequop Mountains (REWP) metamorphic core complex of northeast Nevada to explore reported peak pressure estimates versus geologic field relationships that appear to prohibit deep burial. Relatively high pressure estimates of 6–8 kbar (23–30 km depth, if lithostatic) from the lower section of the Neoproterozoic–Paleozoic passive margin sequence require burial and or repetition of the passive margin sequence by 2–3× stratigraphic depths. Our observations from the least migmatized and/or mylonitized parts of this complex, including field observations, a transect of peak-temperature (Tp) estimates, and critical evaluation of proposed thickening/burial mechanisms cannot account for such deep burial. From Neoproterozoic–Cambrian (Ꞓ) rocks part of a continuous stratigraphic section that transitions ~8 km upsection to unmetamorphosed Permian strata that were not buried, we obtained new quartz-in-garnet barometry via Raman analysis that suggest pressures of ~7 kbar (~26 km). A Tp traverse starting at the same basal Ꞓ rocks reveals a smooth but hot geothermal gradient of ≥40 °C/km that is inconsistent with deep burial. This observation is clearly at odds with thermal gradients implied by high P-T estimates that are all ≤25 °C/km. Remarkably similar discrepancies between pressure estimates and field observations have been discussed for the northern Snake Range metamorphic core complex, ~200 km to the southeast. We argue that a possible reconciliation of long-established field observations versus pressures estimated from a variety of barometry techniques is that the rocks experienced non-lithostatic tectonic overpressure. We illustrate how proposed mechanisms to structurally bury the rocks, as have been invoked to justify published high pressure estimates, are entirely atypical of the Cordillera hinterland and unlike structures interpreted from other analogous orogenic plateau hinterlands. Proposed overpressure mechanisms are relevant in the REWP, including impacts from deviatoric/differential stress considerations, tectonic mode switching, and the autoclave effect driven by dehydration melting. Simple mechanical arguments demonstrate how this overpressure could have been achieved. This study highlights that detailed field and structural restorations of the least strained rocks in an orogen are critical to evaluate the tectonic history of more deformed rocks.

Exhumation of the Coyote Mountains Metamorphic Core Complex (Arizona): Implications for Orogenic Collapse of the Southern North American Cordillera

AbstractA microstructural and thermochronometric analysis of the Coyote Mountains detachment shear zone provides new insight into the collapse of the southern North American Cordillera. The Coyote Mountains is a metamorphic core complex that makes up the northern end of the Baboquivari Mountains in southern Arizona. The Baboquivari Mountains records several episodes of crustal shortening and thickening and regional metamorphism, including the Late Cretaceous‐early Paleogene Laramide orogeny which is locally expressed by the Baboquivari thrust fault. Thrusting and shortening were accompanied by magmatic activity recorded by intrusion of Paleocene muscovite‐biotite‐garnet peraluminous granites such as the ~58 Ma Pan Tak Granite, interpreted as anatectic melts representing the culmination of the Laramide orogeny. Following Laramide crustal shortening, the northern end of the Baboquivari Mountains was exhumed along a top‐to‐the‐north detachment shear zone, which resulted in the formation of the Coyote Mountains metamorphic core complex. Structural and microstructural analysis show that the detachment shear zone evolved under a strong component of noncoaxial (simple shear) deformation, at deformation conditions of ~450 ± 50°C, under a differential stress of ~60 MPa, and a strain rate of 1.5 × 10−11 to 5.0 × 10−13 s−1 at depth of ~11–14 km. Detailed 40Ar/39Ar geochronology of biotite and muscovite, in the context of the deformation conditions determined by quartz microstructures, suggests that the mylonitization associated with the formation of the Coyote Mountains metamorphic core complex started at ~29 Ma (early Oligocene). Apatite fission track ages indicate that the footwall of the Coyote Mountains metamorphic core complex experienced rapid exhumation to the upper crust by ~24 Ma. The fact that mylonitization and rapid extensional exhumation postdates Laramide thickening by ~30 Myr indicates that crustal thickness alone was insufficient to initiate extensional tectonic and required an additional driving force. The timing of mylonitization and rapid exhumation documented here and in other MCCs are consistent with the hypothesis that slab rollback and the effect of a slab window trailing the Mendocino Triple Junction have been critical in driving the development of the MCCs of the southwest.

Reconstruction of Mid‐Cenozoic Extension in the Rincon Mountains Area, Southeastern Arizona, USA, and Geodynamic Implications

AbstractThe Rincon Mountains metamorphic core complex, located east of Tucson, Arizona, consists of an arched footwall of foliated crystalline rocks bounded above by the generally outward dipping, Oligocene‐Miocene San Pedro extensional detachment fault. The southwest trending axes of corrugations in the detachment fault, and in footwall foliation and lithologic layering, parallel mylonitic lineation, and inferred top‐southwest displacement on the fault. An upper plate fault block within a synformal fault groove on the west side of the Rincon Mountains contains a thrust fault that is interpreted as displaced 34‐38 km westward from an original position adjacent to a similar thrust in the footwall of the San Pedro detachment fault. Much of the footwall of the detachment fault in the eastern Rincon Mountains consists of metasedimentary tectonites derived largely from Paleozoic carbonates that were buried beneath Proterozoic crystalline rocks forming the hanging wall of the Laramide Wildhorse Mountain thrust. These tectonites were later exhumed by displacement on the San Pedro detachment fault. Structural reconstruction supports the interpretation that the carbonate tectonites localized extensional faulting along the San Pedro detachment fault at crustal depths where carbonates would be weak and deform by crystal plasticity while quartzo‐feldspathic rocks would be strong and brittle. This weak zone is located adjacent to the greatest width of exposed extension‐parallel mylonitic fabrics in southeastern Arizona and may have been associated with the earliest initiation of extension in the region. Domains of low‐strength carbonates may be an underappreciated influence on extensional tectonics in cratonic southwestern North America.

Crustal structure of the Ruby Mountains metamorphic core complex, Nevada, from passive seismic imaging

Crustal structure of the Ruby Mountains metamorphic core complex, Nevada, from passive seismic imaging

Dike emplacement, footwall rotation, and the transition from magmatic to tectonic extension in the Whipple Mountains metamorphic core complex, southeastern California

AbstractThe Chambers Well dike swarm and associated plutonic/volcanic rocks in the western footwall of the Whipple Detachment Fault (WDF) provide key insight into the evolution of this metamorphic core complex. New structural and geochronologic data suggest that the western 12–15 km of exposed footwall is steeply tilted to the SW, providing a cross‐sectional view of the upper crust, from the Miocene erosion surface to the top of the coeval mylonitization. Ages and compositions of dikes are indistinguishable from adjacent thick volcanic successions. Several kilometers of early Miocene extension (~20.5 to 19.0 Ma) were accommodated by magmatic accretion but transitioned to rapid extensional faulting and tilting at 19.0–18.5 Ma. The subhorizontal WDF in this area initiated as a northeast dipping high‐angle (50–60°) normal fault that breached the surface locally, not in a breakaway tens of kilometers to the west. Large‐scale tilting and differential uplift of the western footwall was in part coeval with mylonitization and dike emplacement and was accomplished by block rotation in the hanging wall of additional normal faults, isostatic uplift, and flow of lower crust from beneath less extended regions to the west. The WDF is likely a composite surface with a western segment that had ceased moving by ~18.5 Ma, cut by successively younger and steeper fault(s) to the east. Perhaps, the most important difference between seismogenic high‐angle normal faults and low‐angle “detachment faults” characteristic of metamorphic core complexes is one of magnitude and rate of total accumulated slip, not of initial failure conditions.

Miocene slip history of the Eagle Eye detachment fault, Harquahala Mountains metamorphic core complex, west-central Arizona

The structural and thermal evolution of major low-angle normal faults in the Colorado River extensional corridor has been a controversial topic since the pioneering studies of metamorphic core complexes in the early 1980s. We present new geo-thermochronometry data from the Harquahala Mountains in west-central Arizona to determine the timing of extension, displacement magnitude, and slip rates along the Eagle Eye detachment fault (EED) during large-magnitude Miocene extension. Zircon and apatite (U-Th)/He data (ZHe and AHe, respectively) from 31 samples along a ~55 km extension-parallel transect indicate active slip along the EED occurred between ~21 ± 1 Ma until ~14 Ma. The spatial extent of ZHe ages and exhumation of the zircon partial retention zone indicated ~44 ± 2 km of total displacement, whereas lithologic similarity and identical U-Pb ages between correlated footwall rocks in the Little Harquahala Mountains and breccia clasts at Bullard Peak in the NE Harcuvar Mountains indicated ~43-45 km of displacement across the EED. AHe and ZHe data indicated slip rates of ~6.7 + 7.8/-2.3 km/Myr, and ~6.6 + 7.1/-2.0 km/Myr, respectively, both consistent with the duration and displacement estimates. The EED initiated as a listric fault with an ~34 ± 9° dip that decreased to ~13 ± 5° below ~7 km depth. Secondary breakaway development and footwall exposure occurred by ~17 Ma, during active EED slip. Lithologic and geo-thermochronometric offset constraints show excellent agreement and provided a rare opportunity to fully resolve the timing, rates, and total displacement magnitudes along a major continental detachment fault.

Jurassic Barrovian metamorphism in a western U.S. Cordilleran metamorphic core complex, Funeral Mountains, California

Large-magnitude retroarc shortening of Cretaceous age is well documented in the Sevier orogenic belt of the western United States, and has been associated with eastward Franciscan subduction that began in the Middle–Late Jurassic, but evidence for major Late Jurassic retroarc shortening has been lacking. Here we report new Lu-Hf garnet geochronology, pressure-temperature ( P - T ) paths, and 40 Ar/ 39 Ar thermochronology data that document tectonic burial of Late Jurassic age in a Barrovian metamorphic terrain, the Funeral Mountains metamorphic core complex, California, located in the hinterland of the Sevier orogenic belt. The P - T paths determined from growth-zoned garnets in upper greenschist facies pelitic schist show steep P - T trajectories consistent with metamorphism during thrust loading. The age of thrust loading is constrained by a five-point Lu-Hf garnet isochron to be 158.2 ± 2.6 Ma (2σ). Partial exhumation, recorded in 146–153 Ma 40 Ar/ 39 Ar muscovite cooling ages, closely followed garnet growth. Late Jurassic Barrovian metamorphism has not been previously recognized in Cordilleran metamorphic core complexes, possibly due to being obscured by Late Cretaceous to Tertiary deformation, magmatism, and metamorphism. This study finds that the period of large-magnitude crustal shortening in the retroarc extended into the Late Jurassic, and may have closely followed the formation of the coherent orogenic system associated with east-dipping Franciscan subduction.

A naturally constrained stress profile through the middle crust in an extensional terrane

We present a method in which paleopiezometry, Ti-in-quartz thermobarometry (TitaniQ), and 2-D thermal modeling are used to construct a naturally constrained stress profile through the middle crust in an area of exhumed mid-crustal rocks. As an example, we examine the footwall of the Whipple Mountains metamorphic core complex (WMCC). Rocks in the WMCC were initially deformed at ~20km depth by distributed ductile shear, and were then progressively overprinted by localized ductile shear zones and eventually by discrete brittle fracture as the footwall was cooled and exhumed toward the brittle–ductile transition (BDT). Increasing strain localization and cooling during exhumation allowed earlier microstructures to be preserved, and rocks in the WMCC therefore represent several points in temperature–stress space (and by inference depth–stress space). We identify enough of these stress–depth points to construct a complete profile of the flow stress through the middle crust to a depth of ~20km, from which we derive regional estimates of the ambient stresses in the brittle upper crust, and the peak strength at the brittle–ductile transition in this region during Miocene extension.Maximum differential stress reached ~136MPa just below the brittle–ductile transition at a depth of ~9km. Stress levels are consistent with Byerlee's law in the upper crust assuming a vertical maximum principal stress and near-hydrostatic pore fluid pressures, and suggest a coefficient of friction on the 25°-dipping Whipple fault of ~0.4. Differential stress decreases to 10–20MPa at 20km depths and ~500°C. For strain rates typical of actively deforming regions (10−12 to 10−15/s), our stress profile is bracketed by the Hirth et al. (2001) flow law for wet quartzite, whereas the flow law of Rutter and Brodie (2004) overestimates the strength of this particular region.

Dynamics of large-magnitude extension in the Whipple Mountains metamorphic core complex

Dynamics of large-magnitude extension in the Whipple Mountains metamorphic core complex

How the Harcuvar Mountains metamorphic core complex became cool: Evidence from apatite (U-Th)/He thermochronometry

Apatite (U-Th)/He (AHe) data from ∼60 km of exposed footwall in the Harcuvar Mountains metamorphic core complex, Arizona, reveal a higher-precision late-stage cooling history than previously reported for this complex and others in the region. Progressively younger AHe ages recorded with distance along the slip direction show two distinct trends separated by a sharp inflection. At structurally shallower levels, ages decrease from ca. 23 to 15 Ma toward the northeast, defining an estimated slip rate of 2.6 +0.1/–0.2 km/ m.y. Samples from the structurally deepest ∼35 km of footwall yield concordant ages of ca. 15 Ma (±2σ error limits). The effects of mineralizing fluids, advection of isotherms toward the surface, and flexure-induced erosion are considered, but are unlikely to have influenced the ages. Therefore, it is suggested that the fault slip rate must have increased ca. 15 Ma to a rate of ∼30 km/m.y. Evidence for an increased slip rate ca. 15 Ma has also been observed in other metamorphic core complexes nearby, and this increase is attributed to the thermal effects of a slab-free window on the extending lithosphere.

Alligator Ridge District, East-Central Nevada: Carlin-Type Gold Mineralization at Shallow Depths

Carlin-type deposits in the Alligator Ridge mining district are present sporadically for 40 km along the north-striking Mooney Basin fault system but are restricted to a 250-m interval of Devonian to Mississippian strata. Their age is bracketed between silicified ca. 45 Ma sedimentary rocks and unaltered 36.5 to 34 Ma volcanic rocks. The silicification is linked to the deposits by its continuity with ore-grade silicification in Devonian-Mississippian strata and by its similar δ 18 O values (~17‰) and trace element signature (As, Sb, Tl, Hg). Eocene reconstruction indicates that the deposits formed at depths of <300 to 800 m. In comparison to most Carlin-type gold deposits, they have lower Au/Ag, Au grades, and contained Au, more abundant jasperoid, and textural evidence for deposition of an amorphous silica precursor in jasperoid. These differences most likely result from their shallow depth of formation. The peak fluid temperature (~230°C) and large δ 18 O H2O value shift from the meteroric water line (~20‰) suggest that ore fluids were derived from depths of 8 km or more. A magnetotelluric survey indicates that the Mooney Basin fault system penetrates to mid-crustal depths. Deep circulation of meteoric water along the Mooney Basin fault system may have been in response to initial uplift of the East Humboldt-Ruby Mountains metamorphic core complex; convection also may have been promoted by increased heat flow associated with large magnitude extension in the core complex and regional magmatism. Ore fluids ascended along the fault system until they encountered impermeable Devonian and Mississippian shales, at which point they moved laterally through permeable strata in the Devonian Guilmette Formation, Devonian-Mississippian Pilot Shale, Mississippian Joana Limestone, and Mississippian Chainman Shale toward erosional windows where they ascended into Eocene fluvial conglomerates and lake sediments. Most gold precipitated by sulfidation of host-rock Fe and mixing with local ground water in zones of lateral fluid flow in reactive strata, such as the Lower Devonian-Mississippian Pilot Shale.

General shear deformation in the Pinaleño Mountains metamorphic core complex, Arizona

Granitic mylonites from the Pinaleño Mountains metamorphic core complex in southeastern Arizona record general shear deformation. The mean kinematic vorticity number ( W m) was estimated using (1) the strain ratio ( R s) and the angle ( Θ) between the long axis of the strain ellipsoid with respect to the high-strain zone boundary and (2) the porphyroclast hyperbolic distribution method. W m for protomylonites and mylonites ranged from 0.6 to 0.9. Ultramylonites record lower W m values (0.1–0.3) suggesting that the incremental vorticity changed during deformation. Three-dimensional strain analysis indicates that deformation in protomylonites and mylonites approximates plane strain and has a monoclinic symmetry. The vorticity path followed by rocks tectonically exhumed in the footwall of major extensional fault systems may evolve from a significant pure shear component early in the deformation towards simple shear as overburden load decreases during uplift.

Crustal structure of the Ruby Mountains and southern Carlin Trend region, Nevada, from magnetotelluric data

We have collected about 150 magnetotelluric (MT) soundings in northeastern Nevada in the region of the Ruby Mountains metamorphic core complex uplift and southern Carlin mineral trend, in an effort to illuminate controls on core complex evolution and deposition of world-class gold deposits. The region has experienced a broad range of tectonic events including several periods of compressional and extensional deformation, which have contributed to the total expression of electrical resistivity. Most of the soundings reside in three east–west profiles across increasing degrees of core uplift to the north (Bald Mountain, Harrison Pass, and Secret Pass latitudes). One short cross-line was also taken to assess an east–west structure to the north of the northern profile. Model resistivity cross-sections were derived from the MT data using a 2-D inversion algorithm, which damps departures of model parameters from an a priori structure. Geological interpretation of the resistivity combines previous seismic, potential field and isotope models, structural and petrological models for regional compression and extension, and detailed structural/stratigraphic interpretations incorporating drilling for petroleum and mineral exploration. To first order, the resistivity structure is one of a moderately conductive, Phanerozoic sedimentary section fundamentally disrupted by intrusion and uplift of resistive crystalline rocks. Late Devonian and early Mississippian shales of the Pilot and Chainman Formations together form an important conductive marker sequence in the stratigraphy and show pronounced increases in conductance (conductivity–thickness product) from east to west. These increases are attributed to graphitization caused by Elko–Sevier era compressional shear deformation and possibly by intrusive heating. The resistive crystalline central massifs adjoin the host stratigraphy across crustal-scale, steeply dipping fault zones. The zones provide pathways to the lower crust for heterogeneous, upper crustal induced, electric current flow. Resistive core complex crust appears steeply bounded under the middle of the neighboring grabens and not to deepen at a shallow angle to arbitrary distances to the west. The numerous crustal breaks imaged with MT may contribute to the low effective elastic thickness ( T e) estimated regionally for the Great Basin and exemplify the mid-crustal, steeply dipping slip zones in which major earthquakes nucleate. An east–west oriented conductor in the crystalline upper crust spans the East Humboldt Range and northern Ruby Mountains. The conductor may be related to nearby graphitic metasediments, with possible alteration by middle Tertiary magmatism. Lower crustal resistivity everywhere under the profiles is low and appears quasi one-dimensional. It is consistent with a low rock porosity (<1 vol.%) containing hypersaline brines and possible water-undersaturated crustal melts, residual to the mostly Miocene regional extension. The resistivity expression of the southern Carlin Trend (CT) in the Pinon Range is not a simple lineament but rather a family of structures attributed to Eocene intrusion, stratal deformation, and alteration/graphitization. Substantial reactivation or overprinting by core complex uplift or Basin–Range extensional events seems likely. We concur with others that the Carlin Trend may result in part from overlap of the large Eocene Northeast Nevada Volcanic Field with Precambrian–Paleozoic deep-water clastic source rocks thickening abruptly to the west of the Pinon Range, and projecting to the north–northwest.

Constraints on crustal composition beneath a metamorphic core complex: results from 3-component wide-angle seismic data along the eastern flank of the Ruby Mountains, Nevada

Metamorphic core complexes expose rocks deformed at deep upper to middle crustal levels during extreme crustal extension. However, mechanisms of crustal extension and exhumation of core complexes remain to be fully understood. Detailed study of crustal velocity structure and inferences about the composition of the crust beneath core complexes (and nearby) provide useful constraints on core-complex evolution. P- and S-wave velocity structures determined from seismic experiments along the eastern flank of the Ruby Mountains metamorphic core complex, Nevada, show that the crust can be divided into three main layers corresponding to the upper, middle and lower crust. We interpreted crustal composition by integrating results of P-wave velocities ( V p), S-wave velocities ( V s), Poisson's ratios ( σ), seismic anisotropy, and reflection character with published geologic maps of the area. Near-surface estimates of V p, V s, σ, and anisotropy of 1.90–4.8 km/s, 1.01–2.75 km/s, 0.25–0.33, and 0.6–2.5%, respectively, are consistent with surface exposures of unconsolidated to consolidated sedimentary rocks, limestone, dolomite, siltstone, sandstone, porous sandstone, conglomerate, and weathered granite. Results from analysis of reflection responses, V p, V s, σ, and anisotropy also indicate that: (1) upper crustal rocks most likely consist of metaquartzite, schist, granite gneiss, and granite–granodiorite with V p of 5.80–6.25 km/s, V s of 3.20–3.72 km/s, σ of 0.22–0.25, and anisotropy of 0.6–2.5%; (2) possible middle crustal rocks are paragranulite, felsic granulite, felsic amphibolite gneiss, granite–granodiorite, and mica–quartz schist with V p of 6.35–6.45 km/s, V s of 3.70–3.75 km/s, and σ of 0.24; and (3) lower crustal rocks most likely consist of granulite- rather than amphibolite-facies rocks with V p of 6.60–6.80 km/s, V s of 3.85–3.92 km/s, σ of 0.24–0.25, and anisotropy of <3%. Our principal conclusions are: (1) significant addition of gabbroic rocks (underplating) is unlikely in the lower crust; (2) lower crustal rocks were stretched into sub-horizontal geometries with aligned minerals during extension, creating seismic lamellae in the lower crust; (3) present-day seismic velocities of highly extended core complex crust and normally extended Basin and Range crust are similar; and (4) orientations of fast shear waves near the surface and in the upper crust are sub-parallel to the regional maximum horizontal compressive stress in the Nevada part of the Basin and Range province.

Cenozoic tectonic evolution of the Ruby Mountains metamorphic core complex and adjacent valleys, northeastern Nevada

Seismic-reflection and borehole data along with crustal-scale refraction/reflection data provide new evidence for the Cenozoic tectonic evolution of the Ruby Mountains metamorphic core complex and Huntington, Ruby, and Lamoille valleys. Analyses of these data suggest: (1) along the western flank of the Ruby Mountains an early stage of upper-crustal extension provided accommodation space for deposition of apparently synextensional strata; (2) the oldest sedimentary rocks in the developing basin along the western flank of the Ruby Mountains are middle Eocene in age, suggesting that active upper-crustal extension and early basin formation in northeastern Nevada began at least by that time; (3) Ruby Valley is bounded by high-angle, east-dipping normal faults on the west and a relatively low-angle, west-dipping normal fault on the east; (4) crustal thicknesses beneath the eastern flank of the Ruby Mountains do not reflect local topographic relief and estimated amounts of extension; and (5) adjacent to the range, maximum thicknesses of basin-fill sedimentary rocks do not directly reflect maximum amounts of exhumation of the Ruby Mountains. Together, these observations suggest that either preexisting crustal roots (subsequently dissipated), or middle- or lower-crustal flow prior to and during extension, were involved in evolution of the core complex.

Controls on pseudotachylyte formation during tectonic exhumation in the South Mountains metamorphic core complex, Arizona

Abstract Pseudotachylyte formed during the later stages of extension in the South Mountains metamorphic core complex. Microstructural and microchemical evidence of pseudotachylyte formation processes is well preserved in granitoid mylonite host rocks because of rapid cooling during tectonic exhumation. Petrographic studies indicate a cataclasite precursor to pseudotachylyte. Transmission electron microscope analysis shows glass in pseudotachylyte, providing evidence of melting. Electron microprobe analyses demonstrate variations in glass, microlite, and crystallite composition with proximity to quartz clasts, indicating reaction between clasts and melt. Amphibole crystallites exhibit sieve textures, interpreted as recording rapid crystallization and subsequent decompression. Decompression is postulated to result from drops in fluid pressure along jogs in the slip surface or following formation of injection veins. Both pseudotachylyte and cataclasite are typically subparallel to C-surfaces in the host mylonite. Cataclasite not associated with pseudotachylyte is composed of quartz + feldspars ± biotite. Pseudotachylyte everywhere contains biotite. The model proposed for formation of pseudotachylyte and cataclasite in foliated granitoid protoliths includes seismic failure along C-surfaces. Where biotite is absent, slip produces cataclasite. Where biotite is present, initial cataclasis may be followed by frictional melting, facilitated by biotite’s low melting temperature and lowering of the melting temperatures of other phases by release of water during biotite breakdown.

The Northern Sacramento Mountains, southwest United States. Part I: Structural profile through a crustal extensional detachment system

Abstract Miocene exhumation of the Sacramento Mountains metamorphic core complex in the Colorado River extensional corridor, southwest United States, produced a range of deformational fabrics: pervasive, synthetic ductile deformation (with associated syntectonic intrusion) occurred at depth, discrete antithetic ductile through to brittle extensional shear zones developed at intermediate crustal levels and brittle cataclasis occurred at upper crustal levels. The region divides into three structural domains on the basis of these fabrics: (1) the eastern domain is dominated by a penetrative, ductile fabric with a consistent mineral alignment and stretching lineation, and sense of shear synthethic with detachment fault motion (top-to-northeast); (2) the central domain is characterized by a continuum of overprinting ductile through to brittle extensional shear zones with motion antithetic (topto-southwest) to detachment faulting; and (3) the western domain, where deformation occurred entirely by brittle cataclasis. Structural and petrofabric data from the Sacramento Mountains suggest that increasing structural depth/temperature is exposed in the direction of tectonic transport (N60E), consistent with detachment faulting. This region is interpreted as a profile through a crustal-scale shear zone in which the central domain of discrete antithetic shear zones formed in response to a combination of non-coaxial shear and vertical shortening, and represents an exhumed brittle-ductile transition. Rapid uplift and cooling of the northern Sacramento Mountains in the Miocene facilitated the preservation of this relatively transient regime.

The Northern Sacramento Mountains, southwest United States. Part II: Exhumation history and detachment faulting

Abstract Thermochronologic and thermobarometric data reveal the timing, distribution and intensity of thermal events associated with detachment faulting in the Sacramento Mountains metamorphic core complex. In the northwest Sacramento Mountains, cooling rates of c. 100°C Ma −1 are associated with Late Cretaceous plutonism followed by cooling of the crust by thermal conduction. Post-Late Cretaceous cooling slowed to c. 1–6°C Ma −1 . Finally, the region records average cooling rates of 38–53°C Ma −1 between c. 20 and 15 Ma. In contrast, the thermal profile of the northeast Sacramento Mountains is dominated by syntectonic Tertiary plutonism followed by very rapid cooling. A granodioritic suite intruded at c. 680°C and c. 3 kbar at c. 20 Ma, records cooling to &lt;100°C by c. 15 Ma. Such rapid cooling and exhumation suggests that unroofing by tectonic denudation was the driving mechanism for the final cooling. The similarity of the miocene cooling profiles between these two areas clearly suggests that the Sacramento Mountains experienced a regional cooling event associated with tectonic unroofing driven by regional Miocene crustal extension. Estimates of the initial angle of the Sacramento Mountains detachment fault using palaeothermal gradients suggest that it was active at a dip of 25°.

Crustal velocity structure beneath the eastern flank of the Ruby Mountains metamorphic core complex: results from normal-incidence to wide-angle seismic data

P-wave velocity structure along the eastern side of the Ruby Mountains metamorphic core complex, determined using 2-D ray-inversion modeling, shows that: (1) the crust can be divided into three layers corresponding to the upper, middle, and lower crust; (2) from east of Ruby Dome to the northern end of the seismic profile, velocities of the upper crust range from 6.12 (±0.1) to 6.20 (±0.1) km/s and near-surface velocities range from 1.90 (±0.1) to 4.80 (±0.1) km/s. From this point to the southern end of the profile, upper crustal velocities range from 5.80 (±0.1) to 6.25 (±0.1) km/s and near-surface velocities range from 3.01 (±0.1) to 4.80 (±0.1) km/s; (3) middle crustal velocities range from 6.35 (±0.15) to 6.45 (±0.15) km/s; (4) lower crustal velocities range from 6.60 (±0.15) to 6.80 (±0.15) km/s; and (5) depths to the Moho vary irregularly between 30.5 and 33.5 km. From interpretation of these results, we conclude that: (1) the transition in metamorphic grade from deep upper crustal rocks south of Harrison Pass to middle crustal rocks north of Harrison Pass does not correlate with an increase in seismic velocity; near-surface basement seismic velocities increase in the central part of the range (near Ruby Dome) well north of the major metamorphic transition; (2) depths to the Moho do not reflect local surface relief; isostatic balance is achieved predominantly in the crust rather than by formation of a local crustal root; and (3) the crust has been modified during extension by intracrustal processes rather than by large-scale magmatic underplating.