Abstract The M 6.5 Monte Cristo Range earthquake that occurred in the central Walker Lane on 15 May 2020 was the largest earthquake in Nevada in 66 yr and resulted in a multidisciplinary scientific field response. The earthquake was the result of left-lateral slip along largely unmapped parts of the Candelaria fault, one of a series of east–northeast-striking faults that comprise the Mina deflection, a major right step in the north–northwest structural grain of the central Walker Lane. We describe the characteristics of the surface rupture and document distinct differences in the style and orientation of fractures produced along the 28 km long rupture zone. Along the western part of the rupture, left-lateral and extensional displacements occurred along northeasterly and north-striking planes that splay off the eastern termination of the mapped Candelaria fault. To the east, extensional and right-lateral displacements occurred along predominantly north-striking planes that project toward well-defined Quaternary and bedrock faults. Although, the largest left-lateral displacement observed was ∼20 cm, the majority of displacements were <5 cm and were distributed across broad zones up to 800 m wide, which are not likely to be preserved in the geologic record. The complex pattern of surface rupture is consistent with a network of faults defined in the shallow subsurface by aftershock seismicity and suggests that slip partitioning between east-striking left-lateral faults and north to northwest-striking right-lateral faults plays an important role in accommodating northwest-directed transtension in the central Walker Lane.
Abstract The relationships between brittle detachment faulting and ductile shear zones in metamorphic core complexes are often ambiguous. Although it is commonly assumed that these two structures are kinematically linked and genetically related, direct observations of this coupling are rare. Here, we conducted a detailed field investigation to probe the connection between a detachment fault and mylonitic shear zone in the Ruby Mountain–East Humboldt Range metamorphic core complex, northeast Nevada. Field observations, along with new and published geochronology, demonstrate that Oligocene top-to-the-west mylonitic shear zones are crosscut by ca. 17 Ma subvertical basalt dikes, and these dikes are in turn truncated by middle Miocene detachment faults. The detachment faults appear to focus in preexisting weak zones in shaley strata and Mesozoic thrust faults. We interpret that the Oligocene mylonitic shear zones were generated in response to domal upwelling during voluminous plutonism and partial melting, which significantly predated the middle Miocene onset of regional extension and detachment slip. Our model simplifies mechanical issues with low-angle detachment faulting because there was an initial dip to the weak zones exploited by the future detachment-fault zone. This mechanism may be important for many apparent low-angle normal faults in the eastern Great Basin. We suggest that the temporal decoupling of mylonitic shearing and detachment faulting may be significant and underappreciated for many of the metamorphic core complexes in the North American Cordillera. In this case, earlier Eocene–Oligocene buoyant doming may have preconditioned the crust to be reactivated by Miocene extension, thus explaining the spatial relationship between structures.
In Figures 13B and 13C on p. 2103, several unit contacts in the footwall of the Independence thrust were shown as thick and red (i.e., a fault), bounding the Ordovician strata (O), because of a conversion error. These lines should be thin and black, instead (i.e., a contact). The corrected figure follows.
<p>The core complexes of western North America are generally thought to exhume deeply buried rocks (as much as 30 km) from the Cordilleran infrastructure, from beneath an inferred orogenic plateau to the surface today. However, how deep these rocks were buried has been intensely debated over the past three decades, especially for the Ruby Mountain-East Humboldt Range (RER) and northern Snake Range core complexes, eastern Nevada: published thermobarometry calculations, including robust modern techniques, suggest deep burial to 2-3x stratigraphic depths (as much as 30 km), whereas generations of field studies support burial only to roughly stratigraphic depths (~12-15 km). This has led to fierce debate that either field geologists are missing major structures or geobarometric estimates may neglect important considerations, such as reaction overstepping. Here we propose that a model of non-lithostatic conditions can resolve both field and petrologic datasets, and therefore the North American core complexes represent an example of tectonic overpressure. Western North America is covered by a remarkably well-characterized ~12-15-km-thick passive margin sequence that allows for careful structural reconstructions. Our observations focus on the RER geology, including new detailed geologic mapping (1:24,000 scale), structural traverses, thermochronology, and peak temperature (Tp) estimates. In particular, peak P-T conditions that suggest deep burial require (1) relatively low geothermal gradients of &#8804;20&#176;C/km and (2) enigmatic structures that are not observed and would be atypical of other Cordilleran fold-thrust belts or even other analogous intra-plateau thrust systems. Instead, our Tp compilation (e.g., Raman spectroscopy of carbonaceous material, Conodont color alteration index, thermochronology) across continuous stratigraphy suggests high geothermal gradients (&#8805;40&#176;C/km) that are consistent with the region being extensively intruded and mineralized&#8212;i.e., the region underwent major Jurassic, Cretaceous, and Eocene intrusive episodes and hosts an Eocene(?) world-class Carlin-type gold deposit&#8212;and matches thermal gradients observed in other eastern Nevada studies and analogous orogens. Systematic mapping does not reveal any structural break across a section of Neoproterozoic to undeformed Permian passive margin strata that was supposedly deeply buried beneath an additional entire stratigraphic section. The approach of using a Tp traverse to test deep burial models allows for self-consistent evaluation of the data. That is, interpretations are based on a trend of temperature variations deduced from numerous measurements rather than relying on a single (or few) pressure data point(s). Our observations suggest that non-lithostatic pressure may have affected Cordilleran core complexes. We explore how the local rheologically heterogeneous rock types and specific tectonic setting may have created conditions favorable for tectonic overpressure in North American core complexes. For example, paleo-stress estimates from across several shear zones demonstrate significant strength variations that may have facilitated mean stress (pressure) perturbations.</p>
Abstract Strongly deformed footwall rocks exposed in metamorphic core complexes (MCC) of the North American Cordillera were exhumed via ductile attenuation, mylonitic shearing, and detachment faulting. Whether these structures accommodated diapiric upwelling or regional extension via low‐angle normal faulting is debated. The Ruby Mountains‐East Humboldt Range MCC, northeast Nevada, records top‐west normal‐sense exhumation of deformed Proterozoic‐Paleozoic stratigraphy and older basement. We conducted 1:24,000‐scale mapping of the southwestern East Humboldt Range, with integrated structural, geochemical, and geochronological analyses to characterize the geometry and kinematics of extension and exhumation of the mylonitized footwall. Bedrock stratigraphy is pervasively intruded by Cretaceous, Eocene, and Oligocene intrusions, but observations of a coherent stratigraphic section show >80% vertical attenuation of Neoproterozoic to Ordovician rocks. These rocks are penetratively sheared with top‐west kinematics. The shear zone thus experienced combined pure‐ and simple‐shear (i.e., general shear) strain. We argue that this shear zone was syn‐/post‐kinematic with respect to Oligocene plutonism because: (a) mylonitic shearing spatially corresponds with preceding Oligocene intrusions; (b) thermochronology reveals that the shear zone experienced substantial cooling and exhumation after Oligocene plutonism; and (c) the mylonites are crosscut by undated, but likely late Oligocene, leucogranite. We propose that Eocene mantle‐derived magmatism and thermal incubation led to Oligocene diapiric upwelling of the middle crust, with ductile stretching focused on the flanks of this upwarp. Regional Basin and Range extension initiated later in the middle Miocene. Therefore, the development of the East Humboldt Range shear zone was not driven by regional extension and coupled detachment faulting.
