Heterogenous late Miocene extension in the northern Walker Lane (California-Nevada, USA) demonstrates vertically decoupled crustal extension

Abstract

The spatial distribution and kinematics of intracontinental deformation provide insight into the dominant mode of continental tectonics: rigid-body motion versus continuum flow. The discrete San Andreas fault defines the western North America plate boundary, but transtensional deformation is distributed hundreds of kilometers eastward across the Walker Lane–Basin and Range provinces. In particular, distributed Basin and Range extension has been encroaching westward onto the relatively stable Sierra Nevada block since the Miocene, but the timing and style of distributed deformation overprinting the stable Sierra Nevada crust remains poorly resolved. Here we bracket the timing, mag­nitude, and kinematics of overprinting Walker Lane and Basin and Range deformation in the Pine Nut Mountains, Nevada (USA), which are the western­most structural and topographic expression of the Basin and Range, with new geologic mapping and 40Ar/39Ar geochronology. Structural mapping suggests that north-striking normal faults developed during the initiation of Basin and Range extension and were later reactivated as northeast-striking oblique-slip faults following the onset of Walker Lane transtensional deformation. Conformable volcanic and sedimentary rocks, with new ages spanning ca. 14.2 Ma to 6.8 Ma, were tilted 30°–36° northwest by east-dipping normal faults. This relationship demonstrates that dip-slip deformation initiated after ca. 6.8 Ma. A retrodeformed cross section across the range suggests that the range experienced 14% extension. Subsequently, Walker Lane transtension initiated, and clockwise rotation of the Carson domain may have been accommodated by northeast-striking left-slip faults. Our work better defines strain patterns at the western extent of the Basin and Range province across an approximately 150-km-long east-west transect that reveals domains of low strain (~15%) in the Carson Range–Pine Nut Mountains and Gillis Range surrounding high-magnitude extension (~150%–180%) in the Singatse and Wassuk Ranges. There is no evidence for irregular crustal thickness variations across this same transect—either in the Mesozoic, prior to extension, or today—which suggests that strain must be accommodated differently at decoupled crustal levels to result in smooth, homogenous crustal thickness values despite the significantly heterogeneous extensional evolution. This example across an ~150 km transect demonstrates that the use of upper-crust extension estimates to constrain pre-extension crustal thickness, assuming pure shear as commonly done for the Mesozoic Nevadaplano orogenic plateau, may not be reliable.

Where did the Arizona‐Plano Go? Protracted Thinning Via Upper‐ to Lower‐Crustal Processes

“…Further, the resolution of the Eocene upper‐crustal cooling period is also poor, as they resolve a mixed age between the pre‐MCC and MCC events. However, pre‐MCC thermochronometric cooling ages are observed in Walker Lane, Nevada (Say & Zuza, 2021) and higher temperature cooling has been documented within the Catalina MCC (Ducea et al., 2020; Jepson et al., 2021), suggesting that this pre‐MCC cooling may be more widespread than previously considered (Singleton et al., 2018). Despite the opaqueness surrounding this pre‐MCC tectonic event, the presence of mixed thermochronometric ages and the lack of normal faulting structures, discussed below, cannot explain the ∼10 km record of crustal thinning based on regional crustal thickness estimates (Figure 3).…”

The Thinnest Crust in South China Associated With the Cretaceous Lithospheric Extension: Evidence From SINOPROBE Seismic Reflection Profiling

“…Given the broadly consistent crustal thickness associated with the smooth Moho (Figures 3b and 4), the lower‐crustal stretching strain should vary to compensate for the heterogeneity of the upper‐crustal strain (Say & Zuza, 2021). The contrasting deformation styles between the upper and lower crust indicate vertically decoupled extension (Say & Zuza, 2021), with a mechanism similar to the two‐layer crustal stretching model (Gans, 1987). This decoupling requires efficient lateral and vertical strain partitioning across the crustal column.…”

“…This decoupling requires efficient lateral and vertical strain partitioning across the crustal column. The presence of decoupled extension—departing from the vertically coherent pure‐shear hypothesis (Best et al., 2009; Coney & Harms, 1984)—suggests that the crustal thinning calculations based solely on the upper‐crustal strain may not be valid (Say & Zuza, 2021).…”

“…These parameters include: the initial structures and inherited rheological heterogeneities (Balázs et al., 2017; Petri et al., 2019), evolving thermal anomalies (Huismans & Beaumont, 2003), strain localization (Frederiksen & Braun, 2001), distributed stretching or flow in the lower crust (Endrun et al., 2011), and rheological and thermal states of the crust and lithospheric mantle (Gueydan et al., 2008; Duretz et al., 2016). In particular, observations from the Basin and Range and the Aegean highlight the role of lower‐crustal stretching or flow in the architecture and evolution of extensional systems, especially in controlling the layered stretching, tectonic decoupling, vertical distribution of finite strain, and differential motion between the crust and mantle (e.g., Brun & Sokoutis, 2010; Endrun et al., 2011; Jolivet et al., 2018; Say & Zuza, 2021; Schulte‐Pelkum et al., 2011).…”

“…Since the proposal of the pioneering models in the 1980s, numerous studies on rift zones and passive margins have focused on the modes and mechanisms of continental lithospheric extension (e.g., Bahadori & Holt, 2019; Best et al., 2009; Buck, 1991). Most studies suggest that neither a pure shear model nor a simple shear model alone is suitable for characterizing the entire processes of lithospheric extension (e.g., Brun & Beslier, 1996; Say & Zuza, 2021), which emphasizes the need to consider the temporal evolution of rift systems (e.g., the multiphase rifting and rift migration models, Brune et al., 2014; Lavier & Manatschal, 2006). Detailed assessments of dynamic processes demonstrate that the extension style depends not only on depth but also on the evolution of extensional systems (e.g., Balázs et al., 2017; Brune et al., 2014; Huismans et al., 2001).…”