Search for: All records

Lithospheric shortening can be described by one of two end-member modes: indentation of the lithosphere and subduction of the lithospheric mantle. Deciphering the difference between these modes is crucial in the interpretation of past and present orogens and in predicting their structural architecture at depth. It is therefore important to establish how observable upper crustal proxies reflect deep lithospheric kinematics and dynamics. Over the last few decades, geological and geophysical data have provided valuable constraints on the northern margin of the Tibetan Plateau. This margin is defined by the Qilian Shan thrust belt, which developed in response to the far-field convergence between the Indian and Eurasian plates. The primary mechanism for this development is the southward subduction of the Asian lithospheric mantle beneath the Tibetan Plateau. We conducted numerical modelling to simulate the kinematics and response of the upper crust to the southward subduction of the lithospheric mantle. Our results show that subduction of the lithospheric mantle can result in upper crustal deformation that matches the records in the Qilian Shan, where pure shear shortening alone does not generate similar upper crust proxies, including the broad width and architecture of the bivergent orogenic wedge, the timing of fault initiation and evolution, seismicity and fault activity, the topography and geomorphology. The geometry of the subducting lithosphere impacts the width and asymmetry of the bivergent orogenic wedge. Our results demonstrate how records of crustal strain can be used to better interpret the deep structural architecture of past and present orogenic wedges. 

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.