Northwesterly displacement of the Sierra Nevada with respect to the Great Basin in the western US Cordillera has been ongoing for the last 12 to 15 Ma. This motion is accommodated by a complicated and incompletely understood belt of deformation along the western margin of the Great Basin that links major, misaligned strike-slip faults in eastern California and western Nevada. Based on previous research and our preliminary work, we have identified that the deformed belt underlies a region of nearly 15,000 km2 and contains large tectonic blocks that both translate toward the northwest as the crust is pulled apart and also undergo vertical-axis rotations of 20° to 90°. In this project, we will provide a better understanding of the dimensions of the tectonic blocks, how far they have moved laterally, to what degree they experienced vertical-axis rotation, and the spatial-temporal pattern of deformation. Much of the western borderland of North America and many parts of other continents around the world reside in similar tectonic settings and record comparable histories of deformation. This study will provide the opportunity to better understand how and to what degree the translation and rotation of large crustal blocks is accommodated as the continental crust is fragmented in response to the relative movement of lithospheric plates.
Crustal response to displacement transfer in structural stepovers linking misaligned segments of large-magnitude transcurrent faults is accommodated by components of translational and rotational displacement and strain and results in complex three-dimensional arrays of structures. The mechanisms by which rotational and translational strain are accommodated either by rigid-blocks and/or by distributed strain and to what degree the translational and rotational processes are coupled in space-time are poorly understood. In this study, we integrate detailed and regional geologic mapping with thermochronologic, structural, and paleomagnetic analysis to unravel the history of transcurrent structures separating the Sierra Nevada and central Great Basin. The objective of the work is to characterize transcurrent and high-angle normal fault displacement, estimate slip on low-angle detachment faults, and assess the relation between translational deformation and differential rotation. Our research results will provide the needed detailed understanding of the spatial and temporal pattern of deformation and supply the constraints to differentiate between coeval and serial translational and rotational deformation histories within the stepover system and provide the means to assess whether or to what degree rotation is accommodated by rigid-body motion or distributed strain processes.
This research project helped delineate the structural mechanisms that allow misaligned strike-slip fault systems (Furnace Creek – Fish Lake Valley and transcurrent faults of the central Walker Lane) to maintain kinematic coordination during long-term deformation. Specifically, faults with large magnitude strike-slip displacement, on the order of tens of kilometers, commonly are not continuous and show steps or offsets in the fault trace. The lateral discontinuity in the faults poses the questions of how displacement is accommodated from one fault system to another and what mechanism(s) exist to transfer displacement through the areas between fault tips. Using a combination of geologic mapping, structural and stratigraphic analysis, geochronology, and paleomagnetic analysis, we documented that displacement transferred through the domains between fault systems is taken up by a complex pattern of strike-slip displacement on high-angle faults, displacement transfer between strike-slip and dip-slip faults, and through vertical axis rotation of large areas. The vertical axis rotation of broad regions is accommodated by decoupling of the structural section undergoing rotation from the underlying basement by regionally pervasive low-angle detachment faults. Low-angle detachment faults work in concert with through-going high-angle faults that serve as both bounding structures for rotational domains and as accommodation structures within the domains. The research project provided a platform for training both undergraduate and graduate students in field techniques and in laboratory practices. The project directly involved four MS students and one Ph.D. student with partial field and laboratory support. Undergraduate students, taking the University of Texas at Dallas field geology course during their senior year, were involved, at no cost to the project, in mapping and field exercises on topics related to the research objectives. The involvement of the undergraduate students in mapping and discussions engendered a heightened sense of relevance for their work and provided insights into how geological research is conducted. Broader impacts for the project arose from involvement of undergraduate science teachers (aspiring for positions in junior and senior high school) in the field work. The teachers worked with graduate students and with the undergraduates in the field. The experience provided them with valuable experience observing and interacting with students and researchers during their three week tours. The involvement of the aspiring science teachers enable a better appreciation of the scientific process. The teachers observe how both graduate and undergraduate students approach and solve geological problems and gain experience that they can carry to their own classrooms.
