Paleomagnetic Approaches to Evaluating Extensional Tectonism in the Western North American Cordillera

Details Paleomagnetic Approaches to Evaluating Extensional Tectonism in the Western North American Cordillera Paleomagnetic Approaches to Evaluating Extensional Tectonism in the Western North American Cordillera

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmgp44a..03g&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #GP44A-03

Mathematics

Logic

1525 Paleomagnetism Applied To Tectonics: Regional, Global, 1527 Paleomagnetism Applied To Geologic Processes, 8109 Continental Tectonics: Extensional (0905), 8111 Continental Tectonics: Strike-Slip And Transform

Scientific paper

The mid-Cenozoic cessation of Mesozoic and early Cenozoic crustal shortening processes in the western Cordillera of North America (WCNA) and profound thermal modification of the western North American lithosphere have resulted in a unique natural laboratory for the study of intracontinental extension. In the Basin and Range Province and related extensional terranes (e.g., Rio Grande rift), numerous paleomagnetic investigations, compiled in a GIS-based format for this paper, have contributed to our understanding of deformation processes related to continental lithosphere extension, and these can be basically divided into two, rather general, research questions. One is the overall contribution of vertical axis crustal rotation to lithosphere extension. The other involves the origin and history of crustal deformation, specifically rotation about a sub-horizontal axis (tilting), associated with slip along regionally extensive normal fault systems, many of which are well-exposed at the surface and presently have very shallow orientations in the crust. Answers to these questions provide important contributions to the kinematic history of lithosphere extension. The magnitude and sense of vertical axis rotations of the crust are evaluated using paleomagnetic declination data, assuming inclination has not changed, which are referenced to either expected field directions (absolute determinations) or to data from the same unit at an "unrotated" location (relative determinations). Slip on normal faults may result in significant tilts of both upper and lower plate rocks to the fault or fault system and paleomagnetic data may, under particular circumstances, be used to estimate tilt magnitude. Several paleomagnetic investigations have provided high-quality data sets to provide the answers to specific questions to local areas. From a regional perspective, the available paleomagnetic data show that the pattern of vertical axis rotations related to extension in the WCNA is heterogeneous. Furthermore, no single mechanism can be applied to explain the observed pattern of crustal rotations. Some data from crystalline rocks in the hanging wall and footwall of normal fault systems show that paleomagnetically significant tilting can occur in these rocks; data from regionally extensive detachment systems show that there may be little geometric modification to the lower plate while upper plate rocks have been translated over considerable distance.

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