USING DETAILED PALEOMAGNETIC STUDIES TO BETTER UNDERSTAND COMPLEX OROGENIC SYSTEMS- THE LEGACY AND INFLUENCE OF JOHN GEISSMAN

Dr. John Geissman has been a pioneer in applying detailed paleomagnetic and rock magnetic investigations to better understand complex orogenic systems. Working over a range of scales in both compressional and extensional settings, John Geissman has shown the power of paleomagnetism to quantify local and regional vertical-axis rotations, which provide key input for kinematic and mechanical models of crustal-scale fault systems. His emphasis on detailed sampling, generating robust datasets, careful demagnetization techniques, and the integration of comprehensive rock magnetic data has been an inspiration to many, including our work in the Cordillera, rooted in Weil’s early collaborations with John on the Precambrian evolution of North America. Example studies from John Geissman’s own work, along with the author’s work on the Sevier fold-thrust and Laramide foreland belts of the western US, will be used to highlight how paleomagnetic, rock magnetism, Anisotropy of Magnetic Susceptibility (AMS), and structural data can be effectively integrated to robustly constrain three-dimensional deformation histories and test various kinematic and mechanical models, including the roles of preexisting crustal architecture, evolving topography, and plate interactions on orogenic wedge development and evolution of map-view fault and paleo-stress/strain patterns. Importantly, good spatial coverage of paleomagnetic sites in combination with analysis of early layer-parallel shortening fabrics and mesoscopic to large-scale structural data can be used to restore paleostress/ strain trajectories to their reference frame at the time of deformation, and therefore better link the dynamic coupling between the plate boundary and the deforming plate interior. This is particularly important for fabrics that develop during early deformation phases in fold-thrust belts and forelands, and undergo subsequent tilting and vertical-axis rotation during differential fault slip and orogenic wedge development.