STRAIN ENVIRONMENT ANALYSIS OF THE LATE JURASSIC CORDILLERAN MARGIN USING THE INDEPENDENCE DIKE SWARM, RIDGECREST, CA

Data on the strain environment along the late Jurassic proto-Pacific margin in western North America are sparse, primarily due to the limited exposure of structures from the period. One important structure is the Independence Dike Swarm (IDS), a 600 km long 148 Ma en-echelon dike swarm extending from the Mojave Desert to the central Sierra Nevada Mountains. Dikes of the IDS have a consistent strike of approximately N 53° W (± 20°) with subvertical dips. However, there is one subset of the IDS – located in the Spangler Hills just south of Ridgecrest, CA – that doesn’t fit this general trend. Within this area, two sets of dikes assumed to be part of the IDS due to their location and general composition are found to be mutually cross-cutting rather than en-echelon. In addition to their mutual cross-cutting relationship, the two sets of dikes also show apparent strike-slip offsets, suggesting that the fractures were tectonically active during dike emplacement. The nearby East Sierran Thrust System (ESTS) is a potential source of tectonic activity. ESTS structures occur alongside IDS dikes, and they were active until around 150 Ma in the Argus and Slate Ranges, which are only a few kilometeters from the field area.

Mapping and preliminary structural analysis suggests that the fractures occupied by the dikes have a shortening direction of approximately N 74° E. These observations are consistent with strain in the ESTS, at least within the Argus and Slate Ranges. Fold planes within the Argus and Slate Ranges generally strike within 10-20 degrees west of north, which is consistent with the shortening direction inferred from the Spangler Hills dikes. While it is difficult to apply conclusions from this small field area to the entirety of the continental margin, these data certainly suggest that the strain environment along the eastern part of the late Jurassic magmatic arc was largely contractional, with little to no transpression or transtension.