EARLY MIOCENE SILVER CREEK CALDERA AS A STRAIN MARKER IN THE COLORADO RIVER EXTENSIONAL CORRIDOR, USA

Silver Creek caldera, source of the 18.8 Ma Peach Spring Tuff, was ripped apart by a major (>100%) pulse of extension soon after it was emplaced. Fragments of the ~25 km wide caldera are strewn along a 60 km long swath of the Colorado River extensional corridor approximately 100 km south of Las Vegas, Nevada. The largest and defining fragment is a west-facing, gently tilted, and deeply eroded 40 km² arc preserved along the western edge of the Black Mountains near Oatman, Arizona. Two other smaller, fault-bounded fragments, preserving south-facing caldera rim segments are present in the hanging wall of the Sacramento Mountains (California) detachment fault 45 km and 52 km to the southwest. The swath suggests an extension direction of 215º which is parallel to the extension direction indicated by slickenlines on the detachment fault (210º-220º), but is inclined ~25º more southerly than the extension direction indicated by ductile fabrics in the footwall of the Sacramento Mountains (240º), and as suggested by the tilt direction of fault blocks in the hanging wall. Decoupling between footwall and hanging wall should be expected in highly extended belts, and may account for the discrepancy. Specifically, an early phase of minor (~5% of the total), ~190°-oriented, dike-accommodated extension in the footwall might may have been associated with higher magnitudes of similarly oriented southerly extension in the hanging wall.

Given that the elevation difference between the reconstructed eastern caldera fragment’s rim and the rim-preserving fragments in the Sacramento Mountains is only on the order of 1 km indicates a very high proportion of extension versus “collapse”. This has implications for mantle - lower crustal dynamics during extension.