Geologic and gravity evidence for thrust emplacement of the Smartville ophiolite

Abstract

Detailed structural relationships at the margins of ophiolites can provide important information about the mode of emplacement of ophiolitic terranes. In this study we use geologic mapping and gravity data to determine the structural and tectonic relations between a Middle to Late Jurassic volcanic arc complex (the Smartville Ophiolite block) and a pre‐Late Jurassic melange terrane in the northern Sierra Nevada foothills. The volcanic arc complex overlies the melange along a complex fault zone. Although this fault zone is steeply dipping wherever it is observed, map patterns suggest that the Smartville Upper Volcanic unit was originally emplaced along a subhorizontal fault that has been subsequently deformed. Two‐dimensional interactive computer analysis of gravity data from over 200 stations shows that the Smartville Upper Volcanic unit is a relatively shallow feature with a maximum thickness ranging from 0.5 to 3 km and that it was thrust as a sheet over the previously deformed melange. Both units were subsequently folded, giving rise to the steeply dipping faults observed at the surface. Recognition of low‐angle thrusting along the northern margin of the Smartville Ophiolite block has important implications for the mode of emplacement of the block. The most plausible mechanism for the thrusting of this particular oceanic island arc sequence onto the continental margin involves a subduction zone dipping westward beneath the Smartville Ophiolite block, similar to the present‐day situation in the Molucca Sea collision zone.