UNRAVELING PALEO-DISPLACEMENT FIELDS IN A COLLAPSED ARC: AN EXAMPLE FROM THE FOOTHILLS TERRANE, SIERRA NEVADA, CALIFORNIA

There is much interest in using geodesic data to evaluate short-term (101 years) displacement fields in arcs and other settings. As these studies mature it will be intriguing to compare these data to the long-term (104 to 106 years) displacement fields inferred from features preserved in ancient arcs. The Foothills terrane, Sierra Nevada preserves the shallow crustal record of arc construction and simultaneous tectonism. An examination of the Jura-Cretaceous history of this arc indicates that heterogeneous rheological behavior of host rocks and magmatic systems strongly control both strain and displacement. Regional displacement processes during arc growth include development of a SW-directed fold and thrust (during which layers were mechanically active), heterogeneous strain, and significant crustal thickening. Lateral translations potentially occurred but are difficult to constrain. Strains were controlled by rock type, deformation mechanisms, local structural setting, and regional gradients. Dramatic along-strike gradients in strain occur even in single formations and are independent of the amount of rigid rotation of markers. Regional displacements are thus best inferred from regional-scale structures and structures in major shear zones and not by outcrop-scale structures, which reflect local mechanical controls. Pluton emplacement in this arc occurred during contraction and crustal thickening. Statistical studies indicate that magmatic centers are independent of regional faults but may show some spatial and temporal clustering. During rise of magma, host rocks were typically displaced by local processes in pluton aureoles or chambers and resulted in vertical, largely downward transport of material. Detailed studies of these aureoles suggest that displacement occurred by downward rigid rotation of layers and strain, sometimes resulting in rim anticlines. This downward displacement during rise of magma is one means by which material was transported towards the developing crustal root. There is much that we can and cannot infer from the rock record regarding the long-term displacement in this arc. Although unraveling paleo-displacement fields is challenging, I want to thank Othmar Tobisch for his encouragement to tackle this undertaking and his gentle help and friendship over the years.