LITHOSPHERIC STRENGTHENING AND INDIRECT TECTONOMAGMATIC LINKS DURING MID-CRETACEOUS OROGENESIS IN THE CENTRAL SIERRA NEVADA

Synthesis of new and published mapping, field data, and geochronology provides an updated reconstruction of mid-Cretaceous orogenesis coeval with an arc flare-up across the central Sierra Nevada (California, USA). Tilted strata, magmatic and solid state fabrics, and shear zones with variable geometries, kinematics, and development timing of reveal that mid-Cretaceous (ca. 105-80 Ma) tectonism was characterized by ~25 million years of spatiotemporally heterogeneous deformation. Arc-normal contraction transitioned gradually to dextral transpression ca. 100-96 Ma, culminating in a marked decrease in intra-arc shortening. Although pluton emplacement must involve significant strain of host rocks, deformation related to pluton emplacement is only locally preserved. Throughout this orogenic episode, tectonism and magmatism show distinct and unrelated spatiotemporal patterns within a broad region in the axial to eastern central Sierra Nevada.

This new synthesis contradicts previous models invoking direct tectonomagmatic links in arcs: magmatism did not control the location, intensity, or kinematics of intra-arc deformation, nor did shear zones control the spatiotemporal patterns of magmatism. Arc and orogenic processes instead interact indirectly by driving the evolution of lithospheric architecture (e.g. thickening; compositional stratification), modifying these same processes in turn. The waning of intra-arc deformation coeval with continued contraction in the Cordilleran foreland indicates that arc lithosphere strengthened as flare-up magmatism proceeded. Potential causes include: lithospheric thickening; increasing additions of competent intrusions that crystallized quickly relative to strain accumulation; disruption of through going, crustal-scale shear zones; and mafic residue accumulation in the deep lithosphere. In addition to changing plate-scale boundary conditions, lithospheric evolution likely played a significant role in the shifting of Late Cretaceous deformation observed within the arc and across-strike of the SW US Cordillera. We highlight the need for a whole-orogen perspective, incorporating the impacts of arc activity and evolving lithospheric architecture, in interpreting paleo-dynamics from continental deformation in convergent margins.