PERVASIVE FOLDING DURING CRETACEOUS DEFORMATION IN THE MINERAL KING PENDANT, SOUTHERN SIERRA NEVADA, CALIFORNIA

The Mineral King pendant is an ~15-km long, northwest-striking body of Late Tr to mid-Cretaceous metavolcanic and metasedimentary rocks that forms a steeply dipping wall rock screen between large mid-Cretaceous plutons of the Sierra Nevada batholith. Pendant rocks have variably developed, NW-striking, steeply dipping, bedding-parallel cleavage and flattening foliation, and steeply NW-plunging stretching lineation. Along-strike facies changes combine with structural imbrication to create a complex map pattern. Contrasts in foliation development correlate primarily with lithology, however, and discrete faults or shear zones are not readily apparent.

Available age dating indicates structural discontinuities resulting from faulting. However, we have observed very few differential shear indicators, and no unambiguous textural evidence for shear zones with significant differential displacement. In particular, we find no evidence for major strike-slip faulting, although such faults may possibly have existed prior to late stage contractional deformation. Kilometer-scale tight to isoclinal synclines that involve 135 Ma rhyolite tuff are exposed on Vandever Mountain and the northeast wall of Mineral King valley, and fold sequences with 100 m amplitudes are exposed in the Tulare Peak and Mineral Peak areas. Outcrop-scale folds are locally common, with both subhorizontal and steeply plunging hinge lines. Top direction indicators show both NE and SW tops, indicating isoclinal folding and bedding transposition at meter scale.

Late stage flattening fabrics are ubiquitous, combined with steeply NW-plunging stretching lineation. Flattening and vertical extension occurred as late as mid-Cretaceous time, as indicated by extensively boudinaged pegmatite dikes dated at 107 Ma, and isoclinally folded aplite dikes dated at 98 Ma (Sisson and Moore, 2013). These dates are concurrent with emplacement of the major late Cretaceous plutons that bound the Mineral King pendant. The observed structures suggest a possible model of large scale fold-thrust imbrication, followed by tilting, isoclinal folding and bedding transposition, and ductile flattening and vertical extension driven by geologically rapid downward return flow of country rock during upward emplacement of granitic plutons.