UC Santa Barbara

The northern Snake Range metamorphic core complex in eastern Nevada provides a unique opportunity to study the older contractional structures that are rarely preserved in Cordilleran core complexes. This study investigates the structurally shallowest (up-dip) portion of the footwall of this core complex to elucidate the early crustal shortening history through detailed geologic mapping. Thrust faults and folds are preserved along the northwestern flank of the northern Snake Range and have not been extensively overprinted by Tertiary extensional deformation. The Lower Cambrian Pioche Shale (Cpi) is interpreted as a detachment surface separating the relatively undeformed Lower Cambrian Prospect Mountain Quartzite (Cpm) from the overlying intensely folded and heterogeneously strained Middle to Upper Cambrian marbles. This detachment surface facilitated map-scale folding in the overlying marble units and a series of small-displacement, discontinuous thrust faults that occasionally cut down section into the Cpm. The variable orientation of marble units observed in the mapped area is inferred to represent the anticlinal closure of the kilometer-scale east-vergent recumbent syncline that characterizes the northern portion of the range. Lower plate strain in the Cpm along the northwestern flank of the range is the lowest magnitude and highest temperature strain observed in the northern Snake Range. The high temperature fabric associated with this strain is characterized by the pervasive alignment of euhedral metamorphic mica and is unrelated to Tertiary extensional deformation. An ~82 Ma pegmatite intrusion cross cuts the fabric but is also locally strained and boudinaged. Due to the observed cross cutting relationships the intrusion is interpreted as being synkinematic with the deformation event and related to the Late Cretaceous metamorphic event that has been previously described in the range. This older deformation event has likely influenced the geometry of lower plate fabrics and the style and magnitude of lower plate strain associated with Tertiary extensional deformation. Any comprehensive study of lower plate strain in the northern Snake Range should address the effect this previously unidentified deformation event may have had on younger extensional deformation.