Displacement and deformation associated with a lateral thrust termination, southern Golden Gate Range, southern Nevada, U.S.A.

Abstract

The Golden Gate thrust offers an excellent opportunity to study displacement and deformation at a lateral thrust tip. Slip across this E-vergent thrust is uniformly about 2.4 km for the southern 2 km of exposed strike length and dies out to zero in the northern 5 km. Constant displacement and uniform hangingwall structure along the southern one-third of exposed strike length suggest that along-strike displacement variation and consequent deformation are localized near the lateral tip. Structures located at and north of the lateral tip are consistent with transpression and later uplift as slip accumulated on the thrust. Fold and fault orientations and calcite twinning strain record possible transpression north of the lateral tip. E-W-striking normal faults at the thrust tip record extension that is interpreted to be the result of the uplift and translation of a structurally continuous, rigid block located north of the normal faults. This block is connected to and was uplifted with the crest of the hangingwall anticline, and was separated by the normal faults from thrust-related folding to the south. The main hangingwall structure is an anticline that in southern exposures trends N-S parallel to the thrust but bends westward and opens into a box fold to the north. The hangingwall anticline superficially resembles a classic fault-propagation fold formed by a migrating ductile bead. However, we are forced to reject the ductile bead hypothesis for the fold because: (1) the geometry of the hangingwall anticline far from the thrust tip cannot have evolved from the geometry at the thrust tip; and (2) twinned calcite strain data from along the thrust suggest that strain was homogeneous and coaxial rather than inhomogeneous and progressively non-coaxial as predicted for a migrating ductile bead. The three-dimensional structural and kinematic relations at the lateral tip of the Golden Gate thrust appear to be the result of deformation around a pinned tip, although the reason that the tip was pinned at its present location is unclear. We interpret the thrust to have propagated relatively quickly to its ultimate extent with little initial displacement, indicating that it was easier to fault the rocks than to fold them.