STRAIN ANAYLSIS AND STRAIN PROFILE OF THE MINERAL KING PENDANT, SOUTHERN SIERRA NEVADA, CALIFORNIA

The Mineral King pendant consists of NW-striking, steeply dipping, Triassic to Cretaceous igneous and sedimentary rocks that were deformed, steeply tilted, and intruded by granitic plutons at ~98 Ma. Well-developed foliation and steeply north-plunging stretching lineation are generally parallel to lithologic layering. Pendant rocks are highly deformed but the strain distribution and importance of pure shear versus simple shear deformation have not previously been investigated.

We collected strain data in an east-west transect across the northern pendant near Timber Gap. Deformed clasts in rhyolitic volcanic rocks from four major units were used for analysis. Two faces perpendicular to foliation were cut in each oriented sample, one parallel and one perpendicular to the stretching lineation. The long and short axes of each clast were measured, along with the clast orientation. Between 25 and 60 deformed clasts were measured from each face of 11 samples, and the data was analyzed using the Rf/phi method. Pumice clasts were used as strain markers in all samples, allowing for comparison of finite strains between units, although syndepositional flattening of all clasts is likely. The Rs values were calculated for each sample and used to construct a strain profile across the pendant.

Flattening strain is broadly distributed across the pendant. Strain is greatest along the western margin with calculated XY strain ellipse axial ratio of 10.5 and XZ ratio of 18.6. Diminishing strain along the eastern margin is apparent with XY and XZ strain ratios of 8.5 and 11.4 respectively. The center of the pendant exhibits diminished strain with XY ratios of 5.2 and XZ ratios of 9.8. The indicated finite strain is predominantly SW-NE flattening with sub-vertical extension.

Locally well-developed ductile flow fabrics around rigid porphyroclasts are generally symmetric, with no evidence for a consistent asymmetry observed in either horizontal or lineation-parallel surfaces. These results suggest predominantly pure shear flattening deformation, with greater strain at the margins of the pendant, diminishing towards the center. This pattern of deformation is consistent with a “return flow” emplacement model, in which screens of country rock are vertically extended and subside downward as surrounding granitic plutons are emplaced.