AN EXAMPLE OF HETEROGENEOUS DEFORMATION AND STRAIN PARTITIONING APPLIED TO CRUSTAL FLOW AND EXHUMATION OF HP AND UHP ROCKS, BLACK HILLS SD, USA

Understanding the nature of heterogeneous ductile flow will elucidate processes such as mixing of rocks of different metamorphic grade. This study examined impure quartzite and schist layers interpreted to have been subject to simultaneous pure and simple shear during emplacement of the Harney Peak granite. Intrusion of a granitic dike at or near peak metamorphic conditions (~ 650 C and 0.30 GPa) allowed preservation of original layer thickness and the amounts of simple shear experienced by layers to be estimated. For quartzite and schist layers, two-dimensional modeling yields a finite strain ellipse ratios of 3.40 and 8.08 respectively. The pure shear component given by the stretch for both layer types is 0.60. The calculated kinematic vorticity numbers for quartzite and schist are 0.64 and 0.90 respectively where 1.00 is simple shear and 0.00 is pure shear. Displacement in the weaker schist is 3.17 times the quartzite layer. Model and outcrop observations indicate the following: 1) The simple shear component in these layer types is partitioned into the weaker schist layer. 2) The entire weak layer becomes the shear zone. 3) There is no localization of strain at the contacts implying that there is coupling between the layers. 4) Thinner interlayers of both types in this exposure tend to take on behavior of the dominant layer type.

During exhumation of HP and UHP rocks weak layers where coupling is maintained the peak metamorphic conditions (assuming preservation) could vary significantly and systematically though the weak layer. Separation (decoupling) weak and strong layers likely depend upon the pure shear component of the deformation. Layers with differing pure shear components localize stain at the contacts allowing a shear zone to develop at the contact. At this stage, layers begin to move independently of one another allowing more dramatic mixing of rocks of different metamorphic grade.