AGU Fall Meeting.
Paper T41F-1301

The Effects of Composition and Finite Strain on Rock Rheology: Constraints From Analysis of Natural Deformation

Horsman, E
eric@geology.wisc.edu
Dept. of Geology and Geophysics, University of Wisconsin - Madison, 1215 W. Dayton St., Madison, WI 53706 United States

Tikoff, B.
basil@geology.wisc.edu
Dept. of Geology and Geophysics, University of Wisconsin - Madison, 1215 W. Dayton St., Madison, WI 53706 United States

Czeck, D.
dyanna@uwm.edu
Dept. of Geosciences, University of Wisconsin - Milwaukee, 3209 N. Maryland Ave., Milwaukee, WI 53211 United States

We present preliminary results of 3-D finite strain analysis across a strain gradient exposed in polymictic lapilli tuffs deformed by the Gem Lake shear zone in the eastern Sierra Nevada, California. The polymictic nature of the tuff allows us to directly examine how different materials behave during the same deformation. The presence of a finite strain gradient across the shear zone allows us to test the hypothesis that material rheology remains unchanged as finite strain increases. The polymictic lapilli tuffs in the field area were deformed by the Gem Lake shear zone at greenschist facies conditions. Field mapping and 3-D finite strain analysis demonstrates that these polymictic lapilli tuffs are more deformed in the center of the shear zone than at its periphery. Four clast types make up the lapilli in the tuff. In order from least to most deformed they are (1) very fine-grained quartz lapilli; (2) very fine-grained intermediate composition (feldspar with subsidiary quartz and oxides) lapilli lacking mica; (3) very fine-grained intermediate composition micaceous lapilli, and (4) highly altered mica-rich lapilli. Examining relative finite strain ratios between different clast types, we find that, as whole-rock finite strain increases, finite strain recorded by individual clast types does not increase proportionately. For example, the ratio of finite strain recorded by mica-rich lapilli relative to that of quartz lapilli increases as whole-rock finite strain increases. The clast type competence hierarchy outlined above appears to be valid across the strain gradient, but the range of competence displayed by the various clast types increases as whole-rock finite strain increases. Thin section analysis suggests that, as whole-rock finite strain increases, fabric development increases more quickly in lithologies recording high amounts of finite strain than in lithologies recording lesser amounts of finite strain. Consequently, we ascribe the relative weakening of intermediate and mica-rich lithologies across the strain gradient to fabric development in those lithologies. These preliminary results suggest that both composition and finite strain (fabric development) affect rheology.

Return to publications.