AN INVESTIGATION OF STRAIN PARTITIONING IN THE PROTEROZOIC TWILIGHT GNEISS, WEST NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO

The ca. 1760 Ma Twilight Gneiss is a major component of the uplifted basement of the West Needle Mountains. This complex consists of interlayered metamorphosed dioritic and gabbroic plutonic rocks that record a complex history of strain related to Proterozoic ductile deformation. General trends of deformational fabrics and structures record NW-SE compression and NE-SW extension, but detailed work on the strain history of these rocks has not been done.

The purpose of this study is to quantify the magnitude and orientation of strain fields in the Twilight Gneiss in order to test the relationship between strain accumulation, rock type, and proximity to high-strain shear zones. This will be used to establish the spatial partitioning of strain accumulation. This information is important in unraveling the tectonic regime and deformational history of these rocks.

Strain values were determined in part by measuring the orientations and geometries of 79 mafic xenoliths in dioritic gneiss at 10 field stations. Averaged xenolith measurements were then used to create a finite strain ellipse for each field station. Strain fields in these rocks are also being evaluated through application of the Fry technique on deformed relict phenocrysts in 11 samples of gneiss.

Preliminary strain data from mafic xenoliths show NE-SW elongation ranging between 99%-230%, and NW-SE shortening ranging between 24%-71%. This data, combined with the data obtained from deformed crystal analysis, will create a spatially representative model of strain accumulation. My complete presentation of the data collected in this ongoing investigation will establish the working hypotheses that are developed from this research.