Joseph L. Allen , Concord University
This project focused on two related studies of fault dynamics along the margin of a hydrocarbon-bearing basin. The goal of this research was to examine the deep structure of an exhumed fault zone and document the geometry and dynamics of early fault development as compared to: (1) the pre-existing structure of mid- to lower-crustal metamorphic fabrics, and (2) the geometry of overprinting brittle faults formed during younger reactivation. The study area is within the southern margin of the Laramide White River uplift in the Rocky Mountain foreland of Colorado. The uplift is a south- to southeast-dipping monocline associated with the east- to northeast-striking Grizzly Creek fault (GCF). The GCF cuts Proterozoic basement and Upper Cambrian through Lower Pennsylvanian strata, and was generated by reactivation of the Proterozoic Grizzly Creek shear zone (GCSZ); a kilometer-thick zone of north-dipping metamorphic foliation, mylonite, and pseudotachylyte. The scientific conclusions are twofold:
1. This study presented the first field and microstructural description of the newly discovered Grizzly Creek shear zone, and concluded that preexisting anisotropy in the Precambrian basement exerted a significant control on earthquake rupture propagation during deformation at mid-crustal depths. The GCSZ developed in amphibolite-grade supracrustal gneisses and granitoids, and is defined as a 0.4- to 0.7-km-wide high-strain zone consisting of a basal mylonite with top-to-south kinematics overlain by mylonitic rocks and transposed foliation oriented 256/51˚NW. The GCSZ is overprinted by hundreds of veins of pseudotachylyte, mylonitic pseudotachylyte, and ultramylonite. Field observations indicate that pseudotachylyte fault veins formed as a result of first-generation rupture through intact rock. Pseudotachylytes are preferentially localized in as many as nine decameter-scale rupture zones dispersed across the width of the shear zone, concordant to foliation. This work generated a conceptual model for the asymmetric development of anisotropic fabric in a thrust-related fault zone in crystalline metamorphic rocks. In this model, progressive tectonic exhumation of hanging wall rocks during thrusting resulted in the development of anisotropic fabric that provided a preferentially weakened zone that accommodated the propagation of ruptures from the seismogenic zone into the middle crust. This early, dynamically generated fabric was subsequently reactivated ~1.5 billion years later during Laramide deformation to form the boundary between a Precambrian uplift and a hydrocarbon-bearing sedimentary basin (Piceance basin).
2. Geologic mapping in the Glenwood Springs and Shoshone 7.5 minute quadrangles redefined the attitude of the GCF and identified the GCSZ as precursor structure that produced fault-related folds during brittle reactivation. In the canyon of Grizzly Creek, a deeply incised tributary of the Colorado River at Glenwood Canyon, the GCF is a south-vergent reverse fault oriented 260/46˚ N with ~200 m of stratigraphic separation. Separation diminishes 2 km to the west of Grizzly Creek where the GCF steepens and dies out into a monocline. The GCF generated an overturned footwall syncline in cover strata, and a gentle hanging wall anticline that is locally broken by a steep splay rooted in the GCF. Along the eastern wall of Grizzly Creek canyon south of the GCF, a west-dipping monocline related to an east-dipping, basement-involved reverse fault strikes ~185˚. The structure is truncated by the GCF and refolded into a large-scale, type 2 superimposed fold in the GCF footwall syncline. We interpret the southern margin of the White River uplift to be a product of Laramide strain partitioning in a compressive stress regime. In this interpretation, deformation initially generated a north-striking, west-directed reverse fault and monocline. During compression, the Proterozoic GCSZ provided an anisotropic zone of weakness that caused deformation to localize along an east-northeast trend. Initially, the footwall of the GCSZ failed along south-dipping lithologic fabrics creating a local north-vergent reverse fault, which was subsequently cut as the GCF developed.
Student and Departmental Impacts:
This project has funded 25 unique students either directly or indirectly. The PRF project resulted in five undergraduate research projects and several short-term field assistantships; it was also used for educational purposes for 19 undergraduates who visited and worked at the field site in 2007, 2009, and 2011 as part of a summer field geology course funded by our university (partial travel funds were provided through this PRF grant). Most of the undergraduate participants were low-income or first-generation college students, and one was an underrepresented minority who was funded by a SUMR fellowship. Participation in the program has resulted in a 100% retention rate; all students who participated either directly or indirectly in the research have either graduated with a B.S. degree, or are still enrolled. Of the 25 students participating in the project 20 have graduated to date and 5 are still enrolled. Of the 20 graduates, 35% are now enrolled in graduate school or have completed a graduate degree, 40% are engaged in petroleum or hydrocarbon-related employment, and the remaining 25% are employed in other industries. More than half of the students in graduate programs are working on theses with relevance to petroleum applications.
This project helped expand shared laboratory infrastructure at Concord University and the acquisition of further external funding (>$375,000) for research and equipment. The nature of the analytical work required for the project informed and justified the acquisition of an electron microprobe, Raman spectrometer, and a micro-X-ray fluorescence spectrometer; the PI obtained the instruments aided by external grants, and secured supplemental external funding to operate the laboratories and hire a full-time Research Assistant Professor. The facilities are now widely used by undergraduate students from disciplines throughout the physical sciences. A short vignette describing this impact was published Fall, 2010 in the Council on Undergraduate Research Quarterly. At this time of this report, further proposals to expand infrastructure are in progress.