THE ROLE OF METAMORPHIC FABRIC IN CONTROLLING MID-CRUSTAL SEISMOGENIC RUPTURE AND UPPER CRUSTAL BRITTLE REACTIVATION IN TWO COLORADO SHEAR ZONES: AN EXAMPLE OF PERMANENT LITHOSPHERIC WEAKENING

The recognition of the tectonic ancestry of the visually striking, sinuous trace of the Appalachian-Ouachita chain by W. A. Thomas (1977; Am. J. Sci.) has led to a renewed interest in tectonic inheritance during the past 30 years. The Appalachian-Ouachita model implies that crustal- and lithospheric-scale basement weaknesses are permanent fabrics controlling the geometry of subsequent orogenic belts, and to a lesser extent rifts, through numerous Wilson cycles. Detailed study of the origin and evolution of deeply exhumed, long-lived lithospheric fracture zones is therefore important to understanding the geodynamics of fault reactivation and modern seismicity below thick orogenic piles and buried continental margins.

In this presentation, we compare and contrast the mid-crustal maturation and upper-crustal reactivation of the Grizzly Creek (GCSZ) and Homestake (HSZ) shear zones in central Colorado. The metamorphic fabric of both shear zones was engraved as a mold for Phanerozoic brittle reactivation by ~1.4 Ga following a complex Paleoproterozoic history, which possibly involved the entire lithosphere for the HSZ. At ~1.4 Ga, composite foliation in the HSZ was transposed into a NE-trending, subvertical belt with complex dip- and oblique-slip in response to regional transpression; in contrast, the GCSZ developed as a N-NW-dipping, S-vergent thrust 55 km NW of the HSZ. Both zones are characterized by the presence of extensive pseudotachylyte systems and spatially isolated mylonite belts. Mutually overprinting brittle and plastic fabrics, including mylonitized pseudotachylyte, suggest a mid-crustal setting where coseismic rupture was coupled to aseismic plastic creep in the mylonite belts. In both shear zones, transposed foliation controlled the geometry, kinematics, and dynamic propagation of brittle ruptures. Both were reactivated under upper-crustal conditions in the Phanerozoic; new brittle faults reactivated the Mesoproterozoic foliation in preference to the ancient fault rocks. In addition, both shear zones mimicked Mesoproterozoic kinematics during Laramide compression, despite an unfavorable, sub-perpendicular orientation to the regional stress field. The shear zones therefore serve as a model for fabric control on localization of seismicity and brittle faulting through time.