TRACKING THE ALONG-STRIKE DEVELOPMENT OF NORMAL-FAULT SYSTEMS: A KEY TO UNDERSTANDING PROCESSES OF LARGE-MAGNITUDE EXTENSION

Ernie Anderson's pioneering work on large-magnitude extension in the Colorado River extensional corridor (CREC) helped inspire a revolution in structure-tectonics that involved reevaluation of fault theory, development of models for low-angle normal faults, and scrutiny of relations between magmatism and extension. Today, detachment faults are widely recognized, but their origin and evolution remain controversial. Major conundrums concern how domains of high-angle normal faults evolve into metamorphic core complexes and how magmatism affects extension. Tracking fault systems along strike from their tips in accommodation zones (AZ's) to their culminations in core complexes offers insight into these questions.

In the CREC volcanic centers are more common in AZ's, as opposed to core complexes, suggesting that voluminous magmatism inhibits development of major normal fault systems. Emplacement of batholiths at depth probably did not induce upper crustal extension. In AZ's, young undeformed plutons that lack preexisting weaknesses may serve as rupture barriers to propagating normal faults while diking accommodates extension.

An elevated brittle-ductile transition (BDT), abrupt strain gradient between the CREC and Colorado Plateau (CP), and developing mid-crustal anisotropies probably conspired to produce core complexes and detachments in the CREC. Voluminous magmatism began 1 to 4 m.y. prior to extension and peppered the crust with abundant subhorizontal intrusions. This probably elevated the BDT. As extension began, a pervasive subhorizontal coaxial fabric formed in the warmed, inflated crust. Severe extension and magmatism elevated the BDT to much shallower levels beneath the CREC compared to the unextended CP. This warped the original subhorizontal fabric to gentle E-dips between the CREC and CP, as evident in reflection profiles. At the onset of extension, high-angle upper-crustal normal faults probably soled into an innocuous BDT. As extension accelerated, some E-dipping faults tapped into the rising welt of mid-crust, exploited the E-dipping fabric, and evolved into major detachments. Paleomagnetic data indicate, however, that negligible vertical-axis rotations accompanied severe displacement gradients on detachments, which challenges concepts of large upper-plate allochthons.