EXTENSION AND MAGMATISM IN THE NORTHERN COLORADO RIVER CORRIDOR: ERNIE'S CONUNDRUMS REVISITED

The pioneering work of Ernie Anderson in the Eldorado Mountains (EM) of southern Nevada stimulated an important and ongoing debate on how continental crust extends. His description of closely spaced, highly-rotated normal faults and his interpretation that they were the consequence of rapid, large magnitude extension were at odds with prevailing views and led to a re-evaluation of Basin and Range extensional tectonics. Subsequent studies in the EM have refined our understanding of the structural and magmatic evolution, but have not changed the fundamental observations, and, if anything, have added to the conundrums that were raised by Ernie's work. These include: 1. Why are the normal faults so closely spaced compared to typical “basin-range” faults? 2. Why did the faults initiate as subvertical fractures, and how did a single generation of faults rotate to such low angles? 3. How was large-magnitude supracrustal extension accommodated at depth? 4. What was the cause of large local variations in the magnitude of upper crustal strain? 5. What triggered the abrupt inception and termination of extension? 6. What was the relationship between extension and magmatic activity? Why was volcanic activity supressed during rapid extension? 7. Do the EM represent a unique style of extension that differs from both “core complexes ” and typical “basin-range” styles? A large caldera complex and long-lived pre- to syn-extensional volcanic centers within the CREC suggests that the presence of magmas at depth played a role in localizing extension and may have thinned the brittle lid. Extension in the EM began at ~15.0 Ma immediately following the peak of volcanism and the area was then stretched by > 100% between 15.0 and 14.1 Ma, during which time volcanic activity virtually ceased. Plate tectonic spreading rates (~ 1 cm/yr in the EM alone) and the northward migration of extension and magmatism within the CREC suggests it represents a failed propagating rift. If extension is rapid, simple mechanical considerations indicate faults should become progressively weaker and can rotate to angles as low as 15° before new tensile fractures develop. The antithetic relationship between eruptive rates and strain rates may reflect enhanced crystallization of mid-crustal magmas during extension. Other conundrums persist.