WAS THE LARAMIDE OROGENY AN EXTENSIONAL EVENT? EVIDENCE FOR EARLY TERTIARY EXTENSIONAL UNROOFING OF SOME OF THE CORDILLERAN METAMORPHIC CORE COMPLEXES

The Laramide Orogeny (~75-45 Ma) is often cited as the classic example of an Andean-type contractional orogeny produced by “flat slab” subduction. The late Cretaceous extinction of the Sierran magmatic arc at ~80 Ma, and the eastward jump of back arc shortening from the “thin skinned” Sevier Belt in Utah/southern Nevada to the “thick skinned” Laramide structures in the Colorado Rockies are widely attributed to the effects of a shallowly-dipping subducting slab. Implicit in this model is that compressive stresses were transmitted from the plate boundary deep into the continental interior and that much of the western Cordillera was in a state of shortening during the Laramide. However, numerous studies have now identified extensional structures and/or evidence for major exhumation of Laramide age in the hinterland of the Sevier Orogenic Belt. This raises the question of whether the Laramide orogeny is truly a contractional orogeny i.e. did it result in a net crustal shortening of the Cordillera, or was the Laramide shortening in the Rockies subordinate to the Laramide extensional collapse of the Sevier highlands.

New thermochronologic data and structural observations from several of the Cordilleran metamorphic core complexes (Central Mojave, Colorado River core complexes, Snake Range) clearly indicate a polyphase exhumational history for footwall rocks. Early (Laramide) exhumation was followed by a prolonged period (20-40 Ma) of isothermal residence and then a renewed period of rapid exhumation during the Miocene (20-15 Ma). The total magnitude (~300°) and rate (30-50°/m.y) of early Tertiary cooling in the core complexes is comparable to the Miocene history, and the cooling commonly shows the same spatial asymmetry. Much of the mylonitic fabrics in these core complexes is likely Laramide as well. These observations suggest: (a) the Miocene core complexes/detachment faults may in part be re-activated Laramide extensional shear zones, and (b) the total magnitude of Laramide extension in the Sevier hinterland probably greatly exceeds the rather modest (~10 km) of Laramide shortening in the foreland.