SOME PUZZLING OBSERVATIONS AND A NEW TAKE ON EXTENSIONAL METAMORPHIC CORE COMPLEX (MCC) DETACHMENT FAULTS

MCC detachment faults have intrigued, obsessed and even divided the structural geology community since they were first defined as extension-related and Cenozoic in age across western U.S. Many models have been proposed for their genesis on land and in the oceans, ranging from their origin as gently-dipping, large-offset, mostly brittle faults, to high angle faults tilted to flat, to faults that roll from steep to flat, to surfaces that represent extensional ductile-brittle transition zones and/or the tops of extending, granite cored gneiss domes. Most of these models have merit and fit observations of various detachment faults mapped in MCC’s.

A variant of these models is proposed here: A MCC detachment fault may represent the rising interface between extensional ductile deformation below and brittle above because of heat added to the upper crust during extension (by magmatism and/or a vertical component of deeper crustal flow). Brittle regime normal faults can become ductilely deformed and become part of the lower plate. The horizon where rising extensional ductile deformation “freezes” becomes the MCC detachment fault.

Two examples of normal faulted sections subsequently deformed (and now considered lower plate) are mapped in the Raft River Mts., UT-ID, and Funeral Mts, Death Valley, CA. In the Rafts, a significant section-omitting fault, mapped by Compton (1972) and named the Mahogany Peaks fault by Wells et al. (1998), post-dates Mesozoic metamorphism and is isoclinally folded with an axial planar fabric developed during intrusion of the 30 Ma Almo granitic complex. The Chloride Cliff fault, Funeral Mts., omits section, but both its hanging and footwall are deformed by the youngest lower plate fabric beneath the Boundary Canyon detachment. In both places, it is hard to find fault-related gouge developed along the overlying detachment faults; their final exhumation is due to motion on younger brittle faults.

Better substantiation of these observations and what they imply for the genesis of MCC faults include study of: 1. faults that omit stratigraphy and then are ductilely deformed during continued extension 2. metamorphosed fault zone rocks 3. overlying detachment faults that are more a transition zone between ductile to brittle behavior rather than a sharp fault with gouge 4. closely spaced thermochronology transects with no thermal breaks.