MCC FAQ (Invited Presentation)

Metamorphic core complexes (MCCs) are common structures in collapsed orogens, slow-spreading oceanic divergent zones, and extended continental margins. Decades of research by geoscientists worldwide have resulted in a wealth of information and yet questions remain about mechanisms, timescales, and broader consequences. Here, we discuss some frequently asked questions (FAQ) about MCC.

1. What is the difference between an MCC and a gneiss dome? Although many gneiss domes form during extension, some are unrelated to MCCs. However, many MCCs contain or entirely comprise gneiss domes; these include (1) migmatite-cored MCC in which partially-molten crust flowed rapidly from deep to shallow levels in extending orogenic crust and (2) MCC in which gneissic rocks are domed beneath a detachment fault that developed in initially thinner crust.
2. How much footwall exhumation is accomplished by detachments? In MCCs with low-magnitude exhumation, displacement on the detachment (brittle and ductile) may account for most of the exhumation. In migmatite-cored MCCs, exhumation by displacement on the detachment is significantly less and much of the exhumation occurs during upward flow of deep crust.
3. What is the role of processes other than extension? Buoyancy contributes to ascent of low-density crust, although numerical models indicate it is a 2^nd order factor. Crustal flow fills gaps created by extension; in the absence of sufficient extension, upward flow is accompanied by downward flow, as seen in Archean terrains but rarely in younger systems. The separation of melt fraction and ascent of granite bodies may be a contributing factor in MCCs, but deep crust generally moves en masse with only minor magmatic differentiation.
4. Are continental margin MCCs common and are they similar to other CC? Recent research has increased recognition of continental margin CCs and has shown that ductile processes driven either by warm thermal regimes (resulting in crustal flow) or hydration (e.g. serpentinization) are associated with CC formation at rifted margins.
5. What is the significance of MCC for lithospheric evolution? MCCs form by movement of material from deep to shallow levels and their development is therefore important for heat transfer, element/volatile cycles, and creation/stabilization of continental and oceanic crust.