CONTROLS ON GNEISS DOME FORMATION: CHICKENS, EGGS AND RED HERRINGS

We consider surficial, tectonic and petrologic controls on gneiss dome formation, using examples from Pacific (accretionary)-type (Cadomian, N. Brittany, France) and collisional-type (Acadian, NH-ME, USA; Variscan, S. Brittany, France) orogens. Controls may include: erosion; lithosphere thinning/delamination/slab breakoff; buoyancy (intracrustal diapirism/escape in subduction zone roll-back); and, melting (volatile phase present and dehydration melting, volume change, rheology, etc.).

Critical to answering the question "Why do gneiss domes form?" is isolating red herrings and distinguishing chickens from eggs. P-T paths are rarely diagnostic of process, although T-t paths may discriminate exhumation mechanisms. Most gneiss domes record evidence of clockwise P-T-t paths. In some examples an erosion-controlled P-T segment may be followed by a near-isothermal decompression segment. This suggests tectonic control, such as a change from contraction to extension, or intracrustal diapirism. Increased buoyancy during melting could initiate diapirism, but this may be a red herring if tectonic controls do not allow the potential of reduced buoyancy to be realized (i.e. effects due to buoyancy are a passive response). Decompression by solid-state diapirism across the Ms-dehydration melting field will involve increased buoyancy (commonly 10-30 vol. % melt), which will enhance diapiric ascent, but this is unlikely to be a trigger for exhumation if melt is retained in the diapir. However, without diapirism, decompression melting can only occur after thinning or exhumation has begun, thus it cannot be a trigger for exhumation. In the anatectic zone, decompression melting is limited because melt mode isopleths are nearly isothermal, except close to the solidus. Thus, decompression melting also may be a red herring. Which of melting or decompression is chicken or egg is a matter of prejudice and may even be moot if dome formation is controlled by other factors. Erosion and/or tectonics (extension or collapse) may be more likely triggers for dome formation, and may even be required. Further, a large contrast in rheology strongly partitions behavior between upper and middle-lower crust, promoting detachment faulting and core complex formation; exhumation may be facilitated by softening associated with melt.