Abstract
Abstract. The formation and exhumation of high and ultra-high-pressure, (U)HP, rocks of crustal origin appears to be ubiquitous during Phanerozoic plate subduction and continental collision events. Exhumation of (U)HP material has been shown in some orogens to have occurred only once, during a single short-lived event; in other cases exhumation appears to have occurred multiple discrete times or during a single, long-lived, protracted event. It is becoming increasingly clear that no single exhumation mechanism dominates in any particular tectonic environment, and the mechanism may change in time and space within the same subduction zone. Subduction zone style and internal force balance change in both time and space, responding to changes in width, steepness, composition of subducting material and velocity of subduction. In order for continental crust, which is relatively buoyant compared to the mantle even when metamorphosed to (U)HP assemblages, to be subducted to (U)HP conditions, it must remain attached to a stronger and denser substrate. Buoyancy and external tectonic forces drive exhumation, although the changing spatial and temporal dominance of different driving forces still remains unclear. Exhumation may involve whole-scale detachment of the terrane from the subducting slab followed by exhumation within a subduction channel (perhaps during continued subduction) or a reversal in motion of the entire plate (eduction) following the removal of a lower part of the subducting slab. Weakening mechanisms that may be responsible for the detachment of deeply subducted crust from its stronger, denser substrate include strain weakening, hydration, melting, grain size reduction and the development of foliation. These may act locally to form narrow high-strain shear zones separating stronger, less-strained crust or may act on the bulk of the subducted material, allowing whole-scale flow. Metamorphic reactions, metastability and the composition of the subducted crust all affect buoyancy and overall strength. Future research directions include identifying temporal and spatial changes in exhumation mechanisms within different tectonic environments, and determining the factors that influence those changes.
Highlights
Global exposures of high- and ultra-high-pressure (HP and UHP) metamorphic rocks mark regions where crustal materials were subducted to mantle depths and exhumed back to the surface at some point in the geological past (Chopin, 1984; Smith, 1984; Ernst et al, 1997)
Rates estimated from the difference between the timing of peak metamorphism and constraints on the cooling path provided by these chronometers vary considerably, from rates on the order of 3–8 cm a−1 estimated for the Dora Maira and Kaghan terranes (Alps and Himalaya; Rubatto and Hermann, 2001; Parrish et al, 2006), with much slower rates, < 1 cm a−1, estimated for the Western Gneiss Region (Kylander-Clark et al, 2008)
More work is still needed to determine which weakening processes dominate at different times and depths within subduction zones, and the amount by which they weaken the crust
Summary
Global exposures of high- and ultra-high-pressure (HP and UHP) metamorphic rocks mark regions where crustal materials were subducted to mantle depths and exhumed back to the surface at some point in the geological past (Chopin, 1984; Smith, 1984; Ernst et al, 1997). These rocks provide insight into the long-term dynamics and physical conditions inside subduction and continental collision zones, insight which is not directly measurable or determinable from real-time geophysical data. A discussion about how these different exhumation drivers are accommodated in different tectonic environments, and how the drivers may change within those environments in both space and time, is presented
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
