HR: 16:45h
AN: V14A-04    [Abstracts]
TI: One View of Dynamic Crustal Rheology During Orogenesis
AU: * Brown, M
EM: mbrown@geol.umd.edu
AF: Univ. Maryland, Lab. for Crustal Petrology, Dept. of Geology, College Park, MD 20742 United States
AB: Orogenic systems control the mechanics of interactions between converging plates, so it is important to understand weakening and hardening mechanisms in relation to the dynamics of orogenesis. Rheology of the ductile crust is influenced by bulk composition and orogenic architecture, thermal profile and enthalpy input, fluid distribution and strain rate. These parameters vary spatially and temporally within and among orogenic systems; retrogression and weakening is only likely if ingress of fluids is allowed. What about the jelly sandwich? Fertile crustal rocks (fresh bread) undergo melting in deeper parts of orogens (bread soaked with jelly), so that during anatexis the solidus becomes a significant additional controlling parameter. The solidus varies with composition and changes position during the dynamic evolution of the thermal structure of an orogen. Important properties of melting systems are viscosity of the melt, rheology of the crystalline framework of grains and permeability of this framework. Permeability is due to an intergranular network of connected pores, compositional layering/fabric and networks of deformation bands; melt distribution is heterogeneous on multiple length scales. Melt segregation and extraction from the source, and ascent and emplacement at a shallow structural level leads to a residual source (dry bread) and melt-rich horizons in sub-solidus crust (jelly puddles). Deformation experiments indicate a dramatic drop in rock strength (to 100-200 MPa) as the melt connectivity transition is approached at 7 vol.% melt, and a more gradual decrease to <1 MPa prior to the drop at the solid-to-liquid transition. The microstructure of anatectic rocks and the magnitude of weakening accompanying melting suggest a limited role for intracrystalline plasticity with increasing vol.% melt and dominance of melt-assisted diffusion creep or diffusion accommodated granular flow. Multiple geophysical datasets are interpreted to indicate >6 but <20 vol.% interconnected melt in deep crust of active orogenic systems, and, based on numerical models, plateau formation and channel flow require a decrease in viscosity consistent with melt volume at the lower end of this range and the resultant weakening. Greywackes and metapelites are the most fertile crustal protoliths, generating 20-50 and 30 vol.% melt respectively at 1 GPa and 1173K. These data suggest that melt is drained from the source during progressive melting, advecting heat to shallower levels in the crust. Field studies of exhumed orogens provide ample evidence of dynamic crustal rheology. These studies show that deformation commonly is laterally, transversely and vertically diachronous, reflecting the spatial and temporal variation in the weakening-to-strengthening cycle as fertile crust melts, melt is drained and the residual source cools. Residual migmatitic granulites show preservation of early fabrics, suggesting that the strain field emergent under subsolidus conditions controlled initial distribution of melt produced by suprasolidus mica breakdown. Studies demonstrate that melt migrates from grain boundaries to mesoscale networks of structures (mm to m) to steeply-inclined conduits (m to dm) that may mimic the apparent strain ellipsoid for syn-deformation melt ascent or be discordant for syn-exhumation melt ascent. Melt loss from lower crust yields residual rocks composed of strong minerals (feldspar, pyroxene and garnet) with only minor melt on grain boundaries. Thus, weakening of lower crust due to melting is followed by its strengthening. Around the brittle-to-viscous transition zone granite accumulates in sub-horizontal tabular plutons, which implies transient presence of significantly weaker layers in shallow orogenic crust; these also are potential detachment horizons.
DE: 3619 Magma genesis and partial melting (1037)
DE: 3660 Metamorphic petrology
DE: 8031 Rheology: crust and lithosphere (8159)
DE: 8110 Continental tectonics: general (0905)
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005