HR: 10:35h
AN: V41D-02    [PDF]
TI: Space-Time Variations in Lower Crustal Rheology and Thermal Structure and its Effects on the Evolution of Arc Crust
AU: * Klepeis, K A
EM: kklepeis@uvm.edu
AF: University of Vermont, Department of Geology, Burlington, VT 05405 United States
AU: Rushmer, T
EM: trushmer@uvm.edu
AF: University of Vermont, Department of Geology, Burlington, VT 05405 United States
AU: Gehrels, G
EM: ggehrels@geo.arizona.edu
AF: University of Arizona, Department of Geosciences, Tucson, AZ 85721 United States
AB: Studies of magmatic arcs in different settings have shown that the rheology and thermal structure of the lower crust are extremely heterogeneous and can change quickly during cycles of magmatism and crustal melting. However, determining the length and time scales of these variations, and their effects on crustal evolution are especially problematic. We present results from a deeply eroded Cretaceous arc exposed in Fiordland, New Zealand that reveal the presence of steep thermal gradients and rheological contrasts in the lower crust ($\sim$45 km paleodepth)on length scales of $\sim$100 km$^{2}$. Metamorphic and geochronologic data collected from near Milford Sound suggest that part of the lower crust experienced rapid cooling from 750$\deg$C$<$T$<$850$\deg$C to T$=$650-700$\deg$C following a period (126-120 Ma) of mafic-intermediate magmatism and lower crustal melting. Cooling was aided by the thrusting of relatively cool pre-existing crust below hot, partially molten new crust and by the efficient extraction of melts out of the lower crust. Simple thermal models of conductive heat loss suggest that the thermal pulse accompanying magmatism near Milford Sound was short-lived (a 3-4 m.y. duration)and had decayed to temperatures of T$=$650-700$\deg$C by $\sim$116 Ma. These models are in accordance with metamorphic ages obtained using U-Pb dates on zircon from pegmatite and tonalite dikes. However, less than 100 km to the south of Milford Sound, U-Pb zircon ages and metamorphic data suggest that lower crustal temperatures there remained at T$=$800$\deg$C until $\sim$108 Ma. These observations indicate that the thermal structure and rheological transitions linked to magmatism and the partial melting of lower crust were much more spatially heterogeneous and transient than previously believed. These variations created localized hot spots in the lower crust that affected deformation patterns during cycles of extension and contraction within the arc. Hot weak zones preferentially developed large (1 km thick) extensional shear zones that thinned the lower crust at $\sim$108 Ma. Structural relationships and geochronology indicate that these lower crustal shear zones formed simultaneously with extensional core complexes and narrow rift basins at mid-upper crustal levels. In contrast areas that were relatively cool at $\sim$108 Ma lack extensional structures and preserve older contractional shear zones that thickened the crust. The Fiordland setting shows that the development of localized hot spots in the lower crust strongly influences deformation partitioning within arc crust. This process helps explain along-strike variability in arc structure, including the development of highly localized zones of extension in the upper crust.
DE: 1035 Geochronology
DE: 1749 Volcanology, geochemistry, and petrology
DE: 7205 Continental crust (1242)
DE: 8110 Continental tectonics--general (0905)
DE: 8160 Rheology--general
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting