Melt Weakening in Crustal-Scale Tectonic Wedging, Southern New England

Partial melting had a critical role in the tectonic wedging of the Gander zone crust by the Avalon terrane in southern New England. Our understanding of this process comes from the recognition (using Nd and Pb isotopes) that both Gander zone and the Avalon zone rocks are present in coastal southern New England. The new tectonic image is one of Neoproterozoic Avalonian crust sandwiched between early Paleozoic upper and Neoproterozoic lower Gander zone crustal blocks. Ductile faults several 100s m thick isolate the Avalon block from the Gander zone gneisses above and below. The faults were texturally softening by the concentration and alignment of phyllosilicates in mylonitic schists and gneisses, but the lower boundary was also softened by the generation of a series of migmatitic liquids. These liquids are now exposed as (1) older foliated migmatites that define the ductile fault zones, and (2) less well-foliated concordant and discordant granitic and pegmatitic dikes in and near these ductile faults. U-Pb SHRIMP ages of zircons from ~290 to 265 Ma date the generation of these melts over a 25 m.y. time span. One-dimensional thermal modeling suggests that this melting began at H2O-saturated minimum-melt temperatures in a crust over-thickened to >50 km. Decompression melting along a clockwise P-T-t path generated new melts as reaction boundaries of vapor-absent muscovite- and biotite-melting were crossed. At the onset of wedging in the late Carboniferous, the cooler Avalon block was much stronger than the partially melted Gander zone rocks. The presence of migmatitic liquids at lithostatic pressure apparently lowered the effective stress in the fault zone, thus lubricating the lower boundary of the Avalon block, and increasing the ductility contrast that allowed the Avalon block to wedge into the Gander zone.