Reading the Record of Ancient Earthquakes At Three Levels in the Crust: Insights From Greenschist-, Amphibolite- and Eclogite-Facies Pseudotachylytes
Pseudotachylyte – glassy rock representing frictional melt – is considered by many geologists to be the only unambiguous indicator of ancient earthquakes. Because its high temperature of formation requires frictional contact between rock surfaces, the presence of fluids in fault zones has been thought to suppress its formation since these fluids would become thermally pressurized during a seismic event and thus reduce the frictional resistance along the slip surface. Natural exposures of pseudotachylyte representing three different levels of the crust reveal that the interplay between fluids and pseudotachylyte formation is in fact more complex. Pseudotachylytes formed at eclogite-facies conditions (ca. 50 km depth) in very dry granulite-facies rocks in the Scandinavian Caledonides in western Norway reveal how earthquakes may provide conduits for aqueous fluids and trigger overstepped metamorphic reactions which in turn change rock rheology. Amphibolite-facies pseudotachylytes in an early Proterozoic fault zone in northern Wisconsin show mutually cross-cutting relationships with both mylonites and quartz-hornblende veins, indicating that alternating episodes of plastic deformation, seismic slip, fracture, and high-temperature fluid flow occurred in the middle crust (ca. 20 km). Greenschist-facies pseudotachylytes and cataclasites along a Cretaceous low-angle normal fault in turbidites on New Zealand’s South Island show that frictional melting can occur even in very hydrous rocks in the upper crust (ca. 10 km) if fracture networks allow fluids to escape the fault zone over the timescale of a seismic event. Collectively, these observations may aid in the interpretation of real-time seismic records and perhaps contribute to seismic hazard assessments in different modern tectonic settings.
Technical Talk: A Geologic Mystery at Brussels Hill, Wisconsin
Brussels Hill in southern Door County, WI is a localized area of pervasively fractured and faulted bedrock in a region of otherwise undeformed lower Silurian dolostone. It has been provisionally interpreted as an eroded impact crater. The area of brecciated bedrock coincides with a prominent flat-topped hill, about 3 km in diameter, that stands 40 m above the surrounding landscape and is capped by glacial till. A gravity survey revealed a small but significant positive Bouguer gravity anomaly near the center of the disturbed zone, pointing to the presence of denser rocks at depth that compensate for the low density of breccias near the surface. Anomalous bodies of glauconite-bearing sandstone within the brecciated dolostone are likely derived from Cambrian strata normally found 400 m deeper in the subsurface in the area. A 103-m drill core into the center of the disturbance was obtained with funding from NASA. The core includes brecciated dolostone to depths of 70 m, much of it vesicular, but with no features diagnostic of shock metamorphism. The lowest 33 m sampled nearly undeformed Ordovician Maquoketa Fm, which is surprising given that materials from greater depth were brought up through this level. While Brussels Hill may have been the site of supersonic impact, an endogenous process such as violent fragmentation in the upper part of a diatreme cannot be ruled out.