Multi-surface Earthquake Rupture Recorded in Pseudotachylyte Vein Geometries, Norumbega Shear Zone, southern Maine

Abstract:

Earthquake rupture surfaces are typically treated as single rupture planes. However, the observation of four linked, non-parallel to sub-parallel slip surfaces on a mining induced earthquake in 2004 shows that rupture geometries may be more complicated (Heesakkers et al., 2011). Multiple pseudotachylyte-bearing fault surfaces are exposed within a 1.1 km wide mylonite zone of the Paleozoic Norumbega fault system. The pseudotachylytes are present in two juxtaposed mylonite zones: the Ray Corner mylonite and a mylonite derived from Scarboro Formation metavolcanics. The Ray Corner mylonite crosscuts pelitic schists of the Cape Elizabeth Formation, at upper greenschist-facies conditions (quartz + feldspar + chlorite + muscovite ± titanite ± pyrite). The pseudotachylyte veins formed late in the deformational history, during a period of predominantly brittle dextral offset. The pseudotachylytes are cryptocrystalline and have rounded porphyroclasts of quartz and feldspar. Microstructural observations show evidence for static and dynamic recrystallization overprinting the primary quench textures, suggesting that previous generations of rupture surfaces have been recycled into the mylonitic fabric (Price et al., 2012). Many of the pseudotachylyte veins have a sharp boundary on one side and are poorly defined on the other, providing insight to the propagation direction. This confirms that the paleo-earthquake ruptures occurred at conditions where quartz and feldspar were able to deform plastically, near the base of the seismogenic zone.

Using differential GPS, we mapped the geometry of pseudotachylyte fault veins, injection veins, and slip surface intersections. At Ray Corner, there are 7 layer-parallel pseudotachylytes in a 4 m wide zone with linking and subsequent oblique pseudotachylytes. Some intersections between pseudotachylytes are dilational, depending on the intersection angle and relative displacement on the two faults. At these sites, pseudotachylyte melt sourced from the two intersecting surfaces fills the zone, and displays swirled compositional banding indicative of melt mixing. As the quench time of thin pseudotachylyte melts is on the order of ≤ 1 s, the two intersecting faults must have ruptured concurrently in order to allow melt mixing between them.