Geometric Complexity of Earthquake Rupture Surfaces Preserved in Pseudotachylyte Networks

Recent earthquakes have demonstrated that rupture may propagate through geometrically complex networks of faults. Ancient exhumed faults have the potential to reveal the details of complex rupture at seismogenic depths. We present a new set of field observational criteria for determining which of a population of pseudotachylyte fault veins formed in the same earthquake and apply it to map rupture networks representing single earthquakes. An exceptional exposure of an exhumed ancient strand of the Norumbega Shear Zone preserves evidence of multistranded earthquake rupture in the deep seismogenic zone of a continental transform fault. Individual fault strands slipped at least 2-18cm, so significant slip is represented by each rupture network. Our data show that synchronously slipped faults intersect at angles of 0 to approximate to 55 degrees, with the opening angles of fault intersections directed toward the dilational quadrants for dextral slip. Multistranded rupture on a fault network instead of rupture of a single fault may result in greater and/or more variable slip and cause slip rake to vary spatially and temporally. Slip on intersecting faults unequivocally means that there will be motion perpendicular to the average fault plane. Modern earthquakes displaying non-double-couple components to focal mechanism solutions and spatially varying rake, slip, and anomalous stress drop may be explained by rupture across fault networks that are too close (spatially and temporally) to be resolved seismically as separate events. Earthquake faults are often treated as simple planar features, but some recent earthquakes have shown that networks of faults can rupture together and produce very complicated patterns of deformation. We found an exposure of an ancient fault zone in Maine, USA, which preserves the pattern of ancient ruptures in the form of fossil earthquake slip surfaces. We mapped the networks of faults that slipped in individual earthquakes to show the geometry of complex earthquake ruptures at middle-crustal depths and predict what seismologists would observe if similar earthquakes are happening today.