Abstract. The Byrd Glacier discontinuity is a major tectonic boundary crossing the Ross Orogen, with crystalline rocks to the north and primarily sedimentary rocks to the south. Most models for the tectonic development of the Ross Orogen in the central Transantarctic Mountains consist of two-dimensional transects across the belt, but do not address the major longitudinal contrast at Byrd Glacier. This paper presents a tectonic model centering on the Byrd Glacier discontinuity. Rifting in the Neoproterozoic produced a crustal promontory in the craton margin to the north of Byrd Glacier. Oblique convergence of a terrane (Beardmore microcontinent) during the latest Neoproterozoic and Early Cambrian was accompanied by subduction along the craton margin of East Antarctica. New data presented herein in support of this hypothesis are U-Pb dates of 545.7 ±6.8 Ma and 531.0 ±7.5 Ma on plutonic rocks from the Britannia Range, directly north of Byrd Glacier. After docking of the terrane, subduction stepped out, and Byrd Group was deposited during the Atdabanian-Botomian across the inner margin of the terrane. Beginning in the upper Botomian, reactivation of the sutured boundaries of the terrane resulted in an outpouring of clastic sediment and folding and faulting of the Byrd Group.

Formation and Douglas Conglomerate with Contortion Schist. These correlations and the lack of equivalent lithologies to the north of Byrd Glacier imply that Byrd Glacier is the northern boundary of the Byrd depositional basin, and reinforces the notion that it marks a major tectonic discontinuity crossing the Ross Orogen.

A paleoseismic study conducted along the Cholame segment of the San Andreas fault provides evidence for three earthquakes and the amount of lateral offset for the most recent event (1857 Fort Tejon earthquake). Excavations at the Las Yeguas (LY4) site include five fault-perpendicular, two parallel, and several handdug trenches. Abruptly truncated sand and silt layers that are not correlative across the fault zone constrain the timing of the penultimate event (L2) between cal. A.D. 1030-1300 and 1390-1460. Vertical offset, shearing, and fracturing of silty sand and gravel units that overlie L2 and historical artifacts that bracket the timing of the MRE (L1) provide evidence that the most recent ground-rupturing event, L1, occurred between cal. A.D. 1390-1460 and ϳ1865. L1 is likely the 1857 Fort Tejon earthquake. Tectonic silt-filled fractures that dissect historic gray-tan silt and sand suggest a ground shaking or a triggered slip event (L0), which occurred after L1. Threedimensional excavation of an alluvial fan edge (unit 4) indicates that 3.0 ‫ע‬ 0.70 m of near-fault brittle slip occurred during the 1857 earthquake at this site.

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