~285 km Since Before 11 Ma Vs. ~30 km Since ~3 Ma: The Hayward-Calaveras Fault Outranks a Part of the San Andreas Fault in Both Age and Offset Distance

Burnham, Kathleen

In depth analysis of correlations between fault lines within the San Andreas-Calaveras-Hayward-Rodgers Creek-Maacama fault as well as the San Gregorio and Pilarcitos faults with map-view / paleogeographic reconstructions. Appendix provides geological locality features correlated to faults discussed in the paper. Bibliography of cited references. A predictive model of the late Cretaceous-Oligocene (~70-30 Ma) positions of the tectonic blocks of the San Andreas fault system is followed herein by refined tentative preliminary sequential paleogeographic reconstructions from ~30 Ma to ~5 Ma. These models juxtapose nearly 75 previously published pairs of correlative features. Eight pairs of correlative features reported by four sets of researchers indicate only ~30 km total lateral offset across the section of the San Andreas fault between its intersecitons with the Pilarcitos an dSan Gregorio faults, and only ~30 km post-Cretaceous lateral offset across the San Andreas fault section between its intersections with the Pilarcitos and Calaveras faults. The Cretaceous-Pleistocene ages of these correlatives indicate dextral offset of this section of the San Andreas fault was not initiated until less than 3-4 Ma. The ~315 km offset of the San Andreas fault at Pinnacles and Neenach volcanics is partitioned between ~30 km offset of the San Andreas fault north of the Calaveras fault intersection, and ~285 km offset of the San Andreas-Calaveras-Hayward-Rodgers Creek-Maacama fault. Offset distance of the Calaveras-Hayward-Rodgers Creek-Maacama-fault leg of this partition is verified by paleogeographic juxtaposition of 130 Ma Skaggs Springs Schist near Healsdburg and Pacheco Pass. Dextral offset of the San Andreas-Calaveras-Hayward-Rodgers Creek-Maacama fault commenced after ~21.3 Ma, and had slipped at least 77 km before 11 Ma. The ~205 km offset of the northernmost San Andreas fault at the Delgada fan and the Wilson Grove Formation is partitioned between ~180 km offset of the San Gregorio-northern San Andreas fault and ~30 km offset of the San Andreas fault south of the San Gregorio fault intersection. Dextral offset of the San Gregorio-northern San Andreas fault began after ~19 Ma and had slipped at least 50 km before 9 Ma. The map-view reconstructions herein accord with the three-dimensional view that the Hayward and Calaveras faults meet at depth beneath the East Bay hills, and thus may be considered to be a single fault at present. The land of the East Bay hills then may be considered to be a fault-sliver or slivers floating within the Hayward-Calaveras fault. By 5 Ma, the San Andreas-Calaveras-Haywards-Rodgers Creek-Maacama fault had slipped ~170 km, while dextral slip across the segment of the San Andreas fault between its intersections with the San Gregorio and Pilarcitos faults, on the San Francisco peninsula, had not yet begun. Since tectonic blocks are, by definition, fault-bounded, it follows that, like the surface of water between icebergs, all of the block-bounding faults must necessarily be connected. This means that as the blocks move, the faults are drastically rearranged. This model may help to clarify the tectonic basis of the ability of rupture to jump from one fault to another during a single earthquake event as noted by UCERF2. Also, if fault length indeed correlates with potential earthquake magnitude, then this interconnection could imply greater earthquake hazard than has been previously postulate for the East Bay area. The San Andreas fault is not a single entity, but a temporary assemblage of disparate fault segments bounding different tectonic blocks and having vastly differing ages and offset distances. The Hayward-Calaveras fault is older and has greater dextral offset than the segment of the San Andreas fault between the Pilarcitos and San Gregorio faults on the San Francisco peninsula. This difference may also have implications of earthquake hazard in the East Bay.

~285 km Since Before 11 Ma Vs. ~30 km Since ~3 Ma: The Hayward-Calaveras Fault Outranks a Part of the San Andreas Fault in Both Age and Offset Distance

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