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Abstract

A vital lesson of plate tectonics is that there is no validity to any assumption that the simplest and therefore most acceptable interpretation demands a proximal rather than a distant origin. (Coombs, 1997, p. 763).

This field trip steps back to provide the very long term and large-scale tectonic history that one might call the broader tectonic context of the 1906 San Francisco earthquake. In effect, the field trip follows a cross section of northern California, with stops that illustrate the geologic history of the region. The field guide also discusses several archaeological stops of significance to California's prehistory. The entire field trip is meant to be taken over a period of four days, with overnight stops in Davis and in Quincy. Day one comprises Stops 1–9; Day 2, Stops 10–18; Day 3, Stops 19–28; and Day 4, Stops 29–33.

Northern California geology is the result of an extended history of active plate margin interactions spanning some 500 million years (m.y.). Over the course of this period, countless numbers of large earthquakes of different types no doubt accompanied tens of thousands of kilometers of movement between tectonic plates and microplates that eventually came together to form the rocks of northern California as we see them today.

From ca. 500–18 million years ago (Ma), subduction, a process still active north of Cape Mendocino, dominated the geologic history of northern California. During this period, several subduction zones and volcanic arcs were active, and subduction zones, whose former positions we will see on our trip, consumed ocean basins thousands of kilometers wide, sweeping together a vast collage of rocks from far-flung locations in the process.

Remnants of the most ancient of these subduction zones and collided blocks (typically called terranes) are preserved in the Sierra Nevada. The most recent subduction history along the North American margin involved the Farallon plate, a plate that lay east of the Pacific plate and was separated from it by a spreading mid-oceanic ridge. The products of this subduction episode are preserved in the rocks of the Coast Ranges, and in the granitic and younger volcanic rocks of the Sierra Nevada ((Figs. 1) and 2).

For the past 18 m.y., the plate margin has been dominated by right-lateral faults of a transform plate margin, the most famous of which, the San Andreas fault, produced the 1906 earthquake. The present plate boundary between the Pacific plate on the west and stable North America on the east is a broad one consisting of two active zones: (1) the San Andreas fault system, whose right-lateral faults occupy the California Coast Ranges; and (2) a zone east of the Sierra Nevada including the right-lateral faulting associated with the Walker Lane and the Eastern California Shear Zone, and, east of the Walker Lane, the extensional faults of the Basin and Range province (Fig. 3).

These zones of active faults, shown in Figure 3, are responsible for generating most of the earthquakes in the area traversed by our field trip. The preexisting complexity of the crust resulting from the earlier tectonic history probably influenced the development and location of these more recent fault zones, in addition to giving us some exceedingly interesting and complex geology to examine on our trip.

This long and complex history requires a great deal of discussion for complete understanding. In this guide, we present first the stops in sequence, including brief descriptions for each site. To augment these brief discussions, we follow the field trip guide with an Appendix in which we discuss the tectonic development of northern California more fully.

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