Phanerozoic continental accretion and the metamorphic evolution of northern and central california

Abstract

The late Paleozoic and younger tectonic fabric of California north of the Transverse Ranges consists of accreted exotic and parautochthonous lithic belts which extend north-south and northwest-southeast, separated from one another by subvertical strike-slip faults and gently to steeply east-dipping thrust faults. Where recognizable, folds in general verge to the west and stratigraphic tops face east, a rough but systematic westward decrease in geologic age and metamorphic intensity may also be recognized. Exotic terranes of uncertain provenance include the central metamorphic belt of the Klamath Mountains, and the Salina-Sur-Obispo composite block of the central Coast Ranges. Pre-Permian sections of the eastern Klamath terrane and portions of the Sierran Calaveras-Melones complex may also represent far-traveled crustal entities. Allochthonous oceanic basement, seamounts, and pelagic sediments are additional transported materials widespread in most of the parautochthonous lithotectonic belts that make up much of the rest of northern and central California, ophiolitic assemblages are especially abundant along the fault zones which mark boundaries between these units. The parautochthonous lithotectonic belts appear to have been constructed in the vicinity of the continental margin and/or an offshore arc, judging from the nature and provenance of their constituent igneous and sedimentary rocks, intimately associated tholeiitic-calc-alkaline complexes indicate that, at least locally, ophiolite generation occurred nearby. The Salinia-Sur-Obispo amalgamated terrane exhibits the effects of fragmentation and removal of crust—now represented by the suture juxtaposing the two blocks. Other areas may have been subjected to similar truncations, but relationships are not as clear. Far-traveled terranes are characterized by ages of formation and recrystallization foreign to surrounding units, the nature of the protoliths and grades of metamorphism developed within them contrast with those of juxtaposed lithotectonic belts. On the other hand, essentially in situ growth of many Californian lithotectonic belts is suggested by a gradually decreasing age of formation and both monotonically decreasing times and grades of metamorphism proceeding towards the Pacific basin. Some parautochthonous units as well as exotic terranes are locally bounded by discontinuous blueschist belts, the latter attesting to at least episodic preservation of the effects of subduction, metamorphism and offloading into the accreting continental margin. Phanerozoic evolution of northern and central California thus apparently consisted of a more or less in situ seaward encroachment of the continental margin relative to the craton involving occasional truncation and loss of section, with a concomitant semi-continuous or episodic incorporation of allochthonous seamounts, oceanic crust, and deep-sea sediments and the less frequent arrival and juxtaposition of exotic microcontinental fragments of problematic but distant source.