CONTRASTING PETROTECTONIC EVOLUTION OF NORTHERN CALIFORNIAN ACCRETIONARY TERRANES

The Cretaceous Sierra Nevada batholith represents massive volcanism-plutonism attending subduction of the Farallon plate beneath North America. Great Valley forearc basin and Franciscan trench sediments represent debris chiefly shed off the Sierran calcalkaline arc; detrital zircon U-Pb data document continuous igneous activity; crustal growth occurred in <90 m.y. Andean arcs are typical of continental margins dominated by head-on underflow.

Late Paleozoic-Early Mesozoic accreted terranes of the Klamaths and Sierran Foothills comprise serpentinized peridotites, disrupted gabbros + basalts, and associated chert-argillite sequences. Mafic-ultramafic complexes display oceanic affinities, whereas terrigenous debris was derived from eastern terranes. Coeval calcalkaline arc rocks are uncommon. Bulk-rock geochemical data suggest a forearc origin for some mafic Klamath metavolcanic rocks interlayered with distal turbidites in the 200-170 Ma North Fork terrane. Detrital zircons in the turbidites have a continental/Eastern Klamath source, so volcanism occurred near the margin. Terrigenous debris and ancient sandstone blocks derived from the Eastern Klamath terrane suggest that the seaward Eastern Hayfork mélange also formed along the continental edge. The coeval Rattlesnake Creek terrane is an outboard ophiolitic mélange. Geochemical data from the Sierran Foothills suggest that mafic volcanic rocks of the 200 Ma Jura-Triassic arc belt also represent oceanic-forearc volcanism. The Calaveras Complex, lying east of the Jura-Triassic arc belt, comprises a thick chert-argillite sequence containing continental debris; it too formed adjacent to the margin. These Late Paleozoic-Early Mesozoic accreted terranes record >230 m.y. of geologic history. We interpret these paired mafic igneous belts and accretionary complexes as typical of margins that underwent transpression + transtension, with minor episodes of subduction.

Differences in igneous and metamorphic development of Cretaceous arc, forearc basin, and trench deposits versus Late Paleozoic-Early Mesozoic mafic arc and sedimentary units result largely from contrasts in relative plate motions. Late Paleozoic-Early Mesozoic terranes chiefly record a history of dominant transpression + transtension, whereas the Cretaceous lithotectonic belts reflect virtually head-on convergence.