Ocean floor accretion and volcanoplutonic arc evolution of the Mesozoic Sierra Nevada

Abstract

Recent studies on the history of the Pacific Ocean floor coupled with structural, paleomagnetic, and paleobiogeographic studies of the North American Cordillera indicate that the Pacific floor moved northward and beneath the California continental margin throughout much of Mesozoic time. Northward movement began by Early Jurassic time and possibly as early as Late Triassic time. The northward movement brought equatorial ocean floor into the western Sierran region as an allochthonous ophiolitic basement terrane. Accretion of the oceanic basement was complex, involving both transcurrent faulting and subduction processes. Native basement consisting of Paleozoic borderland sequences and the North American continental shelf sat inboard of the exotic oceanic basement. The contact between the native and exotic terranes was an active break of long duration, herein named the Sierran Foothills suture. The signature of plate convergence in the Sierran region is a composite volcano-plutonic arc. Voluminous magmatism proceeded semicontinuously from Early Jurassic through Late Cretaceous time. Arc rocks of all ages show compositional variations that reflect the nature of the preexisting basement rocks. Gabbroic-basaltic associations occur primarily within the limits of the oceanic basement, whereas voluminous granitic-rhyolitic assemblages are associated with continental basement. The depositional environments of the volcanic arc rocks varied greatly due to tectonic instability within the arc terrane. These environments consisted of an inherited continental shelf, uplifted and eroded continental and oceanic basement, sedimentary basins receiving epiclastic as well as volcaniclastic turbidites, and newly formed intraarc basins floored by juvenile crust. The Mesozoic plate juncture history along the California margin was complex, being dominated by oblique subduction with probable interludes of transform faulting. The Sierra Nevada coincided with the axis of volcano plutonic arc activity from Early Jurassic through Cretaceous time. No consuming plate juncture of Jurassic or younger age can be identified within the Sierran region. Late Triassic to possibly earliest Jurassic consumption occurred in the western foothills during the accretion of the ophiolitic basement terrane. Jurassic consumption occurred west of the Sierran region, but any subduction complex that may have existed was removed by Jurassic and Cretaceous rifting and transcurrent faulting. Cretaceous consumption in the Coast Ranges is marked by the Franciscan Complex. The Sierra Nevada evolved as one composite arc terrane containing numerous internal structural breaks. Deformation of the arc consisted of large-scale dextral wrench faulting with a family of wrench-related structures, oblique rifting, and the development of fold-thrust welts. The result of these processes is a complex igneous and metamorphic terrane consisting of interleaved volcanic arc and older basement fragments that are cut by highly to only slightly deformed plutons of a wide range of ages. Similar structural complexities are evolving today in modern arc terranes with analogous plate-tectonic settings. The processes that operate along a convergent plate juncture that also has a significant strike-slip component are more complex than junctures with normal or near-normal convergence. The unique tectonic style resulting from prolonged oblique convergence is termed transpression after Harland (1971).

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