SUBDUCTION INITIATION AND SUPRA-SUBDUCTION OPHIOLITES: A VIEW FROM THE WESTERN PACIFIC (IODP EXPEDITIONS 352 AND 366) AND THE CALIFORNIA COAST RANGES

The formation of most (but not all) large ophiolite complexes above subduction zones is new well established, with an emerging consensus that their stratigraphic and structural architecture is linked to nascent subduction zones during subduction initiation. The clearest analogue for this process is found in the Izu-Bonin-Mariana (IBM) arc system of the Western Pacific Ocean. In addition to its classic oceanic island arc complex, the IBM arc preserves in its forearc a record of subduction initiation circa 52 Ma, including a suite of lithologies that are characteristic of classic ophiolite complexes worldwide. IODP Expeditions 352 and 366 explored this record in the last decade, confirming the formation of the IBM forearc during subduction initiation, and documenting its geochemical and stratigraphic correspondence to SSZ-type ophiolites. The occurrence of forearc basalts below boninitic lavas, creating a depleted boninite source region, and the relatively short duration of forearc formation are typical of many SSZ ophiolites. In addition, peridotite xenoliths recovered from serpentine mud volcanoes in the Mariana forearc show reaction relationships consistent with the formation of high-Mg, high-Si boninites.

The Coast Range Ophiolite (CRO) of California is distinct because unlike most SSZ ophiolites it was not emplaced through continental collision, rather it has been exposed by the process of accretionary uplift during prolong underplating of the Franciscan Formation accretionary complex. As a result, it crops out as a series of disconnected fragments, each with its own igneous and structural characteristics. Many of these fragments are similar to IBM forearc associations, e.g., early tholeiitic basalts overlain by boninite. The occurrence of quartz gabbro, quartz diorite, and tonalite intrusive bodies, and felsic volcanic successions in some fragments document the transition to more arc-like settings. And several locations document a late, MORB-like volcanic episode, which may be linked to ridge subduction. We suggest that the IBM forearc represents one end member in a spectrum of subduction initiation settings. The CRO shows that the response to subduction initiation, and the subsequent evolution of SSZ forearc ophiolites, is more varied and includes processes not observed in modern systems.