IS SLAB ROLLBACK THE EXCEPTION OR THE RULE? THE CASE OF THE BROOKS RANGE OPHIOLITE, ALASKA

Most ophiolites found throughout the Cordillera are now recognized as fragments of small ocean basins that opened in the upper plate at convergent boundaries, NOT parts of large, closing ocean basins formed at divergent boundaries. The Brooks Range Ophiolite (BRO) of Alaska provides a well-preserved example of this earliest formative phase of the Cordillera, represented by many other ophiolites, such as the Rocas Verdes of Patagonia. Comparing the composition, magmatic and tectonic emplacement structures, and metamorphic sole of the BRO with other ophiolites of the same age throughout Canada and California, demonstrate that there was a phase of suprasubudction zone (SSZ) spreading along the northern Cordillera during the Middle Jurassic. In Alaska, the opening of the basin was terminated almost immediately by collision with the Ellesmerian continental passive margin, which initiated the Brookian Orogeny. Repeated field, petrologic and geochemical studies of the BRO reveal a basal mantle sequence of mostly dunite and tectonized harzburgite, and a co-magmatic crustal sequence of mostly gabbroic plutons with rare sheeted dikes and volcanic and sedimentary cover units. Distinctive geochemistry of the mantle and crustal sequences indicate a tectonomagmatic setting transitional between MORB and arc types. Magmatic flow fabrics within the BRO reveal the original structural grain of spreading and magmatism. Sub-parallel directions of lattice fabrics in mantle peridotite and shape fabrics in layered gabbro indicate coupled asthenospheric and magmatic flow. Syn- and post-magmatic normal faults, and the rarity of sheeted dikes, indicate amagmatic extension. Crystallization ages (163-170 Ma) are similar to the age of tectonic emplacement. Tectonic emplacement formed a dynamothermal metamorphic sole, which decreases in grade and ductile strain intensity rapidly downward away from the base of the BRO. The metamorphic protolith is transitional with geochemically similar underthrust distal continental margin units. Tectonic transport was top-to-the NNW. The regional implications of these data is that highly oblique or retreating subduction zones formed a series of small SSZ ocean basins throughout the Cordillera, during the Mesozoic. The Caribbean and Scotia Plates provide modern analogs of these processes.