LINKING DEFORMATION IN THE MANTLE WEDGE AND CRUSTAL ACCRETION IN THE SUPRA-SUBDUCTION ZONE JOSEPHINE OPHIOLITE

The Josephine peridotite comprises > 800 km² of tectonized/variably serpentinized harzburgite, dunite, and pyroxenite that represent the upper mantle exposures of the Middle Jurassic Josephine ophiolite (JO). Published results indicate that the JO was generated in a supra-subduction zone, trans-extensional marginal basin setting that evolved by rupturing and rifting of a contemporaneous Middle Jurassic calc-alkaline arc. Published overlapping ages suggest that the rifted arc may have been active along portions of both margins of the JO basin. Thus, the JO and peridotite represent an excellent locality to study the dynamics of mantle flow and accretion of marginal basin crust in the context of rifting and concomitant arc magmatism.

New mapping, structural analysis and olivine LPO data from two regions provide a framework to evaluate potential links between mantle flow patterns and plate separation. Region 1 (latitude of Cave Junction) is underlain by harzburgite with sub-horizontal OPX layers, plastic foliations parallel, oblique and discordant to layers, folds, and outcrop-scale shear zones. Lineations and fold hinges are generally shallowly-plunging with no significant azimuthal clustering. Poles to foliations are moderately plunging also with no significant azimuthal clustering. Domains of sub-horizontal layering are bounded by high-angle plastic shear zones, some of which are characterized by steep dunite bodies. Region 2 (Chrome Ridge) is underlain by foliated peridotite, with a conspicuous lack of OPX layering. Steep, tabular dunites are common. Foliations form a variety of orientations with moderate to shallow dips. Olivine LPO data from region 1 document the ‘high-temperature’ (010)[100] slip system as dominant during deformation and recrystallization, but other slip systems including (100)[001] were operative.

Domains of mantle harzburgite are bounded by dunite shear zones that accommodated movement of individual blocks in relation to each other and acted as zones of basalt accumulation/transport through the upper mantle to the JO spreading axis. Mantle foliation/lineation orientation data are geometrically compatible with hypersolidus flow patterns in the lower crustal sequence of the JO, indicating a link between mantle flow in the wedge and plate separation in the crust.