Possible Mechanisms for the Emplacement of Alpine-Type Serpentinite

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Abstract
A model for the emplacement of alpine-type serpentinite has evolved from studies of an anomalous serpentinite outcrop discovered in northern Colombia during mapping under Harry Hess’s Caribbean Project. According to the model, much alpine-type serpentinite is derived from oceanic crust or uppermost mantle that has been subducted beneath continental crust along continental margins. As the temperature of the subducted slab rises, serpentinite becomes mechanically unstable at about 300° to 350° C, owing to thermally induced dehydration weakening. Because of its low density relative to enclosing rocks and the lubricating effect of interstitial water, the weakened serpentinite migrates upward as protrusions along major faults, and owing to internal shearing and the expansion of water released during dehydration, its velocity may increase as it rises. Eclogite fragments and other high-grade metamorphic rocks torn from deep crustal levels are carried upward with the serpentinite. At shallow crustal levels, steam may be generated within the protrusions, possibly propelling serpentinite to the sea floor explosively. Once at the surface, sheared serpentinite and any associated tectonic inclusions flow downslope as turbidity currents and submarine landslides and are deposited in oceanic trench areas and interrise basins as exotic blocks, olistostromes, and detrital beds. With continuing subduction and sedimentation in the trench area, the sedimentary serpentinite is buried in a mélange of graywacke, deep-sea chert, and spilite. During deformation and metamorphism of this mélange, shearing of the serpentinite may mask original sedimentary features and make a later determination of origin extremely difficult. Alpine-type serpentinites comprise four principal, overlapping groups: (a) serpentinite protrusions—mostly large, elongate bodies bounded by major faults, (b) sedimentary serpentinites—mostly small masses scattered helter-skelter throughout eugeosynclinal sequences, (c) serpentinites associated with large slabs of oceanic crust or upper mantle (ophiolite sequences), and (d) serpentinites associated with high-temperature peridotite protrusions. Serpentinites are highly mobile rocks, and individual masses could have belonged to several of these categories at different stages of their tectonic evolution.

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