SERPENTINITE ENTRAINMENT AND DEHYDRATION DURING FLAT SUBDUCTION AND THE FORMATION OF LARAMIDE-TYPE UPLIFTS

The modern analog for Laramide uplifts occurs in west central Argentina where there is a shutoff of arc volcanism and the presence of active basement cored uplifts of the Sierras Pampeanas (Allmendinger et al. 1986). A causal relationship with flat slab subduction is suggested by the correlation of these characteristic Laramide features with a segment of the South American subduction zone where the Nazca slab is subducting nearly horizontally at about 100 km depth before it descends into the mantle. However, the actual mechanism of this causal relationship remains a topic of active debate. We combine the results from a range of seismic studies, which used data collected during the Chile Argentina Geophysical Experiment (CHARGE) to present a model of the crustal and upper mantle structure in west central Argentina in the vicinity of the flat slab. Beneath the western Sierras Pampeanas region we have identified a layer of unusual mantle above the flat portion of the Nazca slab from regional travel time tomography (Wagner et al., 2007). This mantle layer, 25 km thick, is characterized by low Vp, high Vs, and unusually low Vp/Vs (<1.7) for mantle material. The only known mantle materials that fit these properties is cold, dry, and magnesium rich compositions (e.g., enstatite and fosterite). Based on the distribution of the layer immediately above the flat slab we propose that this layer is serpentinized forearc mantle that was subducted beneath the upper plate during the initial stages of flat subduction, and dehydrated at depth during a transient heating event upon contact with the base of the initially hot upper plate. After the rapid loss of its released fluids by brittle fracturing, the remaining material (mainly enstatite with a small amount of fosterite) has the appropriate seismic properties for this mantle layer. This mechanism is a possible way, not involving extensive partial melting, to make a strong, highly depleted mantle layer that can act as a strong stress guide to transmit stresses up and farther inland into the upper plate resulting in the Sierras Pampeanas uplifts. This layer could remain after cessation of flat slab subduction as a strong layer resistant to subsequent deformation. A possible example of this type of mantle lithosphere, formed by deserpentinization, is beneath the Colorado Plateau in the western U.S.