THE ORIGIN OF HOT, THIN MANTLE LITHOSPHERE IN CONTINENTAL SUBDUCTION ZONE BACKARCS

The North and South American Cordillera are orogenic belts that formed in subduction zone backarcs. Along much of the margin, high surface heat flow, low mantle seismic velocity, high shallow mantle temperatures from xenoliths, and widespread basaltic volcanism suggest that the Cordillera are underlain by hot, anomalously thin mantle lithosphere. The origin of the thin lithosphere is not well-understood.

In this study, we examine the development of thin lithosphere over widths of hundreds of kilometers in subduction zone backarcs which have not been extended. In general, mantle lithosphere is colder, and therefore more dense, than the underlying material, making it gravitationally unstable. In a backarc, lithosphere removal may result from: 1) thermal and mechanical erosion of water-weakened lithosphere by flow in the underlying mantle, and 2) gravitational instability due to lithosphere thickening by backarc contraction. To constrain these processes, we use thermal-mechanical models in which an oceanic plate subducts beneath a continental plate. The sublithospheric mantle has a wet olivine rheology. The lithosphere viscosity is scaled upward by a factor of as much as 10, to approximate a drier, more refractory lithosphere.

Once subduction begins, the subducting plate induces corner flow in the mantle wedge, which produces only limited thinning of the backarc lithosphere. Metasomatism of the lithosphere by hydrous fluids released from the subducting plate may strongly weaken the lithosphere. We examine this effect by reducing the backarc lithosphere viscosity after subduction has begun. Significant (>30 km) lithosphere thinning is observed for a viscosity reduction of more than two orders of magnitude. As slab dehydration occurs mostly at shallow depths, rheological weakening may be important near the volcanic arc. An alternate mechanism for destabilizing lithosphere further into the backarc may be thickening of the lithosphere by backarc contraction. For continental backarcs, radiogenic crustal heating and eclogitization of the lower part of thickened crust may enhance the instability growth rate. Using the numerical models, we investigate the relative importance of rheological weakening and contraction for producing a thin backarc lithosphere.