SUBDUCTION ZONE EARTHQUAKE MECHANISMS AND EXHUMATION OF ROCKS FROM DEPTHS EXCEEDING 300 KM IN COLLISION ZONES

Subduction zones are sites of the downwelling in the mantle; most consist of subducting slabs of oceanic lithosphere, which display earthquakes within them down -- to as deep as 680 km. Subduction also can lead to continental collision, of which the collision between India and Asia is the classic modern example. The resulting Himalayan mountain chain has many fossil counterparts in Asia and Europe, most (all?) of which contain rocks showing evidence of Ultra-High Pressure Metamorphism (ie. they contain coesite or other evidence indicating depths of approximately 100 km or deeper. In this talk, I will address the mechanisms whereby earthquakes can occur in subducting lithosphere and show that the evidence strongly suggests subduction zones are dry below 400 km. I will also give examples of UHPM rocks that have surfaced from more than 300 km.

Earthquakes near the surface are caused by frictional sliding on pre-existing faults or, rarely, by creation of a new fault by brittle shear failure. Neither mechanism can function at depths greater than ~30-50 km because pressure strongly inhibits frictional sliding and temperature enhances flow. Experiments show that deeper earthquakes, those in subduction zones, require a mineral reaction that generates a small amount of a new phase with very low viscosity -- a "fluid" -- which could be a real fluid (e.g. H₂O or melt) or a pseudofluid consisting of a polycrystalline material of nanometric grain size. Work over the last 20 years has delineated that fluid-producing reactions like dehydration of serpentine are the likely mechanism for earthquake nucleation above ~400 km and that transformation-induced faulting of metastable olivine is the likely mechanism below 400 km. I will explore the high-pressure shearing instabilities that are probably the underlying mechanisms of these earthquakes and show how they explain the bimodal distribution of earthquakes with depth, why they stop abruptly before 700 km, that metastable olivine is present in at least 4 subduction zones, and that subducting slabs are probably dry below 400 km.

Over the last 40+ years, rocks have been discovered from progressively greater depths in continental collision zones. In particular, in the 1980's coesite was discovered in Italy and Norway and diamonds in sediments from Kazakhstan, giving rise to the field of Ultra-High Pressure Metamorphism, and implying subduction to more than 120 km and return to the surface. More recently, the use of microstructures has extended the evidence in these terranes to much greater depths of 300 km or more. Some are peridotites, for which the trip may be a one-way voyage up; others are rocks that began at or near the surface, rode down on top of a subducting plate, and returned back up the same way they went down. In each case, they carry a combination of mineral phases and microstructures that disclose the depth of their travels. I will give two examples of rocks that have come up subduction zones from more than 300 km, one of which is a pelitic gneiss which at its greatest depth contained large crystals of stishovite (the high-pressure polymorph of quartz that is stable only at depths greater than 300 km). It is virtually certain that greater subduction also has occurred and most likely is responsible for the "continental" geochemical signal in ocean island basalts. Controversy has swirled around the question of how these high-pressure rocks, generally peridotites and eclogites, were transported to the surface. They generally are surrounded by quartzofeldspathic rocks that may show evidence of migmatization but no evidence whatsoever of high pressure. The question has been: Are the white rocks part of the deeply subducted material or have the high-pressure rocks been somehow incorporated into them after they neared the surface. This question has been soundly answered in the affirmative for the Dabie/Sulu terrane of Eastern China where separation of very large numbers of zircons from the white rocks have shown inclusions of coesite. Thus, in at least that case, the white rocks are clearly part of the subducted complex and almost certainly are the “cork” that brought up the denser rocks by buoyant upwelling.