GREAT NORMAL FAULTS AND THEIR ROLE IN ACCOMMODATING THE RISE OF COLLISIONAL MOUNTAIN BELTS

Within two decades of the dawn of plate tectonics, great normal faults and ultra-high pressure (UHP) metamorphism of continental basement had been discovered within collisional mountain belts. But their significance to resolving the question, “what is the origin of forces driving the rise of collisional mountains,” remains debated. Recent thermochronometric and structural analysis of the main Austroalpine “overthrust” in the Alps (Price et al., 2018, Tectonics, v. 37, p. 724), suggest that it is a great normal fault or detachment that accommodated the rise and lateral emplacement of the ca. 20-km-thick Pennine zone (which includes UHP rocks), interposing it between cratonic Europe and the Adria microplate, between 33 and 18 Ma. The juxtaposition of cratonic Europe with Adria was largely complete by 33 Ma, while most of the Alpine realm still resided near sea level. The development of most Alpine nappe structure, crustal thickening, high topography, and the peripheral Molasse and Lombardy basins (i.e., the hallmarks of Alpine mountain building), thus do not appear to be driven by coeval plate convergence, which in any event does not predict normal motion on the main “overthrust.” Rather, the basic physics of (1) partial subduction of continental crust, viscously coupled to the downgoing slab during plate convergence, followed by (2) buoyancy forces overwhelming coupling due to thermal weakening, and (3) crustal thickening via up-slab motion of continental crust, mainly after convergence, best accounts for the timing and kinematics of Alpine nappe structure and mountain building (e.g. models of Chemenda et al., 1995, Earth and Planetary Science Letters, v. 132, p. 225). As such, contractile structures need not be a direct expression of plate convergence, nor are detachments acting to mitigate high topography or crustal thickness. Great normal faults accommodate (and therefore contribute to) crustal thickening, rather than working to counteract it. As such, forces originating from horizontal plate convergence and high topography would exert little, if any, direct dynamic influence on the mountain building phase. They are instead a precondition and end result, respectively, of mountain building, which is controlled primarily by the competition between viscous coupling and buoyancy forces in the upper mantle.