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B. Surface response to changes in deep structure

The complex structure of the Alps manifests a history of oblique plate convergence punctuated by two slab rupturing events: an Oligo-Miocene event in the Eastern and Central Alps (von Blanckenburg & Davies 1995) and a younger, possibly Plio-Pleistocene event in the Western Alps (Fig. 2, section DD’; Kissling et al. 2006, Fox et al. 2015). Both events appear to coincide with increased deposition rates in the foreland Molasse Basin (Sinclair 1997; Schlunegger & Willet 1999, Kuhlemann et al. 2002, Spiegel et al. 2000, 2002), but deposition lags behind tectonic uplift. The depositional record reflects the competing effects of tectonics, climatics, lithology and erosion (Kühni & Pfiffner 2001), as well as the selective preservation of younger sediments (Willenbring et al. 2010). Linking the latter of these breakoff events to the record of recent uplift and erosion in the Alps is problematic; whereas recent erosion rates in the Central and Western Alps determined by cosmogenic Be-10 from river sediment (Wittmann et al. 2007) are equal to or somewhat less than rock- and surface-uplift rates (0.2-1.1 mm/yr, Kahle et al. 2007), rock-uplift rates exceed erosion rates in the Eastern Alps (Norton et al. 2011). This has been used to argue that the Western Alps are at dynamic steady state in the absence of convergence (Bernet et al. 2001, Wittmann et al. 2007) and assuming isostatic compensation at the current Moho depth. Norton et al. (2011) further speculate that the predominance of erosion in the west may be due to the greater influence of geomorphic inheritance there (glaciation in response to Mio-Pliocene global cooling; Champagnac et al. 2007), whereas the relative importance of uplift in the east may reflect ongoing Adria-Europe convergence.

An enigmatic feature in this context is that topographic relief and average height do not strictly correlate with Moho depth; the Alpine crust is thickest (48-52 km) in a trough just north of and along the Periadriatic Fault (Fig. 4c), whereas the highest mountains and greatest topographic relief occur north and west of this trough, in the vicinity of the External Basement Massifs where the crust is only ~40 km thick. Thus, the Western Alps may not be in dynamic equilibrium after all. Possibly, the parity of rock uplift and erosion rates in anomalously high, isostatically overcompensated topography in the west is a transient response to slab break-off (Fox et al. 2015) and the isostatic compensation depth was instead situated at the lithosphere-asthenosphere boundary (LAB) above the purported slab break (Fig. 2, section DD’). Thus, slab tearing rather than climate change may have triggered exhumation of the External Basement Massifs some 4-6 Myr ago (Vernon et al. 2008, Glotzbach et al. 2011). Distinguishing between these scenarios will not only require clearer images of lithospheric structure in the Western Alps, but also better insight into the physics of surface-slab coupling as provided by lithospheric-scale numerical models (Lechmann et al. 2011, Li et al. 2014).