Abstract

Geological and geophysical evidence reveals a complex tectonic history of pre-orogenic extension and multiple-stage positive and negative orogenic to post-orogenic inversion at Corno Grande, the highest Apennine summit. Corno Grande is made of an upper Triassic-lower Lias sequence of massive shelf dolomites and limestones, peripherally overlain by condensed Meso-Cenozoic pelagic and submarine-slope sediments. Conversely, thicker mid Lias-Miocene successions, fanning outward towards originally deeper basinal areas, are exposed in the adjacent sectors of the Gran Sasso belt. New geological mapping in the Corno Grande-Corno Piccolo area, indicates that early-mid Lias extensional tectonics left Corno Grande as an isolated T-shaped seamount located within the submarine slope connecting the Latium-Abruzzi carbonate platform to the Umbria-Marche basin. A 2 km-long north-dipping segment of the Lias extensional fault system originally surrounding Corno Grande is spectacularly exposed NW of the main top. Just south of this fault, a swarm of E-W sedimentary dykes up to 30 m-thick, filled by mid Lias Corniola-equivalent sediments, suggests a genetic relationship with the Lias tectonics. Other dykes made by upper Lias-Dogger Verde Ammonitico and upper Cretaceous-Oligocene Scaglia sediments are also exposed close to the top and the Calderone Glacier. Although they are a clear proof for tectonic instability and progressive collapse of the Corno Grande seamount, it is unclear whether they also reflect additional pre-orogenic extensional episodes. In order to gain additional strain-trajectory proxies relative to the compressive episodes, I analysed the anisotropy of magnetic susceptibility (AMS) of both sedimentary dykes and strata from 11 different sites located on top and north of Corno Grande. The magnetic fabric of the sedimentary dykes reflects the flow direction of sedimentary mud infilling the extensional fractures, and is not modified by the Mio-Pliocene compressive events. Conversely, the AMS of pelagic sediments cropping out north of Corno Grande mostly reveals a typical sedimentary fabric modified by compressive tectonics. The direction of the magnetic lineation (which in principle should trend normal to the shortening direction) is NE-SW at two sites, and NW-SE at one site. This variability may be due to the presence of the pre-existing extensional discontinuities controlling the geometry of the thrust-sheet ramp and yielding a local strain pattern (instead of the regional one) within the thrust fault hanging-wall. The overall geological/geophysical data set, coupled with published geological information from adjacent areas and paleomagnetism (revealing no rotation at Corno Grande) gives insights on the tectonic evolution during belt development. I suggest that during late Messinian-early Pliocene, the eastward-dipping Lias normal fault of Corno Grande underwent positive inversion due to E-W convergence. In fact geological evidence indicates that the Upper Thrust segment exposed along the Corno Grande eastern wall accommodated top-to-the east shear. Therefore this exposed thrust fault should be a remnant of a late Messinian N-S thrust plane connecting the Montagna dei Fiori-Montagnone ridge to a formerly N-S Gran Sasso chain. Afterwards, in early-mid Pliocene times, N-S shortening occurred at Corno Grande, inducing the positive inversion of the northward-dipping Lias faults. At the same time, the Gran Sasso belt east of Corno Grande underwent 90 degrees counter-clockwise rotation, synchronous with thrust sheet emplacement. Finally, the early Pleistocene to Present post-orogenic extension reactivated some southward-dipping pre-orogenic faults of the Corno Grande seamount.

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