Collaborative Research: Retreating-trench, extension, and accretion tectonics (RETREAT): a multidisciplinary study of the northern Apennines

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0208299CowanThe paradox of how horizontal contraction and extension can occur simultaneously in convergent mountain belts remains a fundamental and largely unresolved problem in continental dynamics. The Apennines represent one of the most accessible "type locality" areas of syn-convergent extension. Rollback - which describes the tendency of a subducting plate to retreat from the orogenic front - is commonly invoked as an explanation for syn-convergent extension, but this idea does not address how the retrograde motion of the subducting plate, which is a mantle-based process, causes horizontal extension in the overlying zone of crustal convergence, especially in light of the large accretionary fluxes typically associated with continental subduction.The goal of the project (project RETREAT) is to develop a self-consistent dynamic model of syn-convergent extension, using the Northern Apennines as a natural laboratory. This part of the Apennine orogen has been the site of relatively steady orthogonal convergence and 2D (plane strain) orogenic deformation since ~30 Ma. GPS measurements indicate that convergence is presently active, and tomography indicates that the full length of subducted slab is still intact to depths of 250 km. Syn-convergent extension has been active since at least 15 Ma. The Northern Apennines are well studied, and all important features of the orogen are onland and thus directly accessible for detailed geological and geophysical research.The specific objectives of project RETREAT are 1) to determine in detail the velocity field across the orogen, including deformation in the orogenic wedge, the motion of lithospheric plates, and the flow fields in the surrounding asthenospheric mantle, and 2) to use this kinematic information to develop and test specific dynamic models for deformation in the orogenic wedge and underlying mantle. The research techniques to be used include; geodesy, tectonic geomorphology, low-temperature thermochronometry, structural geology and tectonic syntheses, seismic studies, and geodynamic modeling. The RETREAT project links together a broad multidisciplinary group with eleven PIs from six institutions, plus some 27 foreign collaborators in Italy, Switzerland, Canada, and France.***