Geologists have known for more than a hundred years, dating back to Edward Suess, that horizontal contraction and extension can occur simultaneously in convergent mountain belts. This paradox remains a fundamental and largely unresolved problem in continental dynamics. The Apennines arguably represents the most accessible “type locality” 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 overall goal of our 4-year multidisciplinary project is to develop a self-consistent dynamic model of syn-convergent extension, using the Northern Apennines as our 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.

Our specific objectives 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 field 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. Our research will focus on the kinematic and dynamic relationships between slab rollback and deformation in the mantle and crust, but we will also address other processes, such as tectonic accretion and underplating, which undoubtedly have an important effect on the deformation observed at the surface.

The RETREAT project links together a broad and talented multidisciplinary group with eleven Principal Investigators from six institutions, plus some 27 foreign collaborators in Italy, Switzerland, Canada, and France. Our 6 research tasks are:

· Geodesy: Determine the present-day horizontal velocity field across the Northern Apennines.

· Tectonic geomorphology: Determine the pattern and rates of long-term uplift and erosion at the surface of the orogen using fluvial and marine terraces, and cosmogenic nuclide dating.

· Low-temperature thermochronometry: Determine the patterns and rates of erosion and tectonic exhumation, and the steadiness of long-term erosion fluxes and topographic relief using (U-Th)/He and fission-track dating.

· Structural geology and tectonic synthesis: Establish the kinematics of materials entering and moving through the Apennine wedge from late Oligocene to the present by constructing regional cross sections that can be sequentially restored back to 30 Ma.

· Seismic studies: Deploy a passive 2D array and dense 1D transect of broadband instruments to resolve: a) the structure and nature of the Moho, 2) the location , internal structure, and strain field of the supra-slab asthenospheric wedge, and 3) the shape, extent, and structure of the subducting plate.

· Geodynamic modeling: Analyze dynamics using numerical models that simulate the lithospheric system at a variety of scales. Finite-element modeling will investigate the upper mantle response to a subducting and retreating slab. Fully-coupled thermomechanical models will test the response of the crust to mantle motion. Distinct element models will be used to test scenarios of crustal deformation in response to accretion, including underplating, and surface erosion and deposition.

To assist collaboration and integration, we have budgeted for 8 semi-annual meetings of PIs, students, post-docs, and collaborators, with meeting locations alternating between the US and Italy. We have also worked hard to ensure that this research will support a large component of undergraduate involvement, specifically in the form of independent research projects. We contend that this approach probably represents the best and most effective form of educational outreach.