We modeled global positioning system measurements of crustal velocity along a N13°E profi le across the southern Adria microplate and south-central Dinarides mountain belt using a one-dimensional elastic dislocation model. We assumed a N77°W fault strike orthogonal to the average azimuth of the measured velocities, but we used a constrained random search algorithm minimizing misfi t to the velocities to determine all other parameters of the model. The model fault plane reaches the surface seaward of mapped SW-verging thrusts of Eocene and perhaps Neogene age along the coastal areas of southern Dalmatia, consistent with SW-migrating deformation in an active fold-and-thrust belt. P-wave tomography shows a NE-dipping high-velocity slab to ~160 km depth, which reaches the surface as Adria, dips gently beneath the foreland, and becomes steep beneath the Dinarides topographic high. The thrust plane is located directly above the shallowly dipping part of the slab. The pattern of precisely located seismicity is broadly consistent with both the tomography and geodesy; deeper earthquakes (down to ~70 km) correlate spatially with the slab, and shallower earthquakes are broadly clustered around the geodetically inferred thrust plane. The model fault geometry and loading rate, ages of subaerially exposed thrusts in the fold-and-thrust belt, and the length of subducted slab are all consistent with Adria-Eurasia collision involving uninterrupted subduction of southern Adria mantle lithosphere beneath Eurasia since Eocene time.
We measured basin-scale erosion rates, using cosmogenic 10 Be concentrations in quartz, from fluvial sediment in rivers draining the coastal mountain ranges of the U.S. Pacific Northwest between 40°and 47°N. Apparent erosion rates are 0.1 to 0.2 mm yr ؊1 throughout the Oregon Coast Ranges north of 43°N, and increase to the south to 0.6 to 1.1 mm yr ؊1 in the northern California coast ranges near 40°N. We propose that these observations display the erosional response to northward-migrating crustal thickening associated with subduction of the Mendocino Triple Junction. North-south variations in erosion rate, range elevation, and metrics of landscape relief and steepness are consistent with the hypotheses that i) their primary cause is northward-migrating crustal thickening; ii) erosion rates are strongly controlled by topographic relief and weakly, if at all, controlled by climate; and iii) the dependence of erosion on relief is nonlinear and obeys a threshold-relief relationship.
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