Jamie T. Buscher

Long-term continental deformation associated with transpressive plate motion: The San Andreas fault

A synthesis of transpressive mountain building, as evidenced by rock uplift and topography along the entire San Andreas fault, reveals a complex crustal response to oblique plate motion. Convergent deformation increases toward the fault but does not correlate with the angle of plate-motion obliquity. The shortening estimated from rock uplift is also insufficient to account for the fault-normal motion based on relative plate velocity. This suggests that near-field convergence is influenced by local structural complexity and is not purely driven by regional transpression, and that the fault-normal component of plate motion is partly accommodated elsewhere. Heterogeneity in deformation and degree of slip partitioning highlight the importance of other factors in shaping transpressive continental deformation, including surface processes, material anisotropy, and strain weakening.

Exhumation in the Chugach‐Kenai Mountain Belt Above the Aleutian Subduction Zone, Southern Alaska

Convergent deformation systems in continental backstops are a common component of well-coupled subduction zones worldwide. The Aleutian megathrust offshore southern Alaska has many attributes in common with convergent subduction zones observed elsewhere, implying that significant permanent shortening could occur within the continental hanging wall. A continuous belt of rugged mountains occurs along the forearc of this subduction zone, from the Kenai Peninsula to the eastern Chugach Mountains near the Copper River, providing further evidence of active shortening. To test for long-term shortening and associated rock uplift along this forearc, we have analyzed bedrock samples from the Chugach and Kenai Mountains using low-temperature thermochronometry. Fourteen new apatite (U-Th)/He ages from this area are older than expected based on the rugged topography, and imply minimal exhumation in the late Cenozoic in response to shortening within the forearc. Ages on the leeward side of this mountain belt are ∼20―40 Ma and imply an average exhumation rate of ∼0.1 mm/a or less. Ages are younger along the coast (∼12―18 Ma), implying slightly more rapid exhumation or deeper incision relative to mean elevation, perhaps due to greater precipitation along the windward flank of the range. Younger ages (∼5 Ma) occur in the southeast near Cordova, and may result from local deformation associated with colliding and underthrusting of the Yakutat microplate. However, all of these cooling ages are older than ages from the Yakutat collision zone and along the transpressional Fairweather fault to the east, indicating that far less exhumation occurs in the backstop above the subduction zone. Based on these older ages and a mass balance interpretation of exhumation, we estimate that the long-term shortening in the forearc mountains above the Aleutian megathrust is less than 1 mm/a.