Concave-outboard convergent margin geometry and modern oroclinal bending
Abstract
The concave-inboard geometry of the trench along most convergent margins is often described as resulting from the depression of the edge of a thin spherical cap. However, several modern subduction zones display concave-outboard trenches that extend for over 500 km in length, such as Cascadia, Bolivia, and northern Japan. In the case of Cascadia, our synthesis of paleomagnetic, structural and GNSS data indicates that the upper plate has been folded from the Miocene to Recent into an orocline with an axial trace that aligns with the apex of the trench concavity. The datasets suggest that oroclinal bending at Cascadia occurs by folding by flexural slip on the orocline limbs and shortening, uplift, and escape within the core of the fold at the Olympic Mountains. We suggest that these processes can produce a concave-outboard trench geometry by inducing relative motion of the forearc towards the arc at the core of the orocline, and opposing rotations of the upper plate on the orocline limbs. Given the spatial coincidence of the Cascadia orocline core with the apex of a broad arch in the subducting slab, we propose that oroclinal bending is promoted and maintained by along-strike variations in plate-boundary tractions resulting from the geometry of the plate interface at depth. We suggest that analogous oroclinal bending processes could contribute to the development of the concave-outboard margin geometry at Bolivia and northern Japan, as both of these examples also exhibit a trench concavity centered on a broad arch in the subducting slab, and each displays evidence for past or present oroclinal bending within the upper plate.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T41J0260M
- Keywords:
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- 7230 Seismicity and tectonics;
- SEISMOLOGY;
- 8158 Plate motions: present and recent;
- TECTONOPHYSICS;
- 8170 Subduction zone processes;
- TECTONOPHYSICS;
- 8488 Volcanic hazards and risks;
- VOLCANOLOGY