MARGIN CURVATURE RELATED TO SUBDUCTION OF OBLIQUE ASEISMIC RIDGES

To reduce frictional forces related to oblique subduction of oblique aseismic ridges (enhanced by flat slab subduction), geometric changes affecting the upper plate must occur (Japas & Ré, 2005). Since minimum friction arises when the ridge is subducted perpendicular to the trench, strike-slip and thrust motions along previous structures would accomodate deformation in the upper plate rotating the margin to this required orientation. In such case deformation would be transfered into the foreland generating changes in the structural style and increasing low depth seismicity (Kley et al., 1999; Gutscher et al., 2002; Gutscher & Peacock, 2003). Reactivation of ancient anisotropies will depend on its trends respect to the convergence direction (Japas & Ré, 2005).

As was pointed out by the same authors, ridge-axis-trend would control the strike of the deformed margin whereas convergence-direction / ridge-axis-trend obliquity relation would impose the width of the rotated continental margin zone and would define the sense of migration of the aseismic ridge along the active plate boundary.

Two situations in which this type of margin transformations would occur will be discussed in here:

1) When the direction of convergence is normal to the continental margin and oblique to ridge axis

2) When the convergence direction is oblique to ridge axis trend and both, oblique to the margin.

Ridge migration along the continental margin and an increase of the deformation zone width in the upper plate would be expected for these both situations.

Two examples of margin accomodation generated by oblique subduction of an oblique aseismic ridge will be analyzed.

REFERENCES

Gutscher,M., Peacock,S., 2003. Thermal models of flat subduction and the rupture zone of great subduction earthquakes. JGR 108 (B1), 2009, 10.1029.

Gutscher,M., Spakman,W., Bijwaard,H, Engdahl,E., (2000). Geodynamics of flat slab subduction: Seismicity and tomographic constraints from the Andean margin. Tectonics 19 (5) : 814-833.

Japas, M. and Ré, G., 2005. Geodynamic impact of arrival and subduction of oblique aseismic ridges. 6th International Symposium on Andean Geodynamics, Extended Abstracts: 408-410.

Kley,J., Monaldi,C., Salfity,J., 1999. Along-strike segmentation of the Andean foreland: Causes and consequences. Tectonophysics 301 : 75-94.