Along-Trench Structural Variations, Seamount Subduction, and Inter-Seismic Coupling at the Central Ecuador Convergent Margin

Abstract:

The structural interpretation of 2D-Pre-stack Depth Migrated Multichannel Seismic Reflection sections collected during the SISTEUR cruise across the Central Ecuadorian convergent margin was combined with multibeam bathymetry, OBS wide-angle tomographic models, a GPS inversion model, and relocated micro-seismicity to decipher the causes of the along-trench variability of the Inter-Seismic Coupling (ISC). Our study shows that the Central Ecuador margin divides in two contrasting segments, the northern “Manta-Puerto López” and southern ”Puerto López-Salinas” segments showing dissimilar long-lived physical properties. The northern segment coincides with a shallow ISC locked zone, and shows a smooth outer-wedge slope scalloped by a gentle, 50 km-wide morphologic re-entrant. No subduction channel is detected across this segment that reveals a large subducted seamount and a 2-4° landward dipping shallow inter-plate contact. In the locked zone, the seamount is in contact with strong (Vp= 5 km/s) oceanic rocks of the margin basement, suggesting that elastic strain can store and trigger a large earthquake. In contrast, the southern margin segment is rather decoupled and shows a highly disrupted outer-wedge seafloor with deep re-entrants and large Mass Transport Deposits. The interplate contact dips landward ~6-7°, and is spotted by isolated seamounts separated by a ~0.5-1 km-thick subduction channel that may act as a lubricant favoring inter-plate creeping. In this segment, subducted seamounts collide against low velocity (Vp=3.5 km/s) margin rocks of a thrust sheet complex deformed by normal faults, so that sufficient elastic strain may not accumulate to trigger a large earthquake. Our study supports that ISC variations along the trench are mainly controlled by the thickness of the SC, the roughness of the subducting plate and stiffness variations of margin basement rocks against which subducted seamounts collide.