Submarine mass wasting processes along the continental slope of the Middle American Trench

This Thesis work presents a regional-scale study of submarine mass-wasting phenomena of the continental slope of a subduction zone. The nature of the study makes it a new, outstanding contribution for two main reasons: 1) The large-scale and interdisciplinary characters of the study conform a comprehensive investigation - unmatched by any other previous study- of land sliding processes along the slope of a tectonically-active convergent margin. 2) The investigation is also unique because it looks into the processes at a subduction zone dominated by tectonic erosion. This type of geological setting represents about 50% of the world subduction zone systems, but it has been overlooked in previous studies of mass wasting processes. The study region is located along a segment of the Middle America Trench (MAT) that extends about 1500 km from the Costa Rica - Panama border to the Guatemala - Mexico boundary. The study investigates the structures of the continental slope of the Pacific-Ocean-side of Central America and the trench-region of the incoming oceanic Cocos plate. We have investigated the distribution of submarine slope failures and their deposits, the type of failures, and their seafloor morphology. We have also investigated possible preconditioning and triggering mechanisms, and the relationship of those mechanisms and the variability in failure type to the tectonic processes of this particular geological setting. Finally, we have made some inferences of the significance of mass wasting processes in the long-term evolution of the slope, compared to other geological settings. The Central America subduction zone has been the locus of intense, continued geoscientific investigation since the late 1970s that culminated with the selection of the region as the focus site for the US-Margins program and the German SFB574 during the first decade of the 21st century. Those two programs included research in a broad range of topics that attempted to advance our understanding of the entire subduction zone system. As a result numerous projects from both communities have benefited from close collaborations. This PhD work is integrated within the research project SFB 574, financed by the DFG, that has as main research goal investigations on “Volatiles and fluids in subduction zones and their impact on climate feedback and trigger mechanisms for natural disasters”. We have analyzed a database containing a compilation of multibeam bathymetry of 7 research cruises, 3 cruises of side-scan sonar imagery and core samples of a dedicated cruise. The database has been assembled in a collaborative effort between both USMargins and SFB 574 communities. Based on seafloor morphology and backscatter imagery, and seismic images we have mapped and classified 147 submarine slope failures in the region. Slope failures vary in their type, abundance and distribution along and across the slope to define six distinct segments along the MAT. The lateral extent of the six segments correlates well with similar along-trench segmentation in the character of the incoming ocean plate, expressed as changes in its relief, age and crustal thickness. We have also found that the six along-margin segments display changes in the across-slope structuring of the different geological elements, including changes in the morphological expression of upper, middle and lower slope, total slope width, and slope dip angle. This structuring of the elements of the slope appears to be related to a longterm evolution caused by the tectonic processes associated to subduction erosion. One segment covers the area of under-thrusting of Cocos Ridge under the shelf-slope offshore Osa Peninsula (southern Costa Rica). Here, 1-km-high narrow, sharp ridges and small conical seamounts festooning Cocos Ridge cause slumps often with rock and debris avalanches from a short, steep continental slope. A second segment occurs offshore central Costa Rica, where large conical seamounts and ridges of 1-3 km high and 40 km wide under-thrust the continental slope causing large re-entries of the slope toe, and furrows across the slope formed by collapse, of previously uplifted upper plate, along steep headwalls behind the under-thrusting seamounts. Failures have generated large slumps, debris flows and rock avalanches containing blocks up to 500 m in diameter. In contrast at a third segment in northern Costa Rica, offshore the North Nicoya Peninsula, a smooth incoming plate is parallel opposite by a continental slope lacking relevant mass wasting structures. The contiguous fourth segment offshore Nicaragua displays a steep middle slope with large translational slides opposite an ocean plate with numerous 1-km-tall seamounts and 100s-meter-high horst and graben relief. Under the fifth segment, offshore El Salvador, subducts a well developed horst and graben relief, but somewhat surprising the segment displays a generally failure-free slope, and only the uppermost slope displays a series of small translation slides The plate under-thrusting the sixth segment offshore Guatemala is similarly characterized by a horst and graben terrain. However, here a steeper slope exhibits frequent, small-scale failures, a few km wide, across the entire segment.