Cruise Plan Christina Massell R/V Melville
We will collect a 20km-wide Seabeam 2000 swath along 2000km of Peru Chile Trench between 15S and 32S. These happen to be the best latitudes to study tectonic landforms, because this part of the trench is sediment-starved, so its structures are not obscured by mud. Existing data include a SeaMARC survey of the Nazca Rise intersection at 15S (Hagen and Moberly, 1994), a regional seismic reflection survey (Schweller et al, 1981), and a pair of MELVILLE Seabeam 2000 swaths collected in 1994 at Dr. Peter Lonsdale's request along the axis 24-20S and 19-17S. From these observations, we see that the trench between Callao and Valparaiso is a good place to address two specific problems during 3 survey days.
"Problem 1:" Effect of a higher outer rise at a trench concavity. The flexural outer rise is exaggerated alongside concavities in the trench plan. The Arica Bight is a major concavity in the plan of the Peru-Chile Trench, and has a higher-than-usual outer rise. We will spend 2 days mapping the entire 80km-wide landward slope of this outer rise (i.e. the seaward slope of the trench around the Arica Bight) to see how the higher rise affects the intensity of faulting and tilting on the seaward slope, and to see how the changing direction of the axis of flexure affects the style of faulting (eg. reactivation of old abyssal hill faults vs. creation of new extensional fractures).
"Problem 2:" Compressional faulting on the lower part of the outer trench slope. The best understood process on the outer slopes of trenches is extensional normal faulting, but along several trenches, the Peru-Chile Trench in particular, there is evidence from earthquakes and seismic reflection profiles that copmpressional thrust faulting takes over as the oceanic crust approaches the trench axis. Presumably some of the compressional inter-plate stress is transmitted from the subduction zone into the exposed part of the oceanic plate. The 1994 MELVILLE data show what appears (from their plan pattern and asymmetric profile) to be excellent examples of lower slope thrusts in the deepest part of the trench at 22S - 24S. To confirm this interpretation and study how these thrusts develop I need additional swath coverage from higher up the slope. A total of 3 new bathymetry and side-scan swaths will be collected between these latitudes, allowing coverage of the entire actively faulting part of the trench slope.