Mega‐Depressions on the Cocos Ridge: Links Between Volcanism, Faults, Hydrothermal Circulation, and Dissolution

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 23; no. 8
• Main Authors: Kluesner, Jared W., Silver, Eli A., Bangs, Nathan L., Ranero, César R., Nale, Stephanie, Gibson, James, McIntosh, Kirk D.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.08.2022; Wiley
• Subjects: Bathymetry; Bottom currents; Carbonates; Circulation; Cocos Ridge; Collapse; Current scouring; dissolution; Dissolving; Drilling; Fault lines; Faults; fluid flow; Geochemistry; Geophysical data; hydrothermal circulation; Indicators; Miocene; Morphology; Ocean floor; Outflow; Pleistocene; Pliocene; pockmarks; Porosity; Scouring; Sediment; Sediments; Seismic data; Seismological data; Volcanism; Water currents; Panama
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2022GC010370

High‐resolution bathymetry and three‐dimensional seismic data along the Cocos Ridge reveal a 245 km2 field of ∼1–4 km in diameter seafloor depressions. The seafloor depressions are part of a two‐tiered honeycomb pattern. The lower‐tier depressions have steep faults that truncate strata with chaotic internal reflections consistent with sediment collapse into the depression. These extend into a lens shaped interval just above igneous basement. Overlying these depressions is a second broader set with rough seafloor morphology with gently dipping boundaries defined by pinch‐out stratigraphic patterns. Drilling results indicate that the lens‐shaped zones that host the deeper depressions represent anomalous regions of high porosity, low velocity, and low density within calcareous rich sediment. Analysis of nannofossils from IODP Site U1414 suggests the collapse structures formed during the late Miocene, whereas the younger shallower depressions likely formed between the early Pliocene and the Pliocene‐Pleistocene boundary. Geochemical and petrological analysis at Site U1414 suggests that hydrothermal circulation during the late Miocene led to carbonate dissolution and collapse. Following collapse, focused fluid‐flow and bottom current scouring resulted in formation of the overlying set of depressions and a honeycomb seafloor morphology. Similar sets of depressions along the Carnegie Ridge to the south support the hypothesis that two‐tiered depressions formed in response to processes that occurred broadly across the Panama Basin between the late Miocene and the Pliocene‐Pleistocene transition. Geochemical results at Site U1414, combined with geophysical data, suggest this two‐tiered system of depressions currently guides ongoing fluid outflow. Plain Language Summary We characterize a vast field of mega seafloor depressions along the Cocos Ridge using three‐dimensional geophysical data tied to drilling results. The mega depressions display distinct two‐tiered structures that form a honeycomb‐like pattern on the seafloor. By imaging faulting and sediment layers within the depressions, we reveal that the older, steeper‐walled depressions formed through collapse processes, whereas the younger, broader depressions likely formed through a combination of fluid‐flow and current scouring. Drilling results indicate that the older collapses formed during the late Miocene, and geochemical results suggest that hydrothermal circulation, likely along ridge‐parallel faults, dissolved carbonate‐rich sediments and led to collapse. In contrast, drilling results suggest the younger depressions formed sometime between the Pliocene and Pliocene‐Pleistocene boundary. Similar depressions imaged within the Panama Basin and along the Carnegie Ridge to the south suggest widespread Galapagos Hotspot volcanism and hydrothermal circulation led to dissolution and collapse of carbonate‐rich sediments across the eastern equatorial Pacific. Fluid‐flow through the existing collapses and water current activity following the closing of the Central American Seaway likely led to the formation of the younger depressions. Drilling results, seismic reflection indicators of fluid pathways, and indicators of seafloor seepage suggests these depressions are actively guiding fluid outflow. Key Points The study provides three‐dimensional seismic tied to drilling results for understanding the development of mega‐depressions along the Cocos Ridge The depressions formed initially through dissolution and collapse, driven by volcanism and hydrothermal circulation The younger depressions likely formed through bottom current activity and fluid‐flow, the latter of which appears to be active today