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ArticleOceanic detachment faults generate compression in extension(Geological Society of America, 2017-08) Parnell-Turner, Ross ; Sohn, Robert A. ; Peirce, Christine ; Reston, Tim J. ; MacLeod, Christopher J. ; Searle, Roger C. ; Simão, Nuno MendesIn extensional geologic systems such as mid-ocean ridges, deformation is typically accommodated by slip on normal faults, where material is pulled apart under tension and stress is released by rupture during earthquakes and magmatic accretion. However, at slowly spreading mid-ocean ridges where the tectonic plates move apart at rates <80 km m.y.–1, these normal faults may roll over to form long-lived, low-angled detachments that exhume mantle rocks and form corrugated domes on the seabed. Here we present the results of a local micro-earthquake study over an active detachment at 13°20′N on the Mid-Atlantic Ridge to show that these features can give rise to reverse-faulting earthquakes in response to plate bending. During a 6 month survey period, we observed a remarkably high rate of seismic activity, with >244,000 events detected along 25 km of the ridge axis, to depths of ∼10 km below seafloor. Surprisingly, the majority of these were reverse-faulting events. Restricted to depths of 3–7 km below seafloor, these reverse events delineate a band of intense compressional seismicity located adjacent to a zone of deeper extensional events. This deformation pattern is consistent with flexural models of plate bending during lithospheric accretion. Our results indicate that the lower portion of the detachment footwall experiences compressive stresses and deforms internally as the fault rolls over to low angles before emerging at the seafloor. These compressive stresses trigger reverse faulting even though the detachment itself is an extensional system.
ArticleDrilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid-Atlantic Ridge 30°N(American Geophysical Union, 2011-07-19) Blackman, Donna K. ; Ildefonse, Benoit ; John, Barbara E. ; Ohara, Y. ; Miller, D. J. ; Abe, Natsue ; Abratis, M. ; Andal, E. S. ; Andreani, Muriel ; Awaji, S. ; Beard, J. S. ; Brunelli, Daniele ; Charney, A. B. ; Christie, D. M. ; Collins, John A. ; Delacour, A. G. ; Delius, H. ; Drouin, M. ; Einaudi, F. ; Escartin, Javier E. ; Frost, B. R. ; Fruh-Green, Gretchen L. ; Fryer, P. B. ; Gee, Jeffrey S. ; Grimes, C. B. ; Halfpenny, A. ; Hansen, H.-E. ; Harris, Amber C. ; Tamura, A. ; Hayman, Nicholas W. ; Hellebrand, Eric ; Hirose, T. ; Hirth, Greg ; Ishimaru, S. ; Johnson, Kevin T. M. ; Karner, G. D. ; Linek, M. ; MacLeod, Christopher J. ; Maeda, J. ; Mason, O..U. ; McCaig, A. M. ; Michibayashi, K. ; Morris, Antony ; Nakagawa, T. ; Nozaka, Toshio ; Rosner, Martin ; Searle, Roger C. ; Suhr, G. ; Tominaga, Masako ; von der Handt, A. ; Yamasaki, T. ; Zhao, XixiExpeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100–220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45° rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises ∼70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.
ArticleHydrothermal vent fields and chemosynthetic biota on the world's deepest seafloor spreading centre(Nature Publishing Group, 2012-01-10) Connelly, Douglas P. ; Copley, Jonathan T. ; Murton, Bramley J. ; Stansfield, Kate ; Tyler, Paul A. ; German, Christopher R. ; Van Dover, Cindy L. ; Amon, Diva ; Furlong, Maaten ; Grindlay, Nancy ; Hayman, Nicholas W. ; Huhnerbach, Veit ; Judge, Maria ; Le Bas, Tim ; McPhail, Stephen ; Meier, Alexandra ; Nakamura, Ko-ichi ; Nye, Verity ; Pebody, Miles ; Pedersen, Rolf B. ; Plouviez, Sophie ; Sands, Carla M. ; Searle, Roger C. ; Stevenson, Peter ; Taws, Sarah ; Wilcox, SallyThe Mid-Cayman spreading centre is an ultraslow-spreading ridge in the Caribbean Sea. Its extreme depth and geographic isolation from other mid-ocean ridges offer insights into the effects of pressure on hydrothermal venting, and the biogeography of vent fauna. Here we report the discovery of two hydrothermal vent fields on the Mid-Cayman spreading centre. The Von Damm Vent Field is located on the upper slopes of an oceanic core complex at a depth of 2,300 m. High-temperature venting in this off-axis setting suggests that the global incidence of vent fields may be underestimated. At a depth of 4,960 m on the Mid-Cayman spreading centre axis, the Beebe Vent Field emits copper-enriched fluids and a buoyant plume that rises 1,100 m, consistent with > 400 °C venting from the world’s deepest known hydrothermal system. At both sites, a new morphospecies of alvinocaridid shrimp dominates faunal assemblages, which exhibit similarities to those of Mid-Atlantic vents.
ArticleSeismicity trends and detachment fault structure at 13 degrees N, Mid-Atlantic Ridge(Geological Society of America, 2020-11-04) Parnell-Turner, Ross ; Sohn, Robert A. ; Peirce, Christine ; Reston, Tim J. ; MacLeod, Christopher J. ; Searle, Roger C. ; Simão, Nuno MendesAt slow-spreading ridges, plate separation is commonly partly accommodated by slip on long-lived detachment faults, exposing upper mantle and lower crustal rocks on the seafloor. However, the mechanics of this process, the subsurface structure, and the interaction of these faults remain largely unknown. We report the results of a network of 56 ocean-bottom seismographs (OBSs), deployed in 2016 at the Mid-Atlantic Ridge near 13°N, that provided dense spatial coverage of two adjacent detachment faults and the intervening ridge axis. Although both detachments exhibited high levels of seismicity, they are separated by an ∼8-km-wide aseismic zone, indicating that they are mechanically decoupled. A linear band of seismic activity, possibly indicating magmatism, crosscuts the 13°30′N domed detachment surface, confirming previous evidence for fault abandonment. Farther south, where the 2016 OBS network spatially overlapped with a similar survey done in 2014, significant changes in the patterns of seismicity between these surveys are observed. These changes suggest that oceanic detachments undergo previously unobserved cycles of stress accumulation and release as plate spreading is accommodated.