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Supplementary material to “Detachments in Oceanic Lithosphere: Deformation, Magmatism, Fluid Flow, and Ecosystems”
25 January 2011
J. Escartín, Institut de Physique du Globe de Paris, CNRS, Paris, France
J. P. Canales, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
Citation:
Escartín, J., and J. P. Canales (2011), Detachments in oceanic lithosphere: Deformation, magmatism, fluid flow, and ecosystems, Eos Trans. AGU, 92(4), 31, doi:10.1029/2011EO040003. [Full Article (pdf)]
Chapman Conference on Detachments in Oceanic lithosphere: Deformation, Magmatism, Fluid Flow and Ecosystems
Agros, Cyprus, 8-15 May 2010
Convenors : J. Escartín1 & J.P. Canales2,
Scientific Committee : D. Blackman3, J. Cann4, M. Cannat1, G. Früh-Green5, B. John6, G. Manatschal7, A. McCaig4, E. Morisseau8, S. Nicolaides8, R. Sohn2, A.-L. Reysenbach9
1 CNRS - Institute de Physique du Globe de Paris, 1 Rue Jussieu, 75238 Paris Cedex 05, France
2 Woods Hole Oceanographic Institution, 360 Woods Hole Rd., Woods Hole, MA 02543, USA
3 Scripps Institution of Oceanography, La Jolla, CA 92093, USA
4 University of Leeds, Leeds, LS2 9JT, UK
5 Eidgenössische Technische Hochschule (ETH), 8092 Zurich, Switzerland
6 University of Wyoming, 1000 E. University Ave., Laramie, WY 82071, USA
7 Ecole et Observatoire des Sciences de la Terre-Université Louis Pasteur (EOST-UPL), 1 Rue Blessig, 67084 Strasbourg, France
8Cyprus Geological Survey Department, Lefkosia, Cyprus
9 Portland State University, 1719 SW 10th Ave., Portland, OR 97201, USA
'CHAPMAN MODEL' OF LITHOSPHERIC ACCRETION ASSOCIATED WITH OCEANIC DETACHMENT FAULTING
This model includes structural elements, architecture and composition of the lithosphere and active processes at the ridge axis. Many elements and details of the model are currently weakly constrained (e.g., size, abundance and distribution of gabbro bodies, hydrothermal flow paths, fault geometry at depth, distribution of alteration).
SUMMARY OF OVERVIEW TALKS BY TOPIC
The Detachment Fault Zone and the Core Complex Beneath it, by M. Cannat and M. Cheadle
Detachment faulting occurs along ~50% of ridge length in the Northern Mid-Atlantic Ridge, providing a mechanism for efficient exhumation of deep-seated mantle and lower crustal rocks.There is a wide range of morphological characteristics of the breakaway, termination, and the overall core complex structure that is not yet understood.The morphology of the exposed fault surface ranges from corrugated domed massifs to elongated hills. The geometry of the fault at the seafloor indicates a very low effective elastic thickness, leading to intense deformation in the footwall. The plausible mechanisms that produce this efficient weakening may include a direct link between exhumation, deformation, hydrothermal circulation, formation of weak alteration phases, and emplacement of magmatic bodies within the core of the detachment.
Detachments also record a long history of efficient strain localization that can span several millions of years. While the deformation history may show an overall evolution from high-temperature, plastic deformation to lower-temperature, brittle deformation, there is an inherent complexity due to the variable lithology/alteration of the host rock, the presence of water in rocks, and the circulation of fluids. The ubiquitous presence of talc-chlorite schists provides the weakening mechanism that allows the fault to operate at low angles and localize strain over long periods of time.
While detachments operate, accretion is extremely asymmetric and the detachment fault takes up to 70-80% of the plate separation. This asymmetric mode of accretion implies a modification of the ridge axis geometry over long periods of time, and a possible variation of the fault geometry and curvature at depth.
Geophysical Framework, by A. Morris and R. Sohn
Geophysical observations constrain the geometry of the detachment at depth and the nature of the footwall’s internal structure. Fault geometry at depth has been inferred at the TAG detachment, where the fault is steep (~70°) below the ridge axis and shallows to ~20° at the seafloor, requiring significant rotation of the footwall. Footwall rotation is also supported by paleomagnetic data from cores drilled at different OCCs, which suggest rotations ranging from 15° to >70°. The magnetic remanence of young lower crustal gabbros sampled in the footwall of oceanic detachments is variable in its complexity, and potentially provide insights into a range of processes and their relative timing (e.g. cooling of magmatic bodies, onset of rotation, timing of serpentinization).
Fluid Flow in Detachment Faults and Core Complexes, by A. McCaig and G. Frueh-Green
Oceanic detachment faulting is associated with hydrothermal fields at the seafloor. Fault-focused flow may produce sites such as TAG, while hydrothermal flow within the footwall may produce high- and low-temperature systems (Logatchev, Lost City). Flow within the fault zone may develop feedbacks between hydrothermal activity, deformation, exhumation, and internal thermal structure. Exhumation of mantle promotes serpentinization. Key aspects affecting fluid-rock interaction are the depth and mechanisms of seawater penetration, the role of detachment faults, and consequences of heterogeneous lithosphere for alteration and mass transfer. Alteration is controlled by rock composition, temperature, kinetics, and variations in activities in elements such as Si, Mg and Fe, and results in a change of physical properties of the rock. Early alteration phases are restricted to pyroxene hydration, while talc assemblages are related to focused fluid flow and high degrees of mass transfer. Late phases of fluid flow involve carbonate fill-in of veins. Pervasive serpentinization is limited to olivine breakdown, with maximum reaction rates at ~250°C. These reactions are a sink of C, B, U and a source of Ca, Ni and Si in the hydrothermal fluids, in addition to H2 and CH4. We still need to understand the flow pathways in peridotite-hosted systems, and obtain estimates of volumes of gabbro rock/melt, as these have consequences for the quantification of mass and heat fluxes as well as the composition of fluids through this lithosphere.
Ecology of Hydrothermal Systems Associated with Detachment Faulting, by D. Kelley and M. Perner
Fluid composition has a profound impact on the active microbial assemblages at hydrothermal vents. Microbially mediated H2 oxidation stimulates CO2 fixation, a process that is not necessarily bound to peridotite-hosted hydrothermal fields, but can also occur in H2-poor systems. Even in comparatively H2-poor vent systems, H2 utilizing microbes can benefit from low H2-concentrations and can have the capacity to consume substantial amounts of H2, with the energy gained being available for driving biomass synthesis. Functional genes encoding key enzymes of H2 oxidizing metabolisms (Hydrogenases) appear to be grouped according to a) H2 concentrations, and b) the amount of admixed oxygenated seawater. This suggests that H2 concentrations can contribute to influencing parts of the H2-oxidizing diversity. 16S rNA genes do not predict metabolism associated with different types of sites as they are not limited to chemosynthetic organisms. Instead analysis of functional genes and turn over rate studies are required.
Moderate to low-temperature fluid-rock reactions in ultramafic rocks result in alkaline fluids with high concentrations of abiogenically produced hydrocarbons that support microbial communities. Considering the global distribution of ultramafic environments and the potential importance of these systems to the origin of life and to models of Earths’ earliest microbial ecosystems, study of Lost City has the potential to yield new discoveries with implications for understanding the linkages between abiotic water-rock reactions and microbial evolution.
Implications for Continental Core Complexes and Detachment Faulting During Continental Breakup, by B. John and G. Manatschal
Continental core complexes share similarities with their oceanic counterparts, including a domal culmination of crust (and mantle), exposure of footwall material, presence of corrugations parallel to extension, asymmetric extension, a complex fault zone with anostomosing shear zones and a down-temperature fault rock history, and extension of up to several 10’s of km over time periods of up to several million years. Detachment faulting may also explain continental thinning and exhumation of the mantle along rifted margins, under conditions of reduced magma supply during extension. These outcrops record pervasive fluid reactions that alter the rocks and contribute to the reactivation of faults along the margins. Faults display a complex structural evolution of the detachment, with ductile deformation coupled with low-temperature deformation. Extension is also associated with serpentinization, as in the case of oceanic faults, which plays a role in the localization and evolution of these fault zones.
The main differences between detachment faulting in the oceans and those in continents and margins are related to fault geometry and overall rheology. Continental detachment faults appear to initiate at a low angle, instead of the high-angle observed at the TAG OCC. The continental faults record deformation in the brittle domain, and development of weak gouges, leading to a fault weakening similar to that of oceanic detachment faults. Rheology in continents is controlled by the mechanical properties of quartz, which result in a much thinner lithosphere than in the oceanic environment.
New Frontiers in Oceanic Core Complex Research, by P. Kelemen and A.-L. Reysenbach
Carbonation of olivine-rich rocks is a possible and major sink of CO2, provided that a sufficient volume of unaltered rock is available, and a sufficiently fast reaction process is established. Major challenges include the cost and feasibility of CO2 transport to storage sites, and the possibility of establishing a self-sustaining reaction process. Potentially, carbonation, which occurs in the oceanic environment and along oceanic detachments where mantle is exhumed, could be enhanced and activated, removing CO2 from the water column and with no need for CO2 transport and injection.
Oceanic detachments also host long-lived, high-and low-temperature hydrothermal systems rich in hydrogen to sustain a unique and complex biosphere. New genetic techniques such as high throughput DNA sequencing are pointing to a much greater diversity and complexity of novel micro-organisms in deep-sea vents, and to a distinct identity of these populations for each vent field. Studies of these environments have implications to understand the possible conditions for subsurface biosphere elsewhere on Earth, and in other planets and moons across the Solar System.