Smoke Without Fire: How Long Can Thermal Cracking Sustain Hydrothermal Circulation in the Absence of Magmatic Heat?

International audienceLong-lived hydrothermal circulation is now well documented along slow and ultraslow spreading mid-ocean ridges, even though these settings only receive a moderate and intermittent supply of magma. This challenges the notion that hydrothermal convection must be sustained by a continuously replenished magma body. Here we investigate the possibility of sustaining hydrothermal circulation by infrequent magmatic intrusions separated by episodes of downward propagation of small cracks enabling fluids to tap heat from deep hot rocks. We focus on cracks nucleating from grain boundary flaws in response to the buildup of cooling stresses at the base of the convection system. We develop an analytical model describing the stable propagation of a percolation front and the associated heat transfer through hydrothermal circulation. Convection ceases when thermoelastic stresses can no longer overcome lithostatic pressure and hydrothermal circulation can no longer mine heat from underlying units. For lithospheric permeabilities greater than ∼10 −15 m 2 and over a wide range of grain and flaw sizes, this occurs within ∼100 kyr to ∼1 Myr from the onset of cracking, after the cracking front has moved by a few kilometers. We validate this analytical prediction by developing two-dimensional numerical models of porous convection subjected to a lower boundary condition representing the dynamics of the cracking front. These models suggest that moderate-to low-temperature hydrothermal venting in off-axis, ultramafic-hosted sites does not require an underlying magma sill at all times, but instead repeated sill intrusions every ∼10-100 kyr, which periodically reinvigorate hydrothermal convection without building a continuous crustal section

Rapid rotation of normal faults due to flexural stresses : an explanation for the global distribution of normal fault dips

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 119 (2014): 3722–3739, doi:10.1002/2013JB010512.We present a mechanical model to explain why most seismically active normal faults have dips much lower (30–60°) than expected from Anderson-Byerlee theory (60–65°). Our model builds on classic finite extension theory but incorporates rotation of the active fault plane as a response to the buildup of bending stresses with increasing extension. We postulate that fault plane rotation acts to minimize the amount of extensional work required to sustain slip on the fault. In an elastic layer, this assumption results in rapid rotation of the active fault plane from ~60° down to 30–45° before fault heave has reached ~50% of the faulted layer thickness. Commensurate but overall slower rotation occurs in faulted layers of finite strength. Fault rotation rates scale as the inverse of the faulted layer thickness, which is in quantitative agreement with 2-D geodynamic simulations that include an elastoplastic description of the lithosphere. We show that fault rotation promotes longer-lived fault extension compared to continued slip on a high-angle normal fault and discuss the implications of such a mechanism for fault evolution in continental rift systems and oceanic spreading centers.This work was supported by NSF grants OCE-1154238 and EAR-1010432.2014-10-2

Modes of extensional faulting controlled by surface processes

We investigate the feedbacks between surface processes and tectonics in an extensional setting by coupling a 2-D geodynamical model with a landscape evolution law. Focusing on the evolution of a single normal fault, we show that surface processes significantly enhance the amount of horizontal extension a fault can accommodate before being abandoned in favor of a new fault. In simulations with very slow erosion rates, a 15 km thick brittle layer extends via a succession of crosscutting short-lived faults (heave 10 km). Using simple scaling arguments, we quantify the effect of surface mass removal on the force balance acting on a growing normal fault. This leads us to propose that the major range-bounding normal faults observed in many continental rifts owe their large offsets to erosional and depositional processes

Mechanism for normal faulting in the subducting plate at the Mariana Trench

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 4309–4317, doi:10.1002/2015GL063917.We investigate the mechanisms of normal fault initiation and evolution in the subducting Pacific Plate near the Mariana Trench, through bathymetry analysis and geodynamic modeling. We model the subducting plate as an elastoplastic slab subjected to tectonic forcing at the trench, including vertical load, bending moment, and horizontal tensional force. In our simulations, normal faults initiate within the outer rise region and reach maximum throw toward the trench. This result holds over a wide range of tectonic forcing and is consistent with observations of the Challenger Deep region, where multibeam bathymetry data indicate faults initiate near the outer rise at 70–110 km from the trench and reach maximum throw at 10–35 km from the trench. However, models require a horizontal tensional force with magnitude comparable to axial vertical load to jointly explain the observed seafloor bathymetry, location of maximum normal fault throw, and prevalence of normal faults dipping toward the trench.This work was supported by the Mariana Trench Project of the South China Sea Institute of Oceanology of Chinese Academy of Sciences, Chinese National 985 project 1350141509, Ministry of Science and Technology 973 project award 2012CB417303, and Chinese Scholarship Council 201406260134.2015-12-0

The role of elasticity in simulating long-term tectonic extension

Author Posting. © Oxford University Press, 2016. This article is posted here by permission of Oxford University Press for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 205 (2016): 728-743, doi:10.1093/gji/ggw044.While elasticity is a defining characteristic of the Earth's lithosphere, it is often ignored in numerical models of long-term tectonic processes in favour of a simpler viscoplastic description. Here we assess the consequences of this assumption on a well-studied geodynamic problem: the growth of normal faults at an extensional plate boundary. We conduct 2-D numerical simulations of extension in elastoplastic and viscoplastic layers using a finite difference, particle-in-cell numerical approach. Our models simulate a range of faulted layer thicknesses and extension rates, allowing us to quantify the role of elasticity on three key observables: fault-induced topography, fault rotation, and fault life span. In agreement with earlier studies, simulations carried out in elastoplastic layers produce rate-independent lithospheric flexure accompanied by rapid fault rotation and an inverse relationship between fault life span and faulted layer thickness. By contrast, models carried out with a viscoplastic lithosphere produce results that may qualitatively resemble the elastoplastic case, but depend strongly on the product of extension rate and layer viscosity U × ηL. When this product is high, fault growth initially generates little deformation of the footwall and hanging wall blocks, resulting in unrealistic, rigid block-offset in topography across the fault. This configuration progressively transitions into a regime where topographic decay associated with flexure is fully accommodated within the numerical domain. In addition, high U × ηL favours the sequential growth of multiple short-offset faults as opposed to a large-offset detachment. We interpret these results by comparing them to an analytical model for the fault-induced flexure of a thin viscous plate. The key to understanding the viscoplastic model results lies in the rate-dependence of the flexural wavelength of a viscous plate, and the strain rate dependence of the force increase associated with footwall and hanging wall bending. This behaviour produces unrealistic deformation patterns that can hinder the geological relevance of long-term rifting models that assume a viscoplastic rheology.This work was supported by NSF grants OCE-11-54238 (JAO, MDB), EAR-10-10432 (MDB) and OCE-11-55098 (GI), as well as a WHOI Deep Exploration Institute grant and start-up support from the University of Idaho (EM)

Magmatic and tectonic extension at the Chile Ridge: Evidence for mantle controls on ridge segmentation

We use data from an extensive multibeam bathymetry survey of the Chile Ridge to study tectonomagmatic processes at the ridge axis. Specifically, we investigate how abyssal hills evolve from axial faults, how variations in magmatic extension influence morphology and faulting along the spreading axis, and how these variations correlate with ridge segmentation. The bathymetry data are used to estimate the fraction of plate separation accommodated by normal faulting, and the remaining fraction of extension, M, is attributed primarily to magmatic accretion. Results show that M ranges from 0.85 to 0.96, systematically increasing from first-order and second-order ridge segment offsets toward segment centers as the depth of ridge axis shoals relative to the flanking highs of the axial valley. Fault spacing, however, does not correlate with ridge geometry, morphology, or M along the Chile Ridge, which suggests the observed increase in tectonic strain toward segment ends is achieved through increased slip on approximately equally spaced faults. Variations in M along the segments follow variations in petrologic indicators of mantle melt fraction, both showing a preferred length scale of 50620 km that persists even along much longer ridge segments. In comparison, mean M and axial relief fail to show significant correlations with distance offsetting the segments. These two findings suggest a form of magmatic segmentation that is partially decoupled from the geometry of the plate boundary. We hypothesize this magmatic segmentation arises from cells of buoyantly upwelling mantle that influence tectonic segmentation from the mantle, up

Quantifying diffuse and discrete venting at the Tour Eiffel vent site, Lucky Strike hydrothermal field

Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 13 (2012): Q04008, doi:10.1029/2011GC003991.The relative heat carried by diffuse versus discrete venting of hydrothermal fluids at mid-ocean ridges is poorly constrained and likely varies among vent sites. Estimates of the proportion of heat carried by diffuse flow range from 0% to 100% of the total axial heat flux. Here, we present an approach that integrates imagery, video, and temperature measurements to accurately estimate this partitioning at a single vent site, Tour Eiffel in the Lucky Strike hydrothermal field along the Mid-Atlantic Ridge. Fluid temperatures, photographic mosaics of the vent site, and video sequences of fluid flow were acquired during the Bathyluck'09 cruise (Fall, 2009) and the Momarsat'10 cruise (Summer, 2010) to the Lucky Strike hydrothermal field by the ROV Victor6000 aboard the French research vessel the “Pourquoi Pas”? (IFREMER, France). We use two optical methods to calculate the velocities of imaged hydrothermal fluids: (1) for diffuse venting, Diffuse Flow Velocimetry tracks the displacement of refractive index anomalies through time, and (2) for discrete jets, Particle Image Velocimetry tracks eddies by cross-correlation of pixel intensities between subsequent images. To circumvent video blurring associated with rapid velocities at vent orifices, exit velocities at discrete vents are calculated from the best fit of the observed velocity field to a model of a steady state turbulent plume where we vary the model vent radius and fluid exit velocity. Our results yield vertical velocities of diffuse effluent between 0.9 cm s−1 and 11.1 cm s−1 for fluid temperatures between 3°C and 33.5°C above that of ambient seawater, and exit velocities of discrete jets between 22 cm s−1 and 119 cm s−1 for fluid temperatures between 200°C and 301°C above ambient seawater. Using the calculated fluid velocities, temperature measurements, and photo mosaics of the actively venting areas, we calculate a heat flux due to diffuse venting from thin fractures of 3.15 ± 2.22 MW, discrete venting of 1.07 ± 0.66 MW, and, by incorporating previous estimates of diffuse heat flux density from Tour Eiffel, diffuse flux from the main sulfide mound of ∼15.6 MW. We estimate that the total integrated heat flux from the Tour Eiffel site is 19.82 ± 2.88 MW and that the ratio of diffuse to discrete heat flux is ∼18. We discuss the implication of these results for the characterization of different vent sites within Lucky Strike and in the context of a compilation of all available measurements of the ratio of diffuse to discrete heat flux.E. Mittelstaedt was supported by the International Research Fellowship Program of the U.S. National Science Foundation (OISE-0757920). Funding for the 2006, 2008, 2009, and 2010 cruises was provided by CNRS/ IFREMER through the MoMAR program (France), by ANR (France), the Mothseim Project NT05–3 42213 to J. Escartín and by grant CTM2010–15216/MAR from the Spanish Ministry of Science to R. Garcia and J. Escartín. T. Barreyre was supported by University Paris Diderot (Paris 7 – France) and Institut de Physique du Globe de Paris (IPGP, France).2012-10-1

Hydrothermally-induced melt lens cooling and segmentation along the axis of fast- and intermediate-spreading centers

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 38 (2011): L14307, doi:10.1029/2011GL047798.The heat output and thermal regime of fast and intermediate spreading centers are strongly controlled by boundary layer processes between the hydrothermal system and the underlying crustal magma chamber (AMC), which remain to be fully understood. Here, we model the interactions between a shallow two-dimensional cellular hydrothermal system at temperatures <700°C, and a deeper AMC at temperatures up to 1200°C. We show that hydrothermal cooling can freeze the AMC in years to decades, unless melt injections occur on commensurate timescales. Moreover, the differential cooling between upflow and downflow zones can segment the AMC into mush and melt regions that alternate on sub-kilometric length scales. These predictions are consistent with along-axis variations in AMC roof depth observed in ophiolites and oceanic settings. In this respect, fine-scale geophysical investigations of the structure of AMCs may help constrain hydrothermal recharge locations associated with active hydrothermal sites

Mid-Ocean Ridges: Geodynamics Written in the Seafloor

International audienceMid-ocean ridges are divergent plate boundaries where new oceanic crust and lithosphere form through a complex interplay of magmatic, hydrothermal and tectonic processes. Magma is generated tens of km below the ridge by adiabatic decompression of upwelling asthenosphere. It ascends and collects at the ridge axis, where it is emplaced as oceanic crust, bringing heat that fuels hydrothermal convection. Repeated intrusions of melt into the lithosphere accommodate a sizeable fraction of plate divergence, thereby modulating the properties of faults that take up the remaining fraction. This chapter uses simple analytical models to draw connections between the thermal state of the sub-ridge asthenosphere, its magmatic productivity, and the thermomechanical properties of young oceanic lithosphere, which deforms in the presence of magma to shape the Earth's seafloor. This provides first-order guidelines to interpret the ~200 Myr-long seafloor record in terms of the transient dynamics of the upper mantle

Mechanical and geological controls on the long-term evolution of normal faults

Thesis: Ph. D., Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 173-186).This thesis investigates the long-term evolution of rift-bounding normal faults. To first order, the observed diversity of extensional tectonic styles reflects differences in the maximum offset that can be accommodated on individual faults during their life span. My main objective is to develop a theoretical framework that explains these differences in terms of a few key mechanical and geological controls. I start by laying out the energy cost associated with slip on a normal fault, which consists of (1) overcoming the frictional resistance on the fault, (2) bending the faulted layer and (3) sustaining the growth of topography. In Chapter 2, I propose that flexural rotation of the active fault plane enables faults to evolve along a path of minimal energy, thereby enhancing their life span. Flexural rotation occurs more rapidly in thinner faulted layers, and can potentially explain the wide range of normal fault dips documented with focal mechanisms. In Chapter 3, I show that surface processes can enhance the life span of continental normal faults by reducing the energy cost associated with topography buildup. In Chapter 4, I focus on lithospheric bending induced by fault growth, which is well described by elasto-plastic flexure models. I demonstrate that numerical models that treat the lithosphere as a visco-plastic solid can properly predict fault evolution only when the rate-dependent viscous flexural wavelength of the lithosphere is accommodated within the numerical domain. In Chapter 5, I consider the interplay of faulting and crustal emplacement at a slow mid-ocean ridge. I show that a depth-variable rate of magma emplacement can reconcile the formation of long-lived detachment faults, which requires a moderate melt supply, and the exhumation of large volumes of lower crustal material. Finally, in Chapter 6 I investigate the three-dimensional interactions between normal faults in a lithosphere of varying thickness. I suggest that large along-axis gradients in lithospheric thickness can prevent the growth of continuous faults along-axis, and instead decouple the modes of faulting at the segment center and at the segment end.by Jean-Arthur Louis Olive.Ph. D