Experimental and seismological constraints on the rheology, evolution, and alteration of the lithosphere at oceanic spreading centers

Abstract

Oceanic spreading centers are sites of magmatic, tectonic, and hydrothermal processes. In this thesis I present experimental and seismological constraints on the evolution of these complex regions of focused crustal accretion and extension. Experimental results from drained, triaxial deformation experiments on partially molten olivine reveal that melt extraction rates are linearly dependent on effective mean stress when the effective mean stress is low and non-linearly dependent on effective mean stress when it is high. Microearthquakes recorded above an inferred magma reservoir along the TAG segment of the Mid-Atlantic Ridge delineate for the first time the arcuate, subsurface structure of a long-lived, active detachment fault. This fault penetrates the entire oceanic crust and forms the high-permeability pathway necessary to sustain long-lived, high-temperature hydrothermal venting in this region. Long-lived detachment faulting exhumes lower crustal and mantle rocks. Residual stresses generated by thermal expansion anisotropy and mismatch in the uplifting, cooling rock trigger grain boundary microfractures if stress intensities at the tips of naturally occurring flaws exceed a critical stress intensity factor. Experimental results coupled with geomechanical models indicate that pervasive grain boundary cracking occurs in mantle peridotite when it is uplifted to within 4 km of the seafloor. Whereas faults provide the high-permeability pathways necessary to sustain high-temperature fluid circulation, grain boundary cracks form the interconnected network required for pervasive alteration of the oceanic lithosphere. This thesis provides fundamental constraints on the rheology, evolution, and alteration of the lithosphere at oceanic spreading centers.

Dependence of seismic coupling on normal fault style along the Northern Mid‐Atlantic Ridge

“…This idea is supported by the independent estimate of a ~2000-m coupled thickness at the TAG detachment fault [DeMartin, 2007], which is greater than at most other sections studied here. We thus propose that, to first-order along slow-spreading ridges, detachment faults are Recent numerical models of seismic cycles on transform and thrust faults based on rate and state friction theory argue that mixed-modes of fault slip may be favored when the characteristic nucleation size of frictional instabilities is comparable to the extent of the seismogenic zone [Liu et al, 2012].…”

“…Interestingly, the b-values determined here are higher than the teleseismic average, which could be partly due to the short time window spanned by the hydro-acoustic data set. Finally, it is noteworthy that these b-values fall between the global mean value for normal faulting events ($0.73) [Schorlemmer et al, 2005] and the b-value determined from a 1 year OBS deployment near the TAG detachment ($0.98) [DeMartin, 2007] ( Figure 6D and see section 5.2).…”

“…DB sections are defined by the occurrence of one or several detachment faults, which have a seafloor expression of finite along-axis extent. While the datasets analyzed in this study do not enable a precise determination of seismic coupling at the scale of an rate yielded R Σ = 2.1×10 15 J/yr/km by summing the moments of ~10,000 earthquakes[DeMartin, 2007]. Further, the Gutenberg-Richter distribution of this catalog has a β-~1000, much smaller than the number of recorded events.…”

Tectonic structure, evolution, and the nature of oceanic core complexes and their detachment fault zones (13°20′N and 13°30′N, Mid Atlantic Ridge)

“…Finally, the width of the chaotic terrain between the scarp and the initiation of corrugations in this study area (4–5 km, Figures and ) is consistent with that of major rift wall fault scarps, degraded by significant mass‐wasting [ Cannat et al ., ], and with associated retreat (Figure ). The mass‐wasted and rotated fault scarp can also undergo late footwall deformation associated with this flexural response of the lithosphere, including “snapping” and antithetic faulting away from the ridge [ Lavier et al ., ], and observed on surface faults [ Escartín et al ., ] and footwall microseismicity [ deMartin et al ., ]. Given that the corrugated surface is subhorizontal and even dips away from the axis, it is necessary that such deformation is accommodated within the chaotic terrain of both the 13°20′N and 13°30′N OCCs, as suggested earlier [ MacLeod et al ., ].…”

Deformation and surface uplift associated with serpentinization at mid‐ocean ridges and subduction zones

“…As discussed in Section 4.2, massive serpentinization ( ε 0 ≈ 40%) in the 3‐D elastic domain is required to cause the observed uplift at TAG (Figure 2). Here we consider a 2‐D inclusion, elongated horizontally along the detachment fault, and analyze the possible effect of serpentinization on both the detachment fault and on normal faults that are typically associated with detachment faults [e.g., Buck , 1988; Lister et al , 1986] and appear to be present at the TAG area [ deMartin , 2007]. Examples of such normal faults are shown in Figure 12a by three parallel lines inclined at the angle of θ = 70° to the horizontal and connecting the seafloor with the detachment fault.…”