Quantitative constraint on footwall rotations at the 15°45′N oceanic core complex, Mid‐Atlantic Ridge: Implications for oceanic detachment fault processes

Abstract

[1] The subsurface geometry of detachment faults at slow spreading mid-ocean ridges is debated: are they planar features that form and slip at low angles, as often inferred for their continental equivalents, or do they initiate at steep angles and then flatten in response to flexural unloading as displacement proceeds, as predicted in "rolling hinge" conceptual models? An essential difference is that significant rotation of the footwall should occur in the rolling hinge but not the planar fault model. This can be tested using paleomagnetism. Previous attempts to address this question have relied upon data from azimuthally unoriented drill cores. Although results are consistent with large rotations having occurred, these interpretations are very nonunique, and other solutions that require minimal rotations are equally permissible. We here present a rigorous analysis of paleomagnetic and structural data from a unique set of azimuthally oriented cores, collected using a seabed rock drill, from the 15°45′N oceanic core complex on the MidAtlantic Ridge. By considering the full paleomagnetic remanence vector in combination with kinematic data from the detachment fault shear zone we are able to quantitatively constrain the geometrically permissible axes and magnitudes of rotation of the detachment fault footwall, for the first time without having to make a priori assumptions about the orientation of the axis. We show that significant rotations (64°± 16°) have indeed occurred, about a gently plunging, near-ridge-parallel axis, robustly supporting the rolling hinge models. We further discuss the geological implications of this result for oceanic detachment fault processes.

Paleointensity estimates from oceanic gabbros: Effects of hydrothermal alteration and cooling rate

“…304/305 samples). The oceanic gabbro layer formed at slow-spreading ridges is considered to be tilted due to detachment faulting (Garcés and Gee, 2007;Morris et al, 2009;MacLeod et al, 2011), and their present remanence inclinations may not reflect the geomagnetic inclination at the time of remanence acquisition. I obtained the mean VADM of 8.2 ± 2.1 10 22 Am 2 after the anisotropy correction (Selkin et al, 2000); before the correction, the mean was 8.3±2.3 10 22 Am 2 (Table 5).…”

Recognizing detachment-mode seafloor spreading in the deep geological past

“…These surfaces represent exposed, now-inactive low-angle fault planes that have been directly responsible for the exhumation of mantle and plutonic lithologies onto the seafloor. Paleomagnetic analyses of OCC footwall sections sampled by scientific ocean drilling along the Mid-Atlantic Ridge91011 have demonstrated that this unroofing is characteristically accommodated by tectonic rotation around ridge-parallel, shallowly plunging axes, consistent with flexural, isostatic rolling-hinge deformation1213.…”

Geologic implications of seafloor character and carbonate lithification imaged on the domal core of Atlantis Massif

“…Hydrothermal systems hosted by this formation could be related to the alteration of the gabbroic core and/or serpentinization of interfingered ultramafic formations (Andreani et al, 2014;McCaig et al, 2010;Blackman et al, 2014). The origin and extent of possible hydrothermal circulation processes in the subsurface of OCCs and their exact fluid pathways have been discussed extensively in previous studies; yet, the direct evidence remains very sparse except for fluid seepage found from seafloor observations (Andreani et al, 2014;Delacour et al, 2008aDelacour et al, , 2008bEscartín et al, 2008;Früh-Green et al, 2003;Karson et al, 2006;Kelley et al, 2001Kelley et al, , 2005Ludwig et al, 2006Ludwig et al, , 2011Macleod et al, 2011;Tucholke et al, 2013). It has been observed that nearly all ultramafic and gabbroic rocks recovered from the upper 800 m of IODP Hole U1309D on the Central Dome, located in close proximity to our prevalent lithified carbonate crust at Argo II track A39 (Fig.…”