Crustal structure of a rifted oceanic core complex and its conjugate side at the MAR at 5°S: implications for melt extraction during detachment faulting and core complex formation.

Planert, Lars, Flueh, Ernst R., Tilmann, Frederik, Grevemeyer, Ingo and Reston, Timothy J. (2010) Crustal structure of a rifted oceanic core complex and its conjugate side at the MAR at 5°S: implications for melt extraction during detachment faulting and core complex formation. Open Access Geophysical Journal International, 181 (1). pp. 113-126. DOI 10.1111/j.1365-246X.2010.04504.x.

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Abstract

We present results of a seismic refraction experiment which determines the crustal and upper-mantle structure of an oceanic core complex (OCC) and its conjugate side located south of the 5°S ridge–transform intersection at the Mid-Atlantic Ridge. The core complex with a corrugated surface has been split by a change in location of active seafloor spreading, resulting in two massifs on either side of the current spreading axis.

We applied a joint tomographic inversion of wide-angle reflected and refracted phases for five intersecting seismic profiles. The obtained velocity models are used to constrain the magmatic evolution of the core complex from the analysis of seismic layer 3 and crustal thickness. An abrupt increase of crustal velocities at shallow depth coincides with the onset of the seafloor corrugations at the exposed footwall. The observed velocity structure is consistent with the presence of gabbros directly beneath the corrugated fault surface. The thickness of the high-velocity body is constrained by PmP reflections to vary along and across axis between <3 and 5 km. The thickest crust is associated with the central phase of detachment faulting at the higher-elevated northern portion of the massif. Beneath the breakaway of the OCC the crust is 2.5 km thick and reveals significantly lower velocities. This implies that the fault initially exhumed low-velocity material overlying the gabbro plutons. In contrast, crust formed at the conjugate side during OCC formation is characterized by an up to 2-km-thick seismic layer 2 overlying a 1.7-km-thick seismic layer 3. Obtained upper-mantle velocities range from 7.3 to 7.9 km s−1 and seem to increase with distance from the median valley. However, velocities of 7.3–7.5 km s−1 beneath the older portions of the OCC may derive from deep fluid circulation and related hydrothermal alteration, which may likely be facilitated by the subsequent rifting.

Our velocity models reveal a strongly asymmetric velocity structure across the ridge axis, associated with the accretion of gabbros into the footwall of the detachment fault and upper-crustal portions concentrated at the conjugate side. Our results do not support a substantial increase in the axial ridge's melt supply related to the final phase of detachment faulting. Hence, the footwall rifting at 5°S may be a generic mechanism of detachment termination under very low melt conditions, as predicted by recent numerical models of Tucholke et al.

Document Type: Article
Additional Information: This article has been published in Geophysical Journal International ©2010 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Keywords: Composition of the oceanic crust, Seismic tomography, Mid-ocean ridge processes, Dynamics and mechanics of faulting, Kinematics of crustal and mantle deformation, Atlantic Ocean
Research affiliation: OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-GDY Marine Geodynamics
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1111/j.1365-246X.2010.04504.x
ISSN: 0956-540X
Projects: Future Ocean
Expeditions/Models/Experiments:
Date Deposited: 23 Mar 2010 11:01
Last Modified: 10 Jul 2018 12:53
URI: http://oceanrep.geomar.de/id/eprint/6951

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