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Footwall rotation in an oceanic core complex quantified using reoriented Integrated Ocean Drilling Program core samples

Footwall rotation in an oceanic core complex quantified using reoriented Integrated Ocean Drilling Program core samples
Footwall rotation in an oceanic core complex quantified using reoriented Integrated Ocean Drilling Program core samples
Oceanic core complexes expose lower crustal and upper mantle rocks on the seafloor by tectonic unroofing in the footwalls of large-slip detachment faults. The common occurrence of these structures in slow and ultra-slow spread oceanic crust suggests that they accommodate a significant component of plate divergence. However, the subsurface geometry of detachment faults in oceanic core complexes remains unclear. Competing models involve either: (a) displacement on planar, low-angle faults with little tectonic rotation; or (b) progressive shallowing by rotation of initially steeply dipping faults as a result of flexural unloading (the “rolling-hinge” model). We address this debate using palaeomagnetic remanences as markers for tectonic rotation within a unique 1.4 km long footwall section of gabbroic rocks recovered by Integrated Ocean Drilling Program (IODP) sampling at Atlantis Massif oceanic core complex on the Mid-Atlantic Ridge (MAR). These rocks contain a complex record of multipolarity magnetizations that are unrelated to alteration and igneous stratigraphy in the sampled section and are inferred to result from progressive cooling of the footwall section over geomagnetic polarity chrons C1r.2r, C1r.1n (Jaramillo) and C1r.1r. For the first time we have independently reoriented drill-core samples of lower crustal gabbros, that were initially azimuthally unconstrained, to a true geographic reference frame by correlating structures in individual core pieces with those identified from oriented imagery of the borehole wall. This allows reorientation of the palaeomagnetic data, placing far more rigorous constraints on the tectonic history than those possible using only palaeomagnetic inclination data. Analysis of the reoriented high temperature reversed component of magnetization indicates a 46° ± 6° anticlockwise rotation of the footwall around a MAR-parallel horizontal axis trending 011° ± 6°. Reoriented lower temperature components of normal and reversed polarity suggest that much of this rotation occurred after the end of the Jaramillo chron (0.99 Ma). The data provide unequivocal confirmation of the key prediction of flexural, rolling-hinge models for oceanic core complexes, whereby oceanic detachment faults initiate at higher dips and rotate to their present day low-angle geometries as displacement increases.

IODP, oceanic core complex, rotation, palaeomagnetism
0012-821X
217-228
Morris, A.
378f6b9c-92e7-4783-b7be-cafca74b5372
Gee, J.S.
a889ff3f-9236-4ca3-80bb-c01131faa432
Pressling, N.
1202b863-d882-4529-b4d0-be2cec9da269
John, B.E.
a4839cce-7cc0-46b4-83ad-3fcafd03ac08
MacLeod, C.J.
1dbf6306-7bba-4fb3-90ec-61b89aee30ba
Grimes, C.B.
47957887-a3b5-45ad-bd14-47db5815dda1
Searle, R.C.
aacbd084-421e-4813-86b7-ac7d84da376f
Morris, A.
378f6b9c-92e7-4783-b7be-cafca74b5372
Gee, J.S.
a889ff3f-9236-4ca3-80bb-c01131faa432
Pressling, N.
1202b863-d882-4529-b4d0-be2cec9da269
John, B.E.
a4839cce-7cc0-46b4-83ad-3fcafd03ac08
MacLeod, C.J.
1dbf6306-7bba-4fb3-90ec-61b89aee30ba
Grimes, C.B.
47957887-a3b5-45ad-bd14-47db5815dda1
Searle, R.C.
aacbd084-421e-4813-86b7-ac7d84da376f

Morris, A., Gee, J.S., Pressling, N., John, B.E., MacLeod, C.J., Grimes, C.B. and Searle, R.C. (2009) Footwall rotation in an oceanic core complex quantified using reoriented Integrated Ocean Drilling Program core samples. Earth and Planetary Science Letters, 287 (1-2), 217-228. (doi:10.1016/j.epsl.2009.08.007).

Record type: Article

Abstract

Oceanic core complexes expose lower crustal and upper mantle rocks on the seafloor by tectonic unroofing in the footwalls of large-slip detachment faults. The common occurrence of these structures in slow and ultra-slow spread oceanic crust suggests that they accommodate a significant component of plate divergence. However, the subsurface geometry of detachment faults in oceanic core complexes remains unclear. Competing models involve either: (a) displacement on planar, low-angle faults with little tectonic rotation; or (b) progressive shallowing by rotation of initially steeply dipping faults as a result of flexural unloading (the “rolling-hinge” model). We address this debate using palaeomagnetic remanences as markers for tectonic rotation within a unique 1.4 km long footwall section of gabbroic rocks recovered by Integrated Ocean Drilling Program (IODP) sampling at Atlantis Massif oceanic core complex on the Mid-Atlantic Ridge (MAR). These rocks contain a complex record of multipolarity magnetizations that are unrelated to alteration and igneous stratigraphy in the sampled section and are inferred to result from progressive cooling of the footwall section over geomagnetic polarity chrons C1r.2r, C1r.1n (Jaramillo) and C1r.1r. For the first time we have independently reoriented drill-core samples of lower crustal gabbros, that were initially azimuthally unconstrained, to a true geographic reference frame by correlating structures in individual core pieces with those identified from oriented imagery of the borehole wall. This allows reorientation of the palaeomagnetic data, placing far more rigorous constraints on the tectonic history than those possible using only palaeomagnetic inclination data. Analysis of the reoriented high temperature reversed component of magnetization indicates a 46° ± 6° anticlockwise rotation of the footwall around a MAR-parallel horizontal axis trending 011° ± 6°. Reoriented lower temperature components of normal and reversed polarity suggest that much of this rotation occurred after the end of the Jaramillo chron (0.99 Ma). The data provide unequivocal confirmation of the key prediction of flexural, rolling-hinge models for oceanic core complexes, whereby oceanic detachment faults initiate at higher dips and rotate to their present day low-angle geometries as displacement increases.

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More information

Published date: 30 September 2009
Keywords: IODP, oceanic core complex, rotation, palaeomagnetism

Identifiers

Local EPrints ID: 68866
URI: http://eprints.soton.ac.uk/id/eprint/68866
ISSN: 0012-821X
PURE UUID: 9908c7bf-f228-4cdc-8313-12a2c96aa299

Catalogue record

Date deposited: 06 Oct 2009
Last modified: 19 Jul 2019 23:48

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