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Far-field tilting of Laurentia during the Ordovician and constraints on the evolution of a slab under an ancient continent

Coakley, Bernard and Gurnis, Michael (1995) Far-field tilting of Laurentia during the Ordovician and constraints on the evolution of a slab under an ancient continent. Journal of Geophysical Research B, 100 (B4). pp. 6313-6327. ISSN 0148-0227. doi:10.1029/94JB02916.

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During a brief period of 10 to 15 million years in the Middle-Ordovician, the Michigan Basin departed from its bull's-eye subsidence pattern and tilted toward the east, opening to the Appalachian basin. This tilting is observed in maps of tectonic subsidence estimated for the Black River and Trenton Formations and extends over 300 km across the Michigan Basin and into eastern Wisconsin. Contours of constant tectonic subsidence rate are approximately parallel to the inferred position of the Laurentian-Iapetus convergent margin. The distance between the inferred position of the subduction zone to the limit of tilting is approximately 1000 km. Three alternative models for the tilting are tested, two relying on the rigidity of the continental lithosphere and a third on the viscous flow generated by a subducted slab. In the first elastic model we assume the edge of the elastic plate is simply loaded from above (by a fold and thrust sheet, volcanic pile, etc.). This model, however, cannot simultaneously satisfy the space of tectonic subsidence and subsidence rate, even for lithospheres which have a rigidity of 10^28 Nm. In the second elastic model, the Laurentian continental margin descends into a trench of an eastward dipping slab, that is, a slab descending under the Iapetus ocean. This process cannot generate any significant far field displacements, even for extremely rigid plates, and must be rejected. For the third model we use finite element solutions of a negatively buoyant slab in a viscous medium with a faulted lithosphere. Such slabs can easily generate not only realistic trenches on the under thrusting plate but also significantly tilt the lithosphere as much as 1000 km from the plate margin. The magnitude and distribution of far-field displacements depend on the age, length, and dip angle of the slab. In contrast to the elastic models, penetration of a west-dipping slab beneath the continent can reproduce both the extent, magnitude, and rate of tectonic subsidence observed in the Trenton and Black River Formations. The observed data are best fit by an old slab (140 Ma), which initially descended at a moderate dip (20°–30°) for 10–15 m.y., which then steepened as the slab penetrated deeper into the mantle. At the position of the previous Ordovician plate margin, there are narrow, block faulted basins which underwent rapid subsidence ∼10 m.y. before the Michigan Basin tilted toward the east. We propose that these earlier subsidence events were caused by the initial descent of the slab under the preexisting Cambro-Ordovician passive margin. The time lag of ∼10 m.y. may be due to the time it takes the slab to penetrate the upper mantle. This result is important for understanding the time evolution of mantle convection and mechanisms for the initiation of subduction.

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Gurnis, Michael0000-0003-1704-597X
Additional Information:© 1995 American Geophysical Union. Manuscript Accepted: 2 November 1994; Manuscript Received: 24 March 1994. B.J.C. was supported by a grant from the Gas Research Institute under contract 5089-260-1810. M.G. is supported by a fellowship from the David and Lucile Packard Foundation and by NSF grant EAR 9496185. Thanks to Greg Nadon, Herb Wang, Jeff Nunn, and Paul Howell for enlightening, continuing discussions. We thank Shijie Zhong for assistance with the finite element modeling. Contribution number 5394, Division of Geological and Planetary Sciences at Caltech.
Group:Seismological Laboratory
Funding AgencyGrant Number
Gas Research Institute5089-260-1810
David and Lucile Packard FoundationUNSPECIFIED
NSFEAR 9496185
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Caltech Division of Geological and Planetary Sciences5394
Issue or Number:B4
Record Number:CaltechAUTHORS:20130311-142835955
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Official Citation:Coakley, B., and M. Gurnis (1995), Far-field tilting of Laurentia during the Ordovician and constraints on the evolution of a slab under an ancient continent, J. Geophys. Res., 100(B4), 6313–6327, doi:10.1029/94JB02916.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:37446
Deposited On:11 Mar 2013 21:53
Last Modified:09 Nov 2021 23:29

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