Flipping detachments: The kinematics of ultraslow spreading ridges.

Reston, Tim (2018) Flipping detachments: The kinematics of ultraslow spreading ridges. Earth and Planetary Science Letters, 503 . pp. 144-157. DOI 10.1016/j.epsl.2018.09.032.

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Abstract

Although the seafloor spreading Hess initially proposed was a virtually amagmatic process, little attention
has been paid to that possibility since. We construct a kinematic framework for virtually amagmatic
and magma-poor Hess-style seafloor spreading, and successfully apply it to processes operating at the
Southwest Indian Ridge (SWIR). The kinematic model is based on symmetric divergence about a rift
axis at depth, with a repeating cycle in which a fault propagates up from the rift axis, develops
into a detachment fault accommodating the plate divergence, migrates beyond the rift axis and is
abandoned when a new fault propagates up through the footwall from the rift axis. We rigorously
explore the controls on the depth, dip and timing of fault initiation and abandonment and use the
kinematic framework to reconstruct the evolution of smooth mantle-dominated seafloor at the SWIR
through symmetric divergence about a fixed rift axis. The model predicts the development of successive
detachments of flipping polarity, as observed, each rooting along a narrow and fixed rift axis at 20 km
depth, the base of the seismically defined brittle lithosphere. The detachments root at 80° (consistent
with constraints on seismicity-defined detachment orientation at oceanic core complexes), and exhume
mantle. Based on the continuity of basement ridges, of magnetic anomalies and of the seismic activity at
the base of the lithosphere, it appears that these exhumation detachments transition laterally into rafting
detachments, transporting fault-bounded volcanic slices up and away from the spreading axis to form the
rougher volcanic seafloor found between mantle-dominated domains. The kinematic framework shows
that increased magmatic divergence requires the detachments to root at shallower depths, consistent
with the seismicity-defined shallowing of the base of the brittle lithosphere moving along the ridge axis
towards the volcanic centres. Only in the immediate vicinity of volcanic centres, where the seismicity
dies out, may magmatism dominate. We conclude that detachment tectonics dominate the process of
ultraslow seafloor spreading as well as much of slow seafloor spreading, totalling about one third of the
global ridge system, and present the first 30 tectonic model for ultraslow seafloor spreading.

Document Type: Article
Keywords: seafloor spreading, detachment faults, plate tectonics
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1016/j.epsl.2018.09.032
ISSN: 0012-821X
Projects: Enrichment
Date Deposited: 11 Oct 2018 07:54
Last Modified: 01 Feb 2019 15:09
URI: http://oceanrep.geomar.de/id/eprint/44454

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