A geological model for the structure of ridge segments in slow spreading ocean crust

Author Posting. © American Geophysical Union, 1994. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 99, no. B6 (1994): 11937–11958, doi:10.1029/94JB00338.First-order (transform) and second-order ridge-axis discontinuities create a fundamental segmentation of the lithosphere along mid-ocean ridges, and in slow spreading crust they commonly are associated with exposure of subvolcanic crust and upper mantle. We analyzed available morphological, gravity, and rock sample data from the Atlantic Ocean to determine whether consistent structural patterns occur at these discontinuities and to constrain the processes that control the patterns. The results show that along their older, inside-corner sides, both first-and second-order discontinuities are characterized by thinned crust and/or mantle exposures as well as by irregular fault patterns and a paucity of volcanic features. Crust on young, outside-corner sides of discontinuities has more normal thickness, regular fault patterns, and common volcanic forms. These patterns are consistent with tectonic thinning of crust at inside corners by low-angle detachment faults as previously suggested for transform discontinuities by Dick et al. [1981] and Karson [1990]. Volcanic upper crust accretes in the hanging wall of the detachment, is stripped from the inside-corner footwall, and is carried to the outside comer. Gravity and morphological data suggest that detachment faulting is a relatively continuous, long-lived process in crust spreading at <25–30 mm/yr, that it rnay be intermittent at intermediate rates of 25–40 mm/yr, and that it is unlikely to occur at faster rates. Detachment surfaces are dissected by later, high-angle faults formed during crustal uplift into the rift mountains; these faults can cut through the entire crust and may be the kinds of faults imaged by seismic reflection profiling over Cretaceous North Atlantic crust. Off-axis variations in gravity anomalies indicate that slow spreading crust experiences cyclic magmatic/amagmatic extension and that a typical cycle is about 2 m.y. long. During magmatic phases the footwall of the detachment fault probably exposes lower crustal gabbros, although these rocks locally may have an unconformable volcanic carapace. During amagmatic extension the detachment may dip steeply through the crust, providing a mechanism whereby upper mantle ultramafic rocks can be exhumed very rapidly, perhaps in as little as 0.5 m.y. Together, detachment faulting and cyclic magmatic/amagmatic extension create strongly heterogeneous lithosphere both along and across isochrons in slow spreading ocean crust.This research was supported by Office of Naval Research grants N00014-90-J-1621 and N00014-91-J-1433 and by National Science Foundation grants OCE 8716713 and OCE 9020408

The Mediterranean Ridge: A mass balance across the fastest growing complex on Earth

Depth migration of seismic reflection profiles across the Mediterranean Ridge accretionary complex between the African and Eurasian blocks illustrates profound variations in the geometry and internal structure along strike. Structural interpretations of four cross sections, together with bathymetric and acoustic surface information and drilling data, are used to volumetrically balance the amount of subduction versus accretion with time. Results suggest the existence of three distinct scenarios, with a jump in décollement in the west, intense backthrusting in the central part between Libya and Crete, and transcurrent tectonism in the east. The onset of accretion coincides with exhumation of thrust sheets (∼19 Ma), followed by rapid sediment accretion with thick, evaporite-bearing incoming successions facilitating outward growth of the wedge. The minimum rate of accretion (20–25% of the total sediment supply) is observed in the central portion where the ridge suffers maximum deformation. Here the indenting leading edge of the African Plate apparently forces the sediment into subduction, or local underplating. In contrast, an estimated 40–60% of the available sedimentary input was accreted in the western domain where collision is less accentuated. The results support the hypothesis that highly destructive forearc collisional events, like slab break off and exhumation of thrust sheets, can be followed by periods of accretion and continuous growth of accretionary wedges

Dupal anomaly in existence 115 Ma ago: Evidence from isotopic study of the Kerguelen Plateau (South Indian Ocean)

The Kerguelen Plateau (South Indian Ocean), whose oldest age has been dated as early Cretaceous, shows geochemical and isotopic features characteristic of OIB-type magmatism. It is probably related to the early stages of activity of the Kerguelen hot spot which is also responsible for the Ninetyeast Ridge. It shows all evidence of being an oceanic plateau with an impressive volume of magmatism. The Nd-Sr isotopic systematics of the Plateau basalts show a large spread of values comparable to the systematics shown by the basalts from the Kerguelen Islands. However, while the archipelago basalts have Pb isotopic variations almost within analytical errors, the Plateau basalts show large Pb isotopic variations which overlap the whole range observed amongst Indian Ocean ridge basalts. Contamination of a deep, enriched OIB-type plume, i.e. the Kerguelen hot spot with characteristic Dupal signature, by a depleted, MORB-type reservoir can account for both the trace-element and isotopic geochemistry of the Kerguelen Plateau basalts. This indicates the existence of the Dupal anomaly already 115 Ma ago. In addition, evidence for its involvement in the genesis of Indian Ocean basalts occurs throughout time, including the present day. This favors the hypothesis of a deep-seated source for this major geochemical anomaly which is then probably responsible for the special features of the Indian Ocean. © 1989.SCOPUS: ar.jinfo:eu-repo/semantics/publishe