A compendium of causes and effects of processes at transform faults and fracture zones

Abstract

A flowchart has been constructed which consolidates into a single conceptual flamework a large number of the phenomena and processes which have been observed or postulated at transform faults (TFs) and fracture zones (FZs). Oceanic ridge-ridge transform faults and their aseismic extensions are emphasized. The upstream end of the flowchart contains large-scale boundary conditions imposed by the position of a particular transform fault in space and time, for example, slip rate. The downstream end of the flowchart contains small-scale features characteristic of individual TFs or FZs, for example, the existence of a FZ-parallel ridge. Features of both the upstream and the downstream ends of the flowchart can be mapped, measured, or otherwise derived by direct observation. The middle of the flow chart includes processes and phenomena which are inferred to result from the upstream boundary conditions and to cause the downstream small-scale features, for example, thermal conduction across the plate boundary. Phenomena in this "black box" in the center of the flowchart cannot be directly observed with present technology but must be inferred. The advantages of ordering knowledge into such a flowchart are that inconsistancies, gaps in understanding, self-limiting phenomena (feedback loops), and previously unrecognized relationships stand out and that the boundary between inference and observation is clearly drawn. INTRODUCTION A vast literature has accumulated describing observations at transform faults and fracture zones and endeavoring to explain these observations. In the present paper an effort has been made to consolidate the numerous hypotheses, interrelationships, and ideas proposed in the literature onto a single sheet of paper (Plate 1). The goal was to provide a conceptual framework of utility to three classes of users: (1) the reader who knows a little about the field and wishes to perceive how his or her tidbit of knowledge fits into the grand scheme of things, (2) the reader who knows nothing about the field who can use Plate 1 as a roadmap from which to plan an assault on the voluminous transform fault literature, and (3) the active researcher in the field who might design experiments to assess the relative importance of various processes and phenomena in Plate 1. In the flowchart of Plate 1, each box represents a phenomenon or process, either observed or inferred. Arrows represent cause and effect: it has been proposed that the phenomenon or process at the upstream end of each arrow influences the phenomenon or process at the downstream end of the arrow. At the far upstream end of the flowchart are those major characteristics or boundary conditions which can be inferred for most transform faults (for example, slip rate). Such boundary conditions grow from either the position of the particular transform fault in the global plate mosaic ("plate tectonic setting") or the geological evolution and development of the particular transform ("plate tectonic history"). The far downstream end of the flowchart contains ...

Sheared continent–ocean margins: an overview

“…Figure 2 reflects the latter definition. Fracture zones extend beyond the junction of sea-floor spreading centers and transforms, known as the ridge-transform intersection (RTI) (Kastens 1987).…”

The Agulhas Ridge, South Atlantic: The Peculiar Structure of a Fracture Zone

“…The depression is filled with well-layered sediments of at least 1000 ms two-way travel-time (TWT) thickness ($ 1000 m using v P ¼ 2000 m/s, Figure 4). The ridge itself is of tectono-magmatic origin (Hartnady and le Roex, 1985;Kastens, 1987) and generally shows only a thin sedimentary cover. In a few places, thicker sediment packages can be identified.…”

Transform Fault

“…It is reasonable to assume, in regions where discontinuities in the oceanic lithosphere are being compressed across their planes, that seawater would not circulate to any significant extent, and this would explain the little serpentinization in such environments as noted by Kusznir and Cooper (2011). By contrast, oceanic fracture zones are places where there is thermal-contraction-driven extension across the transform faults and their fracture zone extensions (Collette, 1974;Turcotte, 1974;Sandwell, 1986;Kastens, 1987;Kumar and Gordon, 2009 and further literature therein). Morphologically, fracture zone valleys may be as impressive as some rift valleys (see esp.…”

“…3b), and fracture zone tectonism may very significantly disrupt the coherence of the oceanic lithosphere thousands of kilometers away from spreading centers. Figure 11 illustrates some of the salient cross-sectional elements of the geology of the fracture zones (for details, see general: DeLong et al, 1977DeLong et al, , 1979; in the oceans : Franchateau et al, 1976;Bonatti, 1978;Bonatti and Hamlyn, 1978;Bonatti et al, 1979;Fox et al, 1980;Hamlyn and Bonatti, 1980;Schouten and White, 1980;Karson and Dick, 1983;Searle, 1983;Bonatti and Crane, 1984;CYAGOR II Group, 1984;Fox and Gallo, 1984;Honnorez et al, 1984;Garfunkel, 1986;Loudenr et al, 1986;Sandwell, 1986;Kastens, 1987;Rutter and Brodie, 1987;Allerton, 1989;Smoot, 1989;Tamsett and Searle, 1990;M€ uller and Roest, 1992;Kastens et al, Encyclopedia of Marine Geosciences DOI 10.1007/978-94-007-6644-0_121-1 # Springer Science+Business Media Dordrecht 2014 1998; Okal and Langenhorst, 2000;Sage et al, 2000;Beutel and Okal, 2003;Behn et al, 2007;Sørensen et al, 2007;in ophiolites, general: Nicolas, 1989, Chap. 5).…”