Transform continental margins — part 1: Concepts and models

Abstract

This paper reviews the geodynamic concepts and models related to transform continental margins, and their implications on the structure of these margins. Simple kinematic models of transform faulting associated with continental rifting and oceanic accretion allow to define three successive stages of evolution, including intracontinental transform faulting, active transform margin, and passive transform margin. Each part of the transform margin experiences these three stages, but the evolution is diachronous along the margin. Both the duration of each stage and the cumulated strike-slip deformation increase from one extremity of the margin (inner corner) to the other (outer corner). Initiation of transform faulting is related to the obliquity between the trend of the lithospheric deformed zone and the relative displacement of the lithospheric plates involved in divergence. In this oblique setting, alternating transform and divergent plate boundaries correspond to spatial partitioning of the deformation. Both obliquity and the timing of partitioning influence the shape of transform margins. Oblique margin can be defined when oblique rifting is followed by oblique oceanic accretion. In this case, no transform margin should exist in the prolongation of the oceanic fracture zones. Vertical displacements along transform margins were mainly studied to explain the formation of marginal ridges. Numerous models were proposed, one of the most used is being based on thermal exchanges between the oceanic and the continental lithospheres across the transform fault. But this model is compatible neither with numerical computation including flexural behavior of the lithosphere nor with timing of vertical displacements and the lack of heating related to the passing of the oceanic accretion axis as recorded by the Côte d'Ivoire-Ghana marginal ridge. Enhanced models are still needed. They should better take into account the erosion on the continental slope, and the level of coupling of the transform continental margin with the adjacent oceanic lithosphere.

Transform continental margins – Part 2: A worldwide review

“…They result from two stages of strike-slip motion along a transform fault, first within continental lithosphere, then between continental and oceanic lithospheres, finally followed by a passive stage after the passage of the oceanic accretion axis along the transform margin (see Figure 1 ; Scrutton 1979;Mascle and Blarez, 1987;Basile 2015). Each end of a transform margin is connected with a divergent margin segment.…”

“…Our study follows up on a previous publication entitled "Transform continental margins -part 1: concepts and models" (Basile, 2015) in which theoretical models of transform margins formation were presented.…”

Transform margins: development, controls and petroleum systems – an introduction

“…The second group interprets transform propagation as unaffected by pre-existing anisotropy (e.g. Taylor et al 2009;Basile 2015). This group has documented that the initial spreading offsets are often non-transform and that transforms develop either after or during the spreading nucleation.…”

Paleogeographic and structural evolution of northwestern Africa and its Atlantic margins since the early Mesozoic

“…Given their transform character, the margins of the equatorial Atlantic Ocean were mainly investigated through the kinematics of ocean opening (e.g., Basile et al, 2005;Moulin et al, 2009;Heine et al, 2013;Basile, 2015) with an emphasis on the fracture zones and the vertical movements induced along the margins (Bouillin et al, 1998;Clift et al, 1998;Bigot-Cormier et al, 2005;Mercier de Lépinay, 2016). The abruptness of the equatorial margins of Africa results from the strong control of strike-slip faults (i.e., future transforms) during rifting.…”