The magnetic fabric of weakly deformed Late Jurassic shales from the southern subalpines chains (French Alps):: evidence for SW-directed tectonic transport direction

Extensive magnetic fabric investigations have been conducted on the Late Jurassic sedimentary black shales (37 sites, 362 samples), the so-called Terres Noires Formation, from the Southern Subalpine Chains (western Alps). This study completes a previous study done in the Northern Subalpine Chains which demonstrated that the Terres Noires Formation preserves a subtle magnetic lineation related to tectonic transport direction. In the Southern Subalpine Chains (Diois, Devoluy, Embrunais, Digne nappe), similar magnetic fabrics have been found. The shape of the magnetic fabric is oblate and locally mirrors the gradient of deformation in the Embrunais footwall. The magnetic foliation is parallel to the bedding or to the slaty cleavage. About 50% of the magnetic lineations have down-dip magnetic directions while the other magnetic lineations are sub-horizontal or parallel to the intersection bedding/cleavage. The tectonic magnetic fabrics recorded in the Embrunais confirm the ability of the Terres Noires to record tectonic transport direction. The magnetic fabric geometry suggests SW-directed tectonic transport in the Southern Subalpine Chains. The Diois appears as a tip zone where transport directions are abruptly clockwise rotated from NW-directed in the Northern Subalpine Chains to SW-directed. The magnetic fabric from the Digne nappe exhibits a fan pattern of transport directions in close agreement with the arcuate shape of the thrust. (C) 1999 Elsevier Science B.V. All rights reserved.