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 Ju rassic sedimentary black shales (37 sites, 362 samples), the so-called Terr es Noires Formation, from the Southern Subalpine Chains (western Alps). Thi s study completes a previous study done in the Northern Subalpine Chains wh ich demonstrated that the Terres Noires Formation preserves a subtle magnet ic lineation related to tectonic transport direction. In the Southern Subal pine Chains (Diois, Devoluy, Embrunais, Digne nappe), similar magnetic fabr ics have been found. The shape of the magnetic fabric is oblate and locally mirrors the gradient of deformation in the Embrunais footwall. The magneti c foliation is parallel to the bedding or to the slaty cleavage. About 50% of the magnetic lineations have down-dip magnetic directions while the othe r magnetic lineations are sub-horizontal or parallel to the intersection be dding/cleavage. The tectonic magnetic fabrics recorded in the Embrunais con firm the ability of the Terres Noires to record tectonic transport directio n. The magnetic fabric geometry suggests SW-directed tectonic transport in the Southern Subalpine Chains. The Diois appears as a tip zone where transp ort directions are abruptly clockwise rotated from NW-directed in the North ern Subalpine Chains to SW-directed. The magnetic fabric from the Digne nap pe exhibits a fan pattern of transport directions in close agreement with t he arcuate shape of the thrust. (C) 1999 Elsevier Science B.V. All rights r eserved.