<p><strong>The Barreme Basin and the Gevaudan diapir - an example of the interplay between compressional tectonics and salt diapirism </strong></p><p><strong>&#160;</strong></p><p><strong>Adam Csicsek and Rod Graham</strong></p><p>Imperial College London</p><p><strong>&#160;</strong></p><p>Our understanding of the role of salt diapirism in determining the finite geometry of fold and thrust belts has grown apace in the last few years, but the interplay between the two remains a significant problem for structural interpretation. The Gevaudan diapir in the fold and thrust belt of the sub-Alpine chain of Haute Provence is well known and has been documented by numerous eminent alpine structural geologists. Graciansky, Dardot, Mascle, Gidon and Lickorish and Ford have all described and illustrated the geometry and evolution of the structure, and Lickorish and Ford&#8217;s interpretation is figured as an example of&#160; diapirism &#160;in a compressional setting by Jackson and Hudec in their text on salt tectonics. We review these various interpretations and present another.</p><p>The differences between the various interpretations say much about the complex interplay of salt diapirism and thin-skinned thrusting and have profound implications for the way we interpret the tectonic and sedimentary evolution of the Barreme basin which lies adjacent to the diapir</p><p>The Barreme basin is a thrust-top fragment of the Provencal foreland basin and has been described in detail from both sedimentological (e.g. Evans and Elliott, 1999) and structural (e.g. Antoni and Meckel, 1997) points of view. Here we make the case that it is also a salt related minibasin - a secondary minibasin developed on a now welded allochthonous Middle Cretaceous salt canopy. &#160;We believe that within the basin it is possible to interpret successive depocentres which may record progressive salt withdrawal. We argue that though thrust loading must be the fundamental driving mechanism responsible for salt movement late in the tectonic history of the region, thrusting has not done much more than modify existing salt related geometry. &#160;&#160;&#160;</p>
<p>The understanding of the evolution of salt structures in passive margins has increased significantly in recent decades, largely driven by advances in seismic reflection imaging in offshore passive margin salt basins. This has provided a new perspective with which to view analogous settings in outcrop. The Subalpine Chains of southeast France is one of these places. This region has undergone a complex tectonic history involving Early to Middle Jurassic rifting related to the opening of the Ligurian Tethys, Late Jurassic to Late Cretaceous passive margin subsidence, and Late Cretaceous to Miocene Alpine shortening. The structures and stratigraphic variations in the area strongly suggest that all of these have provided driving mechanisms for, or been associated with, halokinesis.</p><p>This study investigates the role that salt has played in the tectonic evolution of the Subalpine Chains since its deposition in the Triassic using field observations, structural cross sections and drone photography. The period of Early-Middle Jurassic rifting was associated with reactive salt rise, and halokinesis continued during the subsequent passive margin phase driven by sedimentation in the Vocontian basin. Triassic salt reached the sea bed to form salt glaciers during the Aptian-Albian when salt rise outpaced sedimentation rate.</p><p>Later, during Alpine shortening, SW directed compression was partly partitioned as sinistral strike-slip deformation along a pre-existing salt wall, forming the Rouaine-Daluis fault system. There is a discrepancy between the amounts of thin skinned shortening northwest and southeast of the strike slip system which can probably be attributed to the interplay of Jurassic Provence carbonate platform geometry, subsurface salt distribution and basement architecture. In the thin skinned domain of the Digne arc, salt diapirs and walls, formed during the rifting and passive margin phases, such as those at Chasteuil and Cr&#234;te du Teillon, were tightened and displaced up the slope of the Provence Platform margin. Alpine shortening also squeezed salt to surface to form canopies such as the diapir at G&#233;vaudan.</p><p>Halokinesis has influenced, and has been influenced by the tectonic history of the region. While previous regional shortening estimates have acknowledged the role of Triassic salt as a decollement layer, they do not account for the presence of salt walls and diapirs during Alpine shortening. Consequentially, the amount of strain in the Digne arc has likely been underestimated.</p>
Summary Detailed work in the Provencal sub-Alpine chains shows that salt extrusion can be deduced from the presence of a number of overturned flaps ('megaflaps')which are not internally deformed but show reducedstratigraphic section and commonly contain pinch-outs and unconformities. This has been described before in one Provencal locality (Barles, Graham et al 2012 ) but here we document it additionally at Chasteuil near Castellane and Gevaudan near Barreme. The one-time glaciers produced by salt extrusion are now welded out. Mostly it is difficult to investigate the details of this process, but in the vicinity of the major transcurrent fault that bisects the arcuate fold belt of Provence we propose that evacuation involved lateral salt migration analagous to that of the Roho systems of the Gulf of Mexico, leaving an Eocene secondary minibasin in its wake. We believe that this may be the first description of such a structure in the Alps. It emphasizes that salt movement accompanied all the major tectonic episodes which occurred in the region, in both the Mesozoic passive margin and the Alpine foreland basin which succeeded it
<p>The Upper Triassic evaporites of Western Europe, also known as the &#8216;Keuper&#8217;, are well-known and have been mostly considered as an efficient d&#233;collement level for the thrusts of the external fold-and-thrust belts. Numerous recent studies aimed to reappraise their role, and especially the role of salt tectonics, in the formation of several mountain belts such as the Pyrenees, the Betics, Provence, and the Alps.</p><p>The Western Alps represent a good laboratory to study the role of salt in shaping a mountain belt because it contains areas with (1) no evaporites, (2) evaporites involved as an efficient d&#233;collement level during orogeny and (3) evaporites mobilised in salt tectonics since the Lias rifting. We propose here, based on literature and our recent works regarding salt tectonics in the SW Alps, to present and discuss the different salt-related structural styles observed along-strike the Western Alps. A focus will be done on the SW Alps where evaporites influence their structure during the whole Alpine history from rifting until collision. They were mobilised by the Lias rifting through reactive diapirism. Salt tectonics carried on during the post-rift period by passive diapirism, controlled by sediment loading. A few structures were reactivated during the Oligocene and in places evaporites influenced the structure of the subalpine chains until the Mio-Pliocene.</p><p>Our study shows that evaporites strongly influence the structure of a mountain belt at different scales and that along-strike variations of structural style are observed along the strike of the Western Alps depending on the presence, the amount or the absence of evaporites.</p>
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