<p>We studied the Karwendel mountains of the Northern Calcareous Alps fold-and-thrust belt, that formed during the Upper Cretaceous. The study area exposes one of the principal thrusts that emplaces Triassic sediments on the top of Cretaceous rocks. Based on a detailed structural analysis (Kilian and Ortner, 2019) it has been demonstrated that the folds above the thrust are buckle folds.</p><p>With the finite element modelling, we aimed to create folds with wavelengths comparable to those observed in the field. The model set up is a simple layer cake model based on the stratigraphy of the western Northern Calcareous Alps. We used ABAQUS a finite element software to create the model. The material model is linear elasticity. During modelling, we tested different material characteristics and layer thickness. In all model runs a very weak d&#233;collement (possibly salt) is necessary to produce folds. We further tested the influence of erosion and re-sedimentation on the development of structures. We concluded that the growth of folds having wavelengths comparable to the field examples depends on the thickness of the competent layer, whereas the thickness of the incompetent layer has negligible influence.</p><p>We suggest that fold development in this part of the Northern Calcareous Alps is dependent on the interplay between the growth of evaporite-cored anticlines and surface erosion.</p><p><strong>Reference</strong></p><p>Kilian, S., Ortner, H.: Structural evidence of in-sequence and out-of-sequence thrusting in the Karwendel mountains and the tectonic subdivision of the western Northern Calcareous Alps. Austrian Journal of Earth Sciences, Vienna, 2019, V.112/1, p.62-83, DOI: 10.17738/ajes.2019.0005.</p>
Abstract We investigate the tectonic evolution of the Wetterstein and Mieming mountains in the western Northern Calcareous Alps (NCA) of the European Eastern Alps. In-sequence NW-directed stacking of thrust sheets in this thin-skinned foreland thrust belt lasted from the Hauterivian to the Cenomanian. In the more internal NCA major E-striking intracontinental transform faults dissected the thrust belt at the Albian–Cenomanian boundary that facilitated ascent of mantle melts feeding basanitic dykes and sills. Afterwards, the NCA basement was subducted, and the NCA were transported piggy-back across the tectonically deeper Penninic units. This process was accompanied by renewed Late Cretaceous NW-directed thrusting, and folding of thrusts. During Paleogene collision, N(NE)-directed out-of-sequence thrusts developed that offset the in-sequence thrust. We use this latter observation to revise the existing tectonic subdivision of the western NCA, in which these out-of-sequence thrusts had been used to delimit nappes, locally with young-on-old contacts at the base. We define new units that represent thrust sheets having exclusively old-on-young contacts at their base. Two large thrust sheets build the western NCA: (1) the tectonically deeper Tannheim thrust sheet and (2) the tectonically higher Karwendel thrust sheet. West of the Wetterstein and Mieming mountains, the Imst part of the Karwendel thrust sheet is stacked by an out-of-sequence thrust onto the main body of the Karwendel thrust sheet, which is, in its southeastern part, in lateral contact with the latter across a tear fault.
Abstract We present the results of a field study in the Karwendel mountains in the western Northern Calcareous Alps, where we analysed the boundary between two major thrust sheets in detail in a key outcrop where nappe tectonics had been recognized already at the beginning of the 20th century. We use the macroscopic structural record of thrust sheet transport in the footwall and hanging wall of this boundary, such as folds, foliation and faults. In the footwall, competent stratigraphic units tend to preserve a full record of deformation while incompetent units get pervasively overprinted and only document the youngest deformation. Transport across the thrust persisted throughout the deformation history of the Northern Calcareous Alps from the late Early Cretaceous to the Miocene. As a consequence of transtensive, S-block down strike-slip tectonics, postdating folding of the major thrust, new out-of-sequence thrusts formed that climbed across the step, and ultimately placed units belonging to the footwall of the initial thrust onto its hanging wall. One of these out-of-sequence thrusts had been used to delimit the uppermost large thrust sheet (Inntal thrust sheet) of the western Northern Calcareous against the next, tectonically deeper, (Lechtal) thrust sheet. Based on the structural geometry of the folded thrust and the age of the youngest sediments below the thrust, we redefine the thrust sheets, and name the combined former Inntal- and part of the Lechtal thrust sheet as the new Karwendel thrust sheet and the former Allgäu- and part of the Lechtal thrust sheet as the new Tannheim thrust sheet.
<p>Tectonic subdivisions of larger geologic units reflect the geologic knowledge at the time of creation. In many thrust belts the original subdivisions had been created during the first comprehensive mapping campaigns at the end of the 19<sup>th</sup> to early 20<sup>th</sup> century and reflect the geologic knowledge at that time. Even if many thrusts were identified correctly, no formal framework existed to give guidelines of how to distinguish tectonic units. Nevertheless, these subdivisions are still in use.</p><p>We analyze the thrust sheets of the Northern Calcareous Alps of western Austria and southern Germany and test the implicit assumptions underlying most tectonic subdivisions against field observations:</p><p>Assumption 1: Thrust transport is large and thrusts do not end laterally.&#160;However, several major thrusts do loose stratgraphic offset and end laterally.</p><p>Assumption 2: Allochthons are surrounded by thrusts on all sides. Unfortunately, any fault has been used to delimit allochthons.</p><p>Assumption 3: Thrusting should bring old on young rocks. In some cases, allochthons have been delimited by out-of-sequence thrusts, that stack young on old rocks. In other cases, the allochthon is a mountain-size glide block that was buried by younger sediments, and the trace of the thrust is an unconformity in the field.</p><p>As a consequence we propose a revised tectonic subdivision of the western part of the NCA, that avoids some of the problems discussed here, and is entirely based on the emplacement of old-on-young rocks across thrusts.</p>
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