Abstract
This study describes the inversion of a rift-related graben shoulder during emplacement and transport of a major thrust sheet in the external part of the western Northern Calcareous Alps (NCA). Structural fieldwork was carried out along the Lechtal thrust, separating the tectonically deeper Allgäu thrust sheet from the Lechtal thrust sheet. An irregularly shaped Early Jurassic normal fault and an adjacent basin are present in the immediate footwall of the Lechtal thrust. The Early Jurassic age of the basin formation is indirectly established by thickness differences in the syntectonic deposits. Scaly fabric and small-scale drag folds document top S to top SW kinematics at the normal fault. During Alpine orogeny the Lechtal thrust is forced to recess around the graben shoulder in the footwall. A transpressive, dextral tear-fault develops in the hanging wall to compensate for lateral differences of shortening. The normal fault is too steep for inversion, and a shortcut thrust across the half-graben shoulder developed at a shallower angle. It transports the detached half-graben shoulder into the neighbouring basin. Albian to Paleogene transport of the Lechtal thrust sheet caused deformation below the Lechtal thrust, creating a shear zone with isoclinal folds, break thrusts, boudinaged beds and development of S–C fabric. Kinematic analyses of S–C fabrics and small-scale fold axes along the Lechtal thrust show consistent top NW to top N shortening directions in accordance with Cretaceous to Paleogene thrust directions in the NCA.
Highlights
In many cases complex structural style in fold-and-thrust belts is related to obscured pre-existing normal faults and inverted basins (e.g. Channell et al 1990; May and Eisbacher 1999; Pace et al 2014; Martinez et al 2015)
This study describes the inversion of a rift-related graben shoulder during emplacement and transport of a major thrust sheet in the external part of the western Northern Calcareous Alps (NCA)
Dextral shear accompanies the NW-striking segments. This is indicated by several independent observations: (1) S–C fabrics and fold axes along the NW-striking segments (Fig. 7d) have a different orientation compared to structures along the general NE–SW oriented Lechtal thrust (Fig. 7c); (2) a slice of NorthAlpine Raibl beds is dextrally offset along the western NW-striking segment (Fig. 2); (3) when comparing the two sections of Fig. 3, the trace of the Birkental syncline below the Lechtal thrust is offset dextrally by 900 m across the eastern NW-striking segment
Summary
In many cases complex structural style in fold-and-thrust belts is related to obscured pre-existing normal faults and inverted basins (e.g. Channell et al 1990; May and Eisbacher 1999; Pace et al 2014; Martinez et al 2015). In many cases complex structural style in fold-and-thrust belts is related to obscured pre-existing normal faults and inverted basins Basin inversion has been studied for more than twenty years in numerous fold-and-thrust belts employing fieldwork and analogue modelling, partly because of economic implications and seismic hazard assessment This study describes the inversion of a half-graben shoulder in the footwall of a major thrust (Fig. 1), and discusses timing and kinematics of deformation around this former basin margin. Structural and kinematic data on rift-related normal faulting in the lower thrust sheet and on the emplacement of the overlying thrust sheet during inversion are provided to illustrate the effects of basin geometry with a steep halfgraben shoulder on the structures formed during subsequent shortening
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