Geometry of growth strata in a transpressive fold belt in field and analogue model: Gosau Group at Muttekopf, Northern Calcareous Alps, Austria

Abstract

The thrust sheets of the Northern Calcareous Alps were emplaced during Late Cretaceous thrustdominated transpression expressed by thrust sheets segmented by closely spaced tear faults. Thrust sheet-top sediments were deposited during thrusting and associated fold growth and were controlled by active folding and tearing. We observe two types of angular unconformities: (1) Angular unconformities above folds between tear faults conform with the model of progressive unconformities. Across these unconformities dip decreases upsection.(2) Here, we define progressive unconformities that are related to tear faults and are controlled by both folding and tearing. Across these unconformities both strike and dip change. In growth strata overlying folds dissected by high-angle faults, such unconformities are expected to be common. We used analogue modelling to define the geometry of the tear faults and related unconformities. Within the syn-tectonic sediments, a steep, upward flattening thrust within a broader, roughly tulip-shaped drag zone develops. The thrust roots in the tear fault in pre-tectonic deposits and is curved upward toward the downthrown block. Vertical offset on the thrust is related to differential vertical uplift caused by, for example, growth of folds with different wavelength and amplitude on either side of the tear fault. Formation of progressive unconformities is governed by the relationship between the rates of deposition and vertical growth of a structure. Fault-related progressive unconformities are additionally controlled by the growth of the vertical step across the tear fault. When the rates of vertical growth of two neighbouring folds separated by a tear fault are similar, the rate of growth across the tear fault is small; if the first differ, the latter is high. Episodic tear fault activity may create several angular unconformities attached to a tear fault or allow the generation of angular unconformities near tear faults in sedimentary systems that have a rate of deposition too high to generate classical progressive unconformities between the tear faults.

Deformation around a detached half-graben shoulder during nappe stacking (Northern Calcareous Alps, Austria)

“…8b). Development of NW-striking tear faults in the Late Cretaceous is common in the NCA (Eisbacher and Brandner 1996;Ortner et al 2016), and tear faulting in the Lechtal thrust sheet is required for compensating the lateral differences of shortening. The formation of NE-directed S-C fabrics in the incompetent sediments below the NWstriking segments of the Lechtal thrust (Fig.…”

“…Thrust sheet emplacement and thrust sheet internal folding took place during NW-directed transport (Linzer et al 1995;Eisbacher and Brandner 1996;Ortner 2001Ortner , 2003a. NW-ward thrusting and folding was accompanied by NW-striking, dextral transpressive tear faults (Ortner et al 2016). Thrust-sheet-top deposits accumulated during thrusting and coeval folding in Late Cretaceous to Paleogene times and document growth of individual structures (Ortner 2001;Ortner et al 2016).…”

“…NW-ward thrusting and folding was accompanied by NW-striking, dextral transpressive tear faults (Ortner et al 2016). Thrust-sheet-top deposits accumulated during thrusting and coeval folding in Late Cretaceous to Paleogene times and document growth of individual structures (Ortner 2001;Ortner et al 2016). Thus, shortening did not cease after thrust sheet emplacement.…”

Austrian versus Hungarian bauxites in an Alpine tectonic context: a tribute to Prof. Andrea Mindszenty

“…EISBACHER & BRANDNER 1996). The style of deformation was classified as "thrust-dominated transpression" by ORTNER et al (2016).…”

A geological snapshot from the front of the Northern Calcareous Alps: Well Obermoos TH-1, Salzburg, Austria

“…In the western part of the NCA, the Gosau basins formed in a purely compressional setting (Ortner et al, 2015a). The Salzburg-Reichenhall basin is part of these Gosau basins, which are distributed over nearly the entire length of NCA Wagreich, 1996, 2000).…”