Variscan vs. Alpine structural controls: Karstic proto-conduit development within Palaeozoic marble post-conditioned by Alpine faulting (the Niedźwiedzia Cave, NE Bohemian Massif)

Abstract

Detailed geological mapping and structural analysis of Niedźwiedzia Cave (in the Orlica-Śnieżnik Dome, NE Bohemian Massif, Central Europe) provides new insights into proto-conduit formation in karstic drainage. Mapping has allowed us to document the structure of Variscan-age foliation and non-outcropping, post-Variscan fault zones at meter-scale resolution. The resulting data has been used to build a detailed 3D model of the marble lens hosting the cave, allowing us to define the primary structural guidance trends. We observe that karst development has been controlled by WSW-ENE trending sub-vertical faults and fractures, with N-S to NNW-SSE steeply dipping foliation playing a subsidiary role. Based on micro-movement sensors in the cave and other neotectonic indicators, and the relationship between structural discontinuities and topography, we propose that structural discontinuities have only become favorable for karst development under Alpine stress conditions, with little or no influence of exhumation relaxation. Furthermore, Variscan foliation in the marble only appears to become relevant in karst guidance in mature stages of karst development, with foliation parting, and in the absence of significant faulting or fracturing. This pattern is only disrupted in the presence of tectonic brecciation or comminution along faults or fractures. Structural guidance in the development of the Niedźwedzia Cave has been further modulated by water-table evolution in the overlying Kleśnica river valley. The result is a cave with three distinct water-table controlled levels and a map pattern of passages that grades from a mature N-S dominated orientation in the south (upstream) to a grid-like arrangement in the younger northern segment of the cave (downstream).