Analysis of systematic fracturing in Eocene flsch of the Slovenian coastal region

Marko Vrabec,Galena Jordanova

doi:10.5474/geologija.2017.014

Abstract

We analyse systematic fractures occurring in sandstone beds in Eocene flysch of the Slovenian coastal area. Two nearly perpendicular fracture sets were identified: fractures F1 are generally NW-SE oriented, well-expressed and predominately planar, whereas fractures F2 are NE-SW-striking, shorter, more irregular in shape, and terminate against the F1 set. The average orientation of both sets does not change significantly in a coastal transect crossing all principal structural domains of the area. We analysed fracture spacing with respect to layer thickness and determined fracture spacing index for both fracture sets. We interpret both fracture sets as tensional (Mode I) joints originating in two distinct extensional episodes. Set F1 is older and formed in NE-SW directed tension which we correlate with the well-documented regional post-Dinaric orogen-perpendicular extension of presumably mid-Miocene age. Set F2 formed in NW-SE oriented tension, which is compatible with previously documented NE-SW-striking normal faults occurring in the area, but was so far not documented elsewhere. We interpret that F1 fractures predate folding and thrusting in the coastal belt. Earlier, Eocene-Oligocene Dinaric thrusting therefore did not significantly affect the coastal area, whereas post-F1 shortening, associated with northward indentation and underthrusting of the Adria microplate, did not commence before late Miocene.

Highlights

The territory of Slovenia was affected by polyphase tectonic deformation (e.g. Vrabec et al 2009 and references therein), which produced a complex regional assemblage of tectonic units (Placer, 2008), but is reflected in outcrop-scale structural geometry
We find that spacing of joint sets F1 and F2 in Eocene flysch sandstone of the Slovenian coastal area is reasonably-well correlated to bed thickness (Fig. 5)
The first structural study of systematic fractures in Eocene flysch sandstones in the foreland of the External Dinarides fold-and-thrust belt of the Slovenian coastal area has revealed that the fractures occur in two major sets

Summary

Introduction

The territory of Slovenia was affected by polyphase tectonic deformation (e.g. Vrabec et al 2009 and references therein), which produced a complex regional assemblage of tectonic units (Placer, 2008), but is reflected in outcrop-scale structural geometry. Flysch in the Slovenian coastal area is of distal facies where well-bedded predominately cm-dm thick siliciclastic sandstones are interbedded with marls in approximately equal proportion (Pavšič & Peckmann, 1996) This monotonous sequence is interrupted by a number of distinct 1 – 5 m thick calciturbiditic horizons, derived from the carbonate platform which persisted in front of the foreland basin throughout Eocene (Pavšič & Peckmann, 1996; Placer et al 2004, Vrabec & Rožič, 2014). The youngest tectonic phase documented in the region is the ongoing NNW-SSE directed compression, which probably started in Pliocene (Vrabec & Fodor, 2006; Weber et al, 2010; see Moulin et al, 2016) This phase is mainly manifested by dextral slip on NW-SE-striking faults (Žibret & Vrabec, 2016), which are not common in the Slovenian coastal area

Methods

Discussion and conclusions