Palaeotectonics of the area of Miechów Depression in the Jurassic in relation to the SE segment of the Mid-Polish Trough

Structures reactivated in the basement of the Permian-Mesozoic cover (Fig. 1A) controlled development of the SE segment of Mid-Polish Trough. Facies and thickness patterns in the area of the Laramide Miechów Depression (Złonkiewicz 2006a, b) and adjacent areas verify and complete thesis of analog modeling (McClay et al. 2002; Gutowski & Koyi 2005). They indicate an oblique extension changing from NW to WNW. The NW component affected earlier and stronger the NW part of the area. It caused transtension in reactivated fault systems: the Kraków-Lubliniec (KLF) and Lasocin-Strzelce Dislocation zones (LSDZ). Also an increasing westward tension, earlier and stronger in southern part, may be recognized. Stages of development: I. Hettangian-Bajocian - dextral tension and post-Variscan relaxations. Strike-slip activity of the KLF and LSDZ. The NW- directed stress component down-threw the NW sides of the normal faults: Opoczno-Grójec Fault and the Pilica Fault, rejuvenated on the edge of the Włoszczowa Massif. A slight westward component and relaxations formed a pull-apart rhomboedric system of basins and horsts in down-thrown sides and extended a graben or isolated basin in the LSDZ. II. Bathonian - dextral oblique extension. Dominant stress, directed to NW, supported KLF and LSDZ slip. The coincidence with an increasing westward stress, stronger in the S part, activated a system of en echelone splay faults (strike W-E), posthumous to the Zawiercie Fault (ZF). A second-order system of splay faults (mostly NNW-SSE), bound to ZF, was formed in the Włoszczowa Massif area, leading to formation of grabens and horsts. That pattern was repeated further to the S at the Carpathian front. A basin (W-E) extended at the S border of the Włoszczowa Massif. III. Callovian-Kimmeridgian up to the Hypselocyclum Chron - dextral oblique extension. Increasing westward stress resulted in pull-apart activity and changed a rhombohedric system of horsts and grabens into elongated NW-SE structures. The westward component was stronger in the S part, which resulted in left-hand rotation of the Włoszczowa Massif. The regional uplift, regression and local emersion rebuilt the late-Variscan pattern; an up-thrown Włoszczowa Massif and down-thrown adjacent NW and SE areas. IV. Kimmeridgian from the Divisum Chron onwards-Valanginian - continuation of the palaeotectonic pattern, with dominating westward stress component. Transgression-regression cycle. Extension of the pull-apart structures in the S part of an area. Evolution of the Mid-Polish Trough, extending in front of the East European Craton, depended on varying stress components, produced by extension of rifts in N proto-Atlantic and W Tethys. Palaeotectonic development locates the area of the Miechów Depression between the elevated: Upper Silesian Massif and Holy Cross Mts. (HCM). The HCM belonged to the zone of elevated elements (Gorzów Block, Wielkopolska Ridge, Piotrków Block, Upper San Anticlinorium), typical for the axial part of a basin, separating parallel zones of maximal subsidence. The subsiding zones in the SE segment of the Mid-Polish Trough were located parallel to the edges of HCM (Fig. 1B) in rejuvenated Permian grabens. Observed palaeostress activated a right-hand transfer fault in the deep structure of the Holy Cross Dislocation and formed a positive flower structure in the HCM area.