Keywords : tensor
Iraqi National Journal of Earth Science,
2012, Volume 12, Issue 2, Pages 43-78
The present study involves different aspect of brittle failure structures at Spi Res Anticline within the foreland fold belt of north Iraq. It aims to decipher whether the late Cretaceous orogenic episode has any fracturing consequence to the rock sequence of this period at investigated area. This was accomplished by comparison of various fracture modes between Upper Cretaceous and Tertiary rock sequence exposed at studied area. The study area is prevailed with widespread joints and tabular veins, little mesoscopic faults and scarce of pressure solution surfaces (stylolite). Joint analysis showed two extensional sets (ac and bc) and three shear systems (hko, hol and okl). However, hko acute about a and b system was prevailed and followed by ac and bc sets. The openings of most of joint sets were filled later by calcareous or siliceous precipitations without any kinematic indications, some refers to extension normal with vein walls and for a single growth episode. A little number of striated mesoscopic faults were registered throughout both Cretaceous and Tertiary rocks in study area. Most of them are reverse, others are normal and strike slip. Further, two sets of stylolite seams were found scarcely, peaks of the first oriented parallel with the bedding dip, whereas those of the other set trended parallel to bedding strike. The kinematic analysis of mesoscopic faults and shear joints in study area gave a number of compressive and strike slip stress tensors. The maximum horizontal stress (δmax ) of these tensors lie in N-S, NE-SW, E-W and NW-SE general trends. Accordingly, they were organized into two compressive tectonic regimes. One representing the component normal to the orogenic front, acted in N-S and NE-SW directions. Whereas the other representing the component parallel with the orogenic front, acted in E-W and NW-SE directions. Both components were resulted from oblique collision of Arabian and Eurasian plates. However, these two compressive tectonic regimes are supported by a couple sets of stylolites with their peaks either normal or parallel to the trend of Spi Res Anticline. No brittle failure structures belonging to orogenic episode of Late Cretaceous were recognized in the Upper Cretaceous succession of study area in the present investigation.