Post-Cretaceous to recent stress fields in the SE Moesian Platform (Bulgaria)

Abstract

The reconstruction of the stress fields in the SE Moesian Platform in Bulgaria has been made by means of earthquake fault-plane solutions, tectonic fracture and fold patterns, and physical (electrical) anisotropy in rocks restricted stratigraphically from the Early Cretaceous up to the Late Pliocene. The philosophy of the study is that the recognition of the older stress fields for a given area can be successful if the characteristics of the youngest ones are known. The contemporary stress field is discussed using the fault plane solutions from earthquakes and the kinematics of the activated faults. The reconstruction of the Post-Pliocene paleo-stress field was made by studies of conjugate shear joints systems in Upper Pliocene limestones. Measurements of elements of tectonic fractures were also performed in situ on more than 60 outcrops of rocks aged from Aptian to Pliocene. A limited number of sites were studied for electrical anisotropy using the Azimuthal Vertical Electrical Sounding method. A more complete study of the tectonic meso- and micro-structures has been performed on the Sarmatian sediments in the SE Moesian Platform. This study includes a description of the discovered folds, brittle tectonic analysis and reconstruction of the Post-Sarmatian paleo-stress field. As a result, it was deduced that the compression after the Early Cretaceous period is NE–SW directed. The direction of compression since Sarmatian to Early Quaternary was NW–SE. A clockwise rotation of the main stress axes was established for a number of sites. The contemporary contraction is directed also NW–SE, according to the fault-plane solutions determined for crustal earthquakes in the region. This result is tested using the data from the GPS measurement recently performed in this part of the Balkan Peninsula. It could be suggested that there has been a clockwise rotation of the stress field due to the evolution of curved fold–thrust belt in the south-eastern Carpathians and the collision of the Balkanides with the Moesian Platform.