Rethinking the Tectonic Model of the Caucasus: An Investigation of the Southern Section of the Proposed Borjomi-Kazbegi Fault

The Caucasus region has long been considered to be an example of indenture tectonics. The proposed Borjomi-Kazbegi sinstral fault has been considered the western boundary of the actively indenting wedge. However, an improved seismic network density has led to recent observations noting a lack of seismicity on the proposed Borjomi-Kazbegi fault. These mainly unpublished new seismicity observations call into question the existence of this structure, and with it, the tectonic model of the region. To help clarify this anomaly, geologic and geophysical field research on the southern segment of the proposed Borjomi-Kazbegi fault was performed in the summer of 2005. Because the Borjomi-Kazbegi fault was also proposed to be major crustal structure, a multi-disciplinary approach was utilized for the investigation. The southern section of the Borjomi-Kazbegi fault is part of the Adjara-Trialeti fold thrust belt, composed of volcanic and sedimentary rocks of Early Eocene age and younger. Large scale geologic maps of the Borjomi area show no sinistral displacement of the Early and Middle Eocene sediments. Precise GPS instrumentation was used to map multiple local marker beds across the proposed line of the fault. These marker beds include lithologic contacts, structural folds, quaternary lava deposits and several sills. All localized geology marker beds continue uninterrupted across the proposed fault zone; when combined with the regional data they suggest that the proposed fault does not exist in the Borjomi region. Data from the subsequent magnetic and gravimetric surveys are presently being processed and examined for correlation with the geologic observations. Currently, the field observations support a model that suggests that active shortening in the Borjomi region is accommodated solely by thrust faulting.