EARTHQUAKE FOCAL MECHANISMS, DEFORMATION STATE, AND SEISMOTECTONICS OF THE PAMIR-TIEN SHAN REGION, CENTRAL-ASIA

This paper presents a review of the seismotectonics of the Pamir-Tien Shan collision zone in the Garm region, Tajikistan, based on geologica l structure, seismicity, and focal mechanism solutions. The region is dominated by horizontal compression, manifested by imbricate, low-angl e thrust faults that separate the upper crust into a series of tectoni c sheets. These thrust systems verge northward from the Tajik Depressi on toward the southern Tien Shan and southward toward the northern Pam ir. The pattern of seismicity across the region suggests that similar low-angle thrust faults exist within the crystalline basement as well. In order to reconstruct the present-day stress-strain state of the re gion, we used data from over 15,000 earthquake focal mechanism solutio ns for small earthquakes (M greater than or equal to 1.0) gathered ove r the 27-year period, 1963-1989. The method of reconstruction of the s eismotectonic deformation (STD) field involves dividing the data set i nto small spatial windows and summing the individual focal mechanism s olutions to form an average earthquake mechanism tenser for each cell. The STD state of that cell can then be represented by the orientation of the principal axes of compression and tension, the intensity (or r elative uniformity) of the average mechanism, and the Lode-Nadai coeff icient, which defines the relation between the magnitudes of the princ ipal strain components. We observe throughout the study area a general predominance of subhorizontal compression, manifested in a mixture of thrust and strike-slip deformation; normal-fault deformation is obser ved in only two small portions of the study area. The orientation of p rincipal compression varies from nearly N-S in the northern Pamir and the southern Tien Shan to NW-SE in the Peter the First Range. In gener al, the STD field has a clearly developed ''mosaic'' structure, define d by spatial groupings of nearly uniform orientations of compression a nd tension axes. The boundaries of these groups cannot in most cases b e directly correlated with known geological contacts. The STD structur e also shows some variation with depth, with more or less uniform STD orientation in the upper crust (0-14 lan depth), changing significantl y at greater depths. Examination of various subsets of the focal mecha nism catalog shows the spatial structure of the STD field to be largel y stable with respect to both time and magnitude. The reliability of t he reconstructed STD field was verified using three approaches: (1) co mparison of individual focal mechanism determinations for a large subs et of the data using both Soviet and U.S. algorithms, (2) comparison o f STD reconstruction results using the two independent focal mechanism catalogs, and (3) examination of the effect of spatial sampling on th e results. The dominance of generally north oriented subhorizontal com pression is interpreted to be primarily the result of convergence betw een the Pamir and Tien Shan ranges, in turn caused by the ongoing coll ision of India and Eurasia.