Three-dimensional modeling of the behavior of the oblique convergent boundary of southeast Taiwan: friction and strain partitioning

The Longitudinal Valley Fault (LVF) of eastern Taiwan is the plate boundary between the Philippine Sea plate and Eurasian plate. Analyses of triangula tion networks showed that two distinct deformation zones coexist: thrusting prevails between the Pinanshan Conglomerate massif and the Central Range t o the west, while strike-slip dominates between the Pinanshan massif and th e Coastal Range to the east, Crustal deformation and displacement partition ing along this oblique convergence boundary is investigated by using a 3D d istinct-element model, To constrain our models, we take into account the pr esent-day deformation of southern LVF based on geodetic data, Particular at tention is paid to the role of the major faults that play an important role in the distribution of the regional strain patterns. The overall deformation is examined by separating the transverse and latera l components of the regional deformation. Our models predict that both the dip-slip and strike-slip increase as the friction coefficient decreases in the eastern fault between Coastal Range and Pinanshan Conglomerate massif. For the western fault between Central Range and Pinanshan Conglomerate mass if, the dip-slip increases when the friction coefficient decreases. Convers ely the strike-slip decreases as the friction coefficient decreases. The mo del that provides the best fit in the eastern fault has an effective coeffi cient in the range of 0.1-0.4, The friction coefficient of 0.27 predicts a strike-slip slip of 20.5 mm/yr, similar to the rate of 22 mm/yr towards N35 3 degreesE calculated from geodetic data. Along the western fault, the mode ls that provide the best fit have a friction coefficient of 0.8-1.0. These models predict the average fault-normal shortening of about 15.7 mm/yr. The displacement mentioned above is a 30% larger than that of geodetic measure ments. Internal deformation by numerous minor faults inside the Pinanshan C onglomerate massif accounts for the discrepancy between the model-predicted and the observed displacement rate. Our models predict an average displace ment rate of about 33 mm/yr towards N318 degreesE for reasonable values of the coefficient of friction, These results are consistent with the displace ment rate of 28 +/- 8 mm/yr towards N329 +/- 10 degreesE between the Coasta l Range and the Central Range based on the trilateration networks around th e southern Longitudinal Valley. The friction parameters do affect the orien tation and magnitude of slip along faults by a series of parametric studies , We conclude that the strain partitioning of crustal deformation of southe rn LVF strongly depends on the orientation of oblique convergence, the geom etry of faults, and their friction coefficients. Taking into account both t he simplified assumptions of the 3D numerical simulation and the uncertaint ies of geodetic measurements, our estimates of friction parameters along ma jor discontinuities are only valid to first order. (C) 2001 Elsevier Scienc e B.V. All rights reserved.