[1] Slip rates across active faults and folds show that late Quaternary faulting is distributed across the central Tien Shan, not concentrated at its margins. Nearly every intermontane basin contains Neogene and Quaternary syntectonic strata deformed by Holocene north-south shortening on thrust or reverse faults. In a region that spans two thirds of the north-south width of the central Tien Shan, slip rates on eight faults in five basins range from $0.1 to $3 mm/yr. Fault slip rates are derived from faulted and folded river terraces and from trenches. Radiocarbon, optically stimulated luminescence, and thermoluminescence ages limit ages of terraces and aid in their regional correlation. Monte Carlo simulations that sample from normally distributed and discrete probability distributions for each variable in the slip rate calculations generate most likely slip rate values and 95% confidence limits. Faults in basins appear to merge at relatively shallow depths with crustal-scale ramps that underlie mountain ranges composed of pre-Cenozoic rocks. The sum and overall pattern of late Quaternary rates of shortening are similar to current rates of north-south shortening measured using Global Positioning System geodesy. This similarity suggests that deformation is concentrated along major fault zones near range-basin margins. Such faults, separated by rigid blocks, accommodate most of the shortening in the upper crust.

Paleoseismic data from 11 trenches at seven sites excavated across the southern Johnson Valley, Kickapoo, and Homestead Valley faults that ruptured in the 1992 Landers earthquake, as well as the northern Johnson Valley fault which did not fail in 1992, indicate that the return period for large surface rupturing events in this part of the eastern California shear zone is in the range of 5-15 ka. The inferred slip rates, based on their respective recurrence intervals, are in the range of 0.2-0.6 mm/yr for each of the faults studied. A previous large earthquake ruptured the southern Johnson Valley and Kickapoo faults about 5 ka B.P. The northern Johnson Valley fault also failed at about this time at 5.8 ka B.P. and may have been part of the same rupture. In contrast, the penultimate large earthquake that we identify on the Homestead Valley fault occurred about 15 ka B.P., much earlier than other faults involved in the 1992 rupture. From these observations, combined with paleoseismic work by others after the 1992 earthquake, it appears that previous events along the southern Johnson Valley and Kickapoo faults were different than those of 1992 and may have involved other fault segments. It has been over 5 ka since the most recent rupture on the northern Johnson Valley fault. Therefore, it is surprising that it did not fail in the 1992 rupture. From our observations, dextral shear appears to be distributed across the entire eastern California shear zone, with individual faults taking only a small proportion of the overall slip. Release of this regional strain appears to occur in temporal clusters of large (?) earthquakes, with the 1992 event apparently the most recent of a sequence of late Holocene (0-1 ka) earthquakes that have ruptured the nine faults we have trenched in the southwestern Mojave desert. Previous clusters of earthquake activity occurred in the early (8-9 ka) and middle (5-6 ka) Holocene, and possibly the latest Pleistocene (ϳ15 ka).

The Min Shan region, located along the eastern margin of the Tibetan Plateau north of the Sichuan Basin, provides an important natural laboratory in which to study the rates and patterns of deformation and their relationship to mountain building at the margin of the plateau. The topographic margin of the plateau is coincident with a north-trending mountain range, the Min Shan, that stands nearly 2 km above the mean elevation of the plateau (~3500 m in this region). We exploit the preservation of a series of variably deformed Quaternary sediments along the western flank of the range to investigate the Pleistocene-Holocene deformation field within the Min Shan region. Mapping and field observations of remnant alluvial fans of late Pleistocene age indicate that deformation within the Min Shan involved substantial (~10°), rapid, down-to-the-northwest tilting. The geometry of the deposits and the partial preservation of an erosion surface beneath the basin suggest that much of the modern relief of the Min Shan relative to the Tibetan Plateau is a consequence of this late Pleistocene tilting. Rates of tilting inferred from luminescence dating of interbedded loess have been remarkably rapid (~10-8 rad/yr). Similarly rapid rates of Holocene differential rock uplift are inferred from tilted lacustrine sediments in the southwestern part of the range. The range is bounded on the west by the Min Jiang fault zone, an east-vergent reverse fault. However, Holocene alluvial terraces in headwaters of the Min River are preserved across the fault in several places, indicating that displacement rates on the Min Jiang fault are <1 mm/yr. Active faulting only occurs along the eastern foot of the range (Huya fault) for a short distance (~60 km), despite 3 km of relief on the eastern range front. The relationship between these structures and the tilting observed in the Min Jiang basin is enigmatic; the faults do not appear to exert a strong control on the rates and pattern of deformation within the basin. A simple flexural model demonstrates that rates of tilting on the western flank of the Min Shan are too high to be simply attributed to an isostatic response to surficial loading and unloading of the lithosphere. Present-day horizontal shortening across the Min Shan is geodetically determined to be less than 2-3 mm/yr, suggesting that only a small part of the observed tilting can be attributed to horizontal shortening. Thus, tilting and concomitant differential rock uplift in the Min Shan appear to require an additional driving component. We suggest that Quaternary deformation along the western Min Shan may reflect the surface response to thickening of a weak lower crust at the margin of the Tibetan Plateau.

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