The Greater Caucasus fold-thrust belt is the northernmost expression of the Caucasus orogen and is accreted to the southern margin of the Precambrian Scythian platform. This paper presents new data of zircon LA-ICP-MS U-Pb geochronology of the Svaneti segment plutons of this orogen. The data indicate three major stages of plutonic magmatic activity that correspond to major orogenic events in the region: 1-Ordovician (Caledonian orogenesis), 2-Late Carboniferous (Variscan orogenesis) and 3-Middle Jurassic (Cimmerian orogenesis). In the Early Ordovician (ca. 480 Ma), the granitoid protoliths crystallsized, wich later, during the Variscan orogeny, deformed and metamorphosed (biotite orthogneisses). At the second stage, during Late Variscan orogenesis, Upper Carboniferous granodiorite-granite plutons (318-312 Ma) were emplaced within Caledonian migmatised gneissic infrastructure of the Pass and Elbrus subzones. At the third stage, the Middle Jurassic monzo-syenite plutonic activity is preserved in the Paleozoic-Triassic Dizi series. The ages of the Middle Jurassic plutons gradually decreases from north to south (177, 168, 164 Ma), which indicates the time and direction of the Dizi basin closure. Combining of the spatial analysis with chronological constraints provided by these plutons leads to the conclusion that the northern margin of Neotethys subduction, were proceeding from south to north.

The Chi-Chi earthquake rupture that mostly follows the mountain front in central Taiwan bends east/northeast into hills in the Tachia River valley. We found no major pre-existing fault systems in this hill area. However, based on river terrace morphology and sequences, the lower Tachia River valley appears to have been uplifted more rapidly than the lower Taan River valley in the past ten thousands of years, which is consistent with the bend of the earthquake rupture. The earthquake rupture (and associated uplift) also coincides with: (1) straight mountain-front escarpment south of the Tachia River valley; (2) two fault/fold-scarps on the Holocene terraces, and abnormal sideward inclination and downstream flattening of these terraces in the Tachia River valley; (3) local development of non-laterized terraces in the Taan River valley; (4) abnormal rise and narrowing of the Taan/ Tachia drainage divide. These geomorphic anomalies, however, are either local or diffusive; features (1) to (3) might have been interpreted as of flu vial origin. Also, the link between these features and the movement of the Chelungpu-Sanyi fault, the causative fault of the earthquake, might have been uncertain. We thus argue that the pre-location of this portion of the Chi-Chi earthquake rupture would have been difficult, even though we could have roughly delineated a potential coseismic deformational zone in the region.