Accreted terranes of Central Taiwan delineated by magnetic surveying and digital terrain analysis
© Lewis Charles, Sheng-Wen Chen
Vertical magnetic intensity and total magnetic intensity were surveyed at 128 stations along a thirteen kilometer stretch of the South Cross Island Highway between the villages of Tienchih and Yako in south - central Taiwan. This area forms part of the "upthrust slate belt" of Taiwan. From outcrop studies, magnetic profiles and digital terrain analysis, several faults have been identified that might be related to the activity associated with the Lishan Fault, an important northeast - trending fault in Taiwan. These crustal breaks correlate with positive departures from major magnetic minima and north-northeast striking lineaments, as mapped from the digital terrain model (DTM) of this area. Furthermore, some of the anomalies have short wavelengths, suggesting an overprint of near-surface sources on deeper seated features. Associated with these mostly combination high-angle reverse - strike-slip faults are outcrops of green metadiabases, breccias, metasiltstones, metasandstones, and purple tuffs. Red cherts, white marble, and dark green serpentinite are also common in outcrops. Even though other igneous rocks east-southeast of this area have been found in other formations such as the Yako Formation, all of the basaltic ndesitic rocks in this study belong to the Eocene Pilushan or "Hsinkao" Formation, form part of the Backbone Range of Taiwan that consists of an upthrusted slate belt, and represent island arc fragments that have been thrust upwards to elevations of approximately 2,400-2,700 m. These island arc terranes were probably affiliated with the Manila Trench in the proto South China Sea and were formed prior to the onset of collision between the Philippine Plate and the Eurasia Plate that resulted in the uplift responsible for forming the present-day island of Taiwan. Due to their encasement in Eocene metamorphic rocks, it is possible that these terranes represent fragments of an island arc that is older than the Miocene Luzon arc, or they may represent earlier volcanism that prior to this study has been unrecognized in the Luzon arc system. Magnetic surveying and digital terrain analysis can be important aids in identifying active faults that bound these tectonostratigraphic units and can be used in terrane analysis to explain anomalous occurrences of igneous rocks in compressional orogenic belts such as Taiwan.