To achieve an improved over all under standing of the tectonics of Taiwan, an island-wide magnetic survey, especially in the mostly inaccessible mountain ranges, was carried out from July 2003 to February 2004. In total, 6063 magnetic points were surveyed at an average spacing of 2 km. After data reductions, a new magnetic anomaly map of Taiwan was constructed. The map shows the range of magnetic anomalies mostly distributing between -400 to 400 nT. Anomalies in the southern part of Taiwan are much higher than those in the north whilst anomalies in the western plains area are higher than those of the mountain ranges.
High-frequency magnetic anomalies, generally more complex and localized, are found in west central (south of 24°N) and south western Taiwan. Smooth and gentle magnetic anomalies lie over a major part of the mountain ranges, except in the southern segment of the Central Range. According to the amplitude of magnetic anomalies and their coverage, this implies that a broad extrusive ultramafite body could exist beneath the southern segment of the Central Range.