A Regional Gravity Map for the Subduction-Collision Zone Near Taiwan

  • Author(s): Horng-Yuan Yen, Wen-Tzong Liang, Ban-Yuan Kuo, Yih-Hsiung Yeh, Char-Shuan Liu, Donald Reed, Neil Lundberg, Fu-Chen Su, Hou-Sheng Chung
  • DOI:

    10.3319/TAO.1995.6.2.233(T)

Abstract

To delineate the tectonic character of the subduction-collision zone in the marine area near Taiwan, we have constructed a regional gravity map by reconciling the shipboard data of 15 cruises that have surveyed the region of 20-26¢XN and 119-124¢XE over the past 20 years. Among the 15 cruises, three have known, reliable base-tie information. The absolute level for the other 12¡¦s cruises are adjusted in such a way that their crossover errors with respect to the other three are minimized. The root-mean-square difference of the totally 865 crossovers in the final data set is 6.0 mgal, representing an acceptable quality considering the multi-agency nature of the source and the rugged seafloor. The distribution of the ship tracks is uneven, with the better resolution around Taiwan but a progressive undersampling toward the east and northwest. To make a regional map, we interpolate both the marine data and the 640 free-air anomaly measurements on Taiwan and its offshore islands into 5 minute spaced grids using a minimum curvature technique. The SEASAT altimetry derived gravity values are placed ono part of the map¡¦s boundary as an additional constraint in the minimum curvature calculation. On the map, the free-air anomaly ranges from -250 to 320 mgal, primarily reflecting the topography, with the maximum and minimum at the high mountains of Taiwan and the Ryukyu Trench-Taiwan intersection, respectively. The Lutao-Lanyu volcanic arc is manifested as a prominent, continuous high from the southern border of the map, merging northerly with the Coastal Range in eastern Taiwan. The Hengchun Ridge shows a much more subdued gravity anomaly than that of the volcanic arc at a similar elevation. The map also delineates the North Luzon Trough as a forearc basin to the west of the arc, the Nanao Basin at the Ryukyu Trench, and to a less extenet the Gagua Ridge at 123¢XE. to illustrate how the marine gravity data constrain crustal structure, a 2-D gravity analysis is performed with a free-air anomaly profile across the Ryukyu Trench taken from this map.

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