Aeromagnetic surveys offshore of the western Nansei Islands, Japan, were conducted by the Geological Survey of Japan (GSJ) from 1982 to 1989 (Okuma et al., 1991). The surveys cover the area from the Tunghai Shelf in the East China Sea to the Okinawa Trough west of the Nansei Islands (Ryukyu Arc) (Figs. 1 and 2). Although the area, especially the Tunghai Shelf, was regarded as a potential field for hydrocarbon resources, few explorations were conducted in the area. The aeromagnetic surveys, therefore, were conducted. Then, we performed a three-dimensional two-layer magnetic modeling to obtain further detailed information about the basement structure of the area. This paper outlines the results of the modeling in addition to describing characteristics of the magnetic anomalies in the area.
Fig.1 Location map of the study area (Okuma et al., 1998)
The area bounded by solid red lines shows the study area. Vertical exaggeration is 12.5:1. Topography data was compiled from the terrain data 'KS-110' by Geographical Survey Institute of Japan, bathymetry data by the Hydrographic Department, Maritime Safety Agency, Japan and ETOPO-5 data by NOAA.
Fig.2 Bird's-eye view map of the topography of the the study area (Okuma et al., 1998)
See also Fig. 1.
Fig. 3 Outline of the geological structure in and around the East China Sea (after Japan Natural Gas Association and Japan Offshore Petroleum Development Association (1982))
1: Fukien-Reyongnam Belt, 2: Goto Belt, 3: Senkaku Belt, 4: Ryukyu Geanticline, 5: Ryukyu Trench; A: Goto Sedimentary Basin, B: Tokai Sedimentary Basin, C: Senkaku Sedimentary Basin, D: Okinawa Trough, E: Ryukyu Fore-arc Sedimentary Basin.
In the study area, two major geological structures are known for having hydrocarbon potential; the Tunghai Shelf and the Okinawa Trough (Fig. 2).
Wageman et al. (1970) suggested the existence of a large sedimentary basin, the Taiwan Basin, in the Tunghai Shelf, which is bounded on the northwestern and southeastern sides by the Fukien-Ryeongnam Belt (Fukien-Reinan Massif) and Goto-Senkaku Belt (Taiwan-Sinzi Folded Zone), respectively (Fig. 3). Since then, the basin has been regarded as a potential field for oil and gas resources.
The Taiwan Basin is divided into three basins; the Goto Basin, the Tokai Basin, and the Senkaku Basin from northeast to southwest (Kizaki, 1986; Fig. 3). Thick sediments have been deposited almost continuously since the Paleogene with a hiatus in the Middle Miocene (Aiba and Sekiya, 1979). Recently hydrocarbon explorations such as seismic reflection surveys and drilling have been conducted mainly in the Goto Basin and the Tokai Basin, resulting in shows of oil and gas. Oil and gas shows also have been confirmed in several drill holes close to the continent in the East China Sea (Guangding, 1989). However, the detailed basement structure of the area, especially of the Senkaku Basin, remains unclear because of insufficient explorations.
The Goto-Senkaku Belt lies along the outer edge of the Tunghai Shelf between Kyushu Island and Taiwan, and consist of two parts, the Goto Belt and the Senkaku Belt (Aiba and Sekiya, 1979; Fig. 3). The Goto Belt comprises Paleogene to early Miocene sediments associated with welded tuff and granitic rocks of Miocene age, and is mostly overlain by Pliocene sediments (Kizaki, 1986). Miocene granitic rocks are also exposed in the Senkaku Islands (Matsumoto and Tsuji, 1973). The Goto-Senkaku Belt was uplifted in the Middle Miocene and in some parts to Pliocene time and dammed flows of sediments from the continent.
The Okinawa Trough is a back-arc basin associated with northwestward subduction of the Philippine Sea Plate at the Ryukyu Trench (Fig. 2) and is bounded on the northwest and southeast edges by the Goto-Senkaku Belt and the Ryukyu Arc, respectively (Fig. 3). The bathymetry of the trough becomes deeper towards southwest and exceeds 2,000 m offshore of northern Ishigaki Island (Fig. 2). Hydrocarbon explorations have been conducted mainly in shallow water areas; the northeastern part of the trough and the Ryukyu Arc area (Aiba and Sekiya, 1979). Geological and geophysical investigations (e.g. Oshima et al., 1988) also have been conducted to reveal the origin of the trough. Many step faults and their resultant roll-over anticlines are commonly observed on seismic profiles in the west flank of the trough north of Okinawa Island (Aiba and Sekiya, 1979). Covering the trough are thick Pliocene and Pleistocene sediments and the maximum depth of acoustic basement exceeds 5,000 m.
Fig. 4 Shaded-relief map of IGRF-residual total intensity aeromagnetic anomalies of the study area (Okuma et al., 1998)
The aeromagnetic surveys by GSJ were flown along NW-SE flight lines at an altitude of 460 m above sea level and spaced 5 km, apart, especially 2.5 km apart over the Senkaku Islands (Okuma et al., 1991). Contour interval is 25 nT. Bathymetric contours are in meters. Inclination of the light source, 45 degrees; Declination, 0 degrees.
Fig. 5 Shaded-relief map of reduction-to-the-pole anomalies, calculated from the IGRF-residual total intensity aeromagnetic anomalies (Fig. 3) (Okuma et al., 1998)
H1-H4: Magnetic highs, L1-L2: Magnetic lows. See also Fig. 4.
Generally, magnetic anomalies trend northeast to southwest along the topography (Fig. 4). Magnetic anomalies, which have long-wavelength of more than several tens km and small-amplitude of less than 50 nT, are predominantly distributed in the Tunghai Shelf and west flank of the Okinawa Trough. From the total intensity magnetic anomalies (Fig. 4), a reduction to the pole anomaly map was made through a method in frequency domain (Fig. 5). This map clearly shows a correlation between the bathymetry and magnetic highs at the margin of the Tunghai Shelf.
A large number of magnetic highs (H2) are dominant over the Goto Belt, whereas few intense anomalies are obvious over the Senkaku Belt, except in and around Sekibi-sho Island. This implies a difference in past volcanic activities between the Goto Belt and the Senkaku Belt; more intense activities on the Goto Belt. Magnetic lows (L1) lie west of the Goto-Senkaku Belt in the shelf, where exist the Tokai and Senkaku Basins. Magnetic high area (H1) is distributed north of Uotsuri Island in the shelf, which is inferred to correspond to local uplifts of the basement of the Senkaku Basin.
A magnetic low area (L2) ranges along the west flank of the Okinawa Trough but is limited to the areas from the northeastern edge of the survey area to the northwest of Okinawa Island.
Magnetic highs (H3, H4) can be observed in the Okinawa Trough near the southwestern and northeastern edges of the survey area. Some of these anomalies at the axis of the trough correspond to knolls which are assumed to be magnetized in a direction of the present EarthÕs magnetic field (Oshima et al., 1988).
Fig.6 Magnetic basement depth map of the study area, estimated by a three-dimensional two-layer model inversion (Okuma et al., 1998)
The magnetic basement is assumed to be magnetized in a direction of the present EarthÕs magnetic field (Inclination=38.5°N, Declination=4°W) in the area, with a uniform magnetization intensity of 2.0 A/m. The absolute mean error is 14.9 nT. Contour interval is 1, 000 m. See also Fig. 4.
Fig. 7 Cross section (A-B) on the magnetic basement depth map (Fig. 6)
The 4.5-6.0 km/s layer, corresponding to the acoustic basement of a multi-channel seismic reflection profile, and the 6.2-6.4 km/s layer, probably a granitic layer, are from a seismic refraction profile along a two-dimensional P-wave velocity model (Hirata et al. , 1991) of the 1988 DELP cruise.
As we described before, long-wavelength and small-amplitude magnetic anomalies are predominantly distributed in the Tunghai Shelf and west flank of the Okinawa Trough. These anomalies may correspond to uplift and subsidence of the geologic basement, composed probably of granitic rocks. Therefore, we applied a three-dimensional two-layer model inversion (Gerard and Debeglia, 1975; Okuma et al., 1989) to interpret magnetic anomalies of the study area. First, an average depth of the interface, 3.9 km below sea level, between the non-magnetic upper layer and magnetic basement was estimated from a slope of radially averaged power spectrum of the reduction to pole anomalies (Fig. 5). Then, the deviation from the average depth (i.e. magnetic basement depth) was calculated over the survey area by an iterative convergence method, using pseudogravity and reduction to the pole anomalies. We assumed the magnetic basement is magnetized in a direction of the present EarthÕs magnetic field (Inclination=38.5°N, Declination=4°W) in the area, with a uniform magnetization intensity. We employed 2.0 A/m for the magnetization intensity, taking account of magnetic anomalies caused by volcanic rocks in the Goto-Senkaku belts and the Okinawa Trough in addition to ones caused by granitic rocks. As magnetization intensities of rocks vary from point to point actually, resultant magnetic basement depths in this study don't always coincide with the acoustic basement. In our case (Figs. 6 and 7), the magnetic basement in the southern Okinawa Trough is deeper than the top of the 4.5 - 6.0 km/s layer, the acoustic basement of a multi-channel seismic reflection profile, and shallower than the top of the 6.0 - 6.4 km/s layer (Hirata et al, 1991), probably a granitic layer.
The magnetic basement attains a depth of less than 2 km below sea level on the Goto Belt (Fig. 6). The result implies the existence of magnetized bodies near the sea bottom in this area. They may be related to Middle Miocene volcanic and intrusive rocks of the Goto Belt. Around Sekibi-sho Island, the magnetic basement also rises to a depth of shallower than 2 km below sea level (Fig. 6). However, the uplifted area is limited to near the island, compared to the one in the Goto Belt. This result suggests a difference in geology between the Goto and Senkaku Belt.
The magnetic basement is situated at a depth of 1 km below sea level to the north of Uotsuri Island in the Tunghai Shelf (Fig. 6). This magnetic basement high corresponds to one of large anticlinal structures with gentle dips in the Senkaku Basin, suggested by seismic reflection data (Tokai Univ., 1969). Hydrocarbon accumulations are expected in those structures but intensive seismic investigations are necessary to confirm more detail structures in this area.
The magnetic basement also rises up to near the sea bottom along the east flank of the Okinawa Trough, especially offshore of northern Okinawa Island and near the Miyako Seamount). They correspond to volcanic knolls and intrusive rocks beneath the sea bottom.
The magnetic basement subsides mainly in the Tunghai Shelf and the west flank of the Okinawa Trough (Fig. 6). A magnetic basement low extends in the NNE - SSW or NE - SW direction at a maximum depth of 10 km below sea level in the Tunghai Shelf, though a small magnetic basement high, 100 km north of Sekibi-sho island, interrupts the extension locally. This corresponds to the southern edge of the Tokai Basin and the eastern part of the Senkaku Basin.
The maximum depth of the magnetic basement reaches 10 km below sea level in the west flank of the Okinawa Trough, 150 km offshore of northwest Okinawa Island. A magnetic basement low traverses the Goto Belt in the NW - SE direction and corresponds to a subsidence of the acoustic basement in the northern part of this area, where thick deltaic or submarine fan sediments are deposited (Aiba and Sekiya, 1979). There has a possibility of hydrocarbon accumulations in those thick sediments because of high heat flows, though the sediments seem to be too young to generate them.
Results
A three-dimensional two-layer model inversion was applied to analyze magnetic anomalies in the Tunghai Shelf and the Okinawa Trough. The resultant magnetic basement map gave constraints of the basement structure of the area. This map shows the existence of a local uplifted zone in the Senkaku Basin, magnetic basement depths of 10 km below sea level in the Tokai Basin and in the west flank of the Okinawa Trough north of Okinawa Island.
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