Magnetic lineations of early Cretaceous age in the western equatorial Pacific Ocean
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Bathymetry and magnetic studies (part of the Trans Indian Ocean Geotraverse investigations) in the northeastern Indian Ocean revealed seafloor topographic features, magnetic lineations (19 through 32B) and abandoned spreading centers. The seafloor topography of the Ninetyeast Ridge is relatively wider and shallower south of 15°S. The magnetic anomalies indicate nine fracture zones. Two of them are newly identified. Some of the fracture zones are reflected in the bathymetry. Abandoned spreading centers between 86°E Fracture Zone (FZ) and 92°E FZ are interpreted as the western extensions of the Wharton Ridge. They ceased spreading along with other spreading centers in the Wharton Basin soon after the formation of magnetic anomaly 19 (around 42 Ma) and merged the Indian and Australian plates as single Indo‐Australian plate. The pattern of magnetic lineations between 86°E FZ and 90°E FZ indicate a series of southerly ridge jumps at anomalies 30, 26 (Royer et al., 1991 and other workers) and 19. These ridge jumps transferred portions of the Antarctic plate to the Indian plate. The captured portions and offset along 86°E FZ between India‐Antartica Ridge and Wharton Ridge resulted in an anomalous extra oceanic crust between 86°E FZ and Ninetyeast Ridge spanning 11° in latitude.
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Magnetic total intensity observations off the coast of California to 133°W delineate additional offsets of the magnetic anomaly lineations not previously reported. The lack of topographic expression of the minor magnetic anomaly offsets suggests that either the sedimentary cover masks small displacements or the source of the magnetic anomalies lies in the deeper part of the oceanic crust. A distinct change in amplitude, wavelength, and trend of the magnetic lineations occurs at approximately 124°20′W. This change coincides with a hiatus in the pattern of the magnetic lineations and supports the contention that the process of sea-floor genesis was episodic rather than a continuous event.
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New shipborne surveys provide a closely spaced magnetic anomaly dataset covering the East Subbasin (ESB) of the South China Sea (SCS). Magnetic anomalies of seafloor spreading are identified using the dataset supplemented with previous data and age constraints from recent International Ocean Discovery Program Expeditions 349 and 367/368 holes. We present a high-resolution oceanic crustal age model and associated magnetic lineations of the ESB based on identified magnetic anomaly picks. Seafloor spreading in the ESB initiated at ~30 Ma (C11n) and terminated at ~16 Ma (C5Br). The spreading direction has experienced a gradual counterclockwise rotation between C6Cr and C5Er and a significant counterclockwise rotation at C5Dr. The spreading rotations reorganized the orientation and segmentation of the spreading ridge, resulting in the formation of a series of S-shaped fracture zones. The interpretation of the magnetic lineations reveals that three southward ridge jumps occurred at C9r, C8n, and C7n and a synchronous jump occurred at C5Dr. Three southward ridge jumps contributed to a total difference of ~184 km in the distance between the two flanks and left the paired magnetic lineations C10r–C7r on the present-day north flank. The synchronous jump caused the spreading ridge to rotate rapidly counterclockwise and obliquely intersect the existing seafloor. We postulate that these ridge jumps and rotations are common processes during seafloor spreading reorientation and are dynamic responses to the plate or microplate tectonics around the SCS.
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The nature of the Jurassic Quiet Zone (JQZ), a region of low-amplitude oceanic magnetic anomalies, has been a long-standing debate with implications for the history and behavior of the Earth's geomagnetic field and plate tectonics. To understand the origin of the JQZ, we studied high-resolution sea surface magnetic anomalies from the Hawaiian magnetic lineations and correlated them with the Japanese magnetic lineations. The comparison shows the following: (i) excellent correlation of anomaly shapes from M29 to M42; (ii) remarkable similarity of anomaly amplitude envelope, which decreases back in time from M19 to M38, with a minimum at M41, then increases back in time from M42; and (iii) refined locations of pre-M25 lineations in the Hawaiian lineation set. Based on these correlations, our study presents evidence of regionally and possibly globally coherent pre-M29 magnetic anomalies in the JQZ and a robust extension of Hawaiian isochrons back to M42 in the Pacific crust.
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Abstract Magnetic surveys by a Deep-tow Three-Component Magnetometer (DTCM) were conducted in the northeastern part of the Japan Basin and the central part of the Tsushima (Ulleung) Basin. Magnetic lineations are recognized clearly in the former area, whereas they were not recognized by previous studies in the latter area. The high-quality vector magnetic anomaly data obtained by DTCM enables the precise determination of the strikes of magnetic lineations and the positions of magnetic boundaries. Magnetic anomalies measured by DTCM show the characteristics of linear magnetic anomalies in both basins. The strikes of magnetic lineations are N47°E in the Japan Basin and N82°E in the Tsushima Basin. The estimated magnetization intensities of magnetic source models constructed from the amplitudes of analytic signal calculated from vector anomalies and the crustal structures determined by seismic studies are similar to those of typical extrusive basalt in both basins. The observed anomalies in the Japan Basin contain a short wavelength anomaly which cannot be explained by the model. Their ages may be chrons C5Cr (16.726–17.277 Ma), C5Dn (17.277–17.615 Ma), C5Dr (17.615–18.281 Ma), and sub-chron C5Dr.1n which was identified by a paleomagnetic study. The estimated half-spreading rate is 2.0 cm/yr, which is slower than that estimated by previous study. The observed anomalies in the Tsushima Basin show that there is a partial magnetization high. This may indicate that not all of the sources of magnetic lineations in the Tsushima Basin changed to low magnetization by the effect of thick sediment cover and the intrusions of a large amount of dikes after the formation.
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Magnetic and seismic reflection data of the Japan Basin with a good quality of ships positioning were selected to make a magnetic anomaly and reflective basement contour maps. The magnetic contour map reveals a number of magnetic anomaly lineations. We identified one additional lineation group trending N70°-75°E in the western side of the two another lineations trending N30°-35°E, N65°E formerly identified by other researches. The trends of these anomalies are qualified to be real ones by use of modified semblance analyses. Age identification of these magnetic anomaly lineations is achieved by introducing a basement magnetization model based on the Cenozoic magnetic reversal time-scale referring to the age constraints of other works. Albeit the Japan Basin has a smooth topography of 3, 000-3, 700 m water depth, the reflective basement map shows fairly rugged topography. The reflective basement topography has no correlation to the magnetic anomaly patterns of the present study area. Therefore we adopt an assumption of a flat magnetic basement model referring to recent seismic refraction studies of this area. Using variable half spreading rates, layer thickness, intensity of natural remanent plus induced magnetization, and skewness parameters, a possible range of ages of the formation of these magnetic anomalies is determined. Among a number of possibilities, we assign the age of these anomalies to 13-24 Ma. The older age 22-24 Ma and a half spreading rate 8.0 cm/yr are the best fit to the eastern area. The central area shows an age 22-24 Ma and a slower half spreading rate 7.5 cm/yr. The youngest age 13-15 Ma, and faster half spreading rate 10.0 cm/yr fit to the western anomaly group. We conclude that the formation of the Japan Basin proceeded toward west changing its spreading direction.
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