Northwest Sub-basin Research Articles

Cenozoic Evolution Model of the Sea‐Floor Spreading in South China Sea: New Constraints from High Resolution Geophysical Data

AbstractBased on the analysis of new high resolution gravity and magnetic data, combined with swath bathymetry, the Cenozoic evolution model of the South China Sea is revised. Two major sea‐floor spreading episodes were revealed with different dynamic mechanism, separated by a remarkable tectonic‐sedimentary event near 25 Ma. Sea‐floor spreading started firstly in the East and Northwest Sub‐basins from ~33.5 Ma to 25 Ma, and formed the original oceanic crust with E‐W or NEE‐oriented magnetic anomalies in the northern and southern side of the East Sub‐basin nowadays and whole Northwest Sub‐basin. Since 25 Ma the sea‐floor spreading continued with different direction until 16.5 Ma. The later NW‐SE‐oriented spreading made the NE‐oriented magnetic anomalies in the central East Sub‐basin and Southwest Sub‐basin. Different tectonic areas developed during these two episodic spreading in the oceanic basin. The whole oceanic basin could be divided into three subregions from north to south. The northern and southern sub‐regions are related to the first episodic sea‐floor spreading, while the central one was formed during the later sea‐floor spreading. And the central sub‐region could be further separated into six segments by NW‐NNW‐oriented transform faults. Evidence from magnetic anomalies showed that the sea‐floor spreading in the later episode began in the east segments with C6c anomaly (23.5 Ma), and continued with C6b (22.8 Ma) in the middle segments, and with C5e (18.5 Ma) in the west. We indicated that the central part of the East Sub‐basin and Southwest Sub‐basin experienced progressive rifting from east to west and the spreading direction changed in each segment between 25 Ma and 16.5 Ma. A NNW‐trending transform fault separated the East and Southwest Sub‐basins. It not only cuts the magnetic anomalies with different directions, but also makes a nearly 95 km sinistral offset of the spreading ridge. Before the collision of the Luzon arc with the Eurasia Plate, the South China Sea was a gulf‐shape marginal sea opening to the Pacific Ocean, resulting from westward propagation of a spreading center into the Sunda Continent, as in the case of the western Gulf of Aden.

Cenozoic tectono-sedimentary characteristics and extension model of the Northwest Sub-basin, South China Sea

Based on the interpretations of three seismic profiles and one wide-angle seismic profile across the Northwest Sub-basin, South China Sea, stratigraphic sequences, deformation characteristics and an extension model for this sub-basin have been worked out. Three tectonic-stratigraphic units are determined. Detailed analyses of extension show that the event occurred mainly during the Paleogene and resulted in the formation of half-grabens or grabens distributed symmetrically around the spreading center. Sediments are characterized by chaotic and discontinuous reflectors, indicating clastic sediments. Farther to the southwest, the sub-basin features mainly continental rifting instead of sea-floor spreading. The rifting would have been controlled by the shape of the massif and developed just along the northern edge of the Zhongsha-Xisha Block, rather than joined the Xisha Trough. After 25 Ma, a southward ridge jump triggered the opening of the Southwest Sub-basin. The NW-directed stress caused by the sea-floor spreading of the Northwest Sub-basin may have prevented the continuous opening of the sub-basin. After that the Northwest Sub-basin experienced thermal cooling and exhibited broad subsidence. The deep crustal structure shown by the velocity model from a wide-angle seismic profile is also symmetrical around the spreading center, which indicates that the Northwest Sub-basin might have opened in a pure shear model.

Seismic stratigraphy of the Qiongdongnan deep sea channel system, northwest South China Sea

Based on more than 4000 km 2D seismic data and seismic stratigraphic analysis, we discussed the extent and formation mechanism of the Qiongdongnan deep sea channel. The Qiongdongnan deep sea channel is a large incised channel which extends from the east boundary of the Yinggehai Basin, through the whole Qiongdongnan and the Xisha trough, and terminates in the western part of the northwest subbasin of South China Sea. It is more than 570 km long and 4-8 km wide. The chaotic (or continuous) middle (or high) amplitude, middle (or high) continuity seismic facies of the channel reflect the different lithological distribution of the channel. The channel formed as a complex result of global sea level drop during early Pliocene, large scale of sediment supply to the Yinggehai Basin, inversion event of the Red River strike-slip fault, and tilted direction of the Qiongdongnan Basin. The large scale of sediment supply from Red River caused the shelf break of the Yinggehai Basin to move torwards the S and SE direction and developed large scale of prograding wedge from the Miocene, and the inversion of the Red River strike-slip fault induced the sediment slump which formed the Qiongdongnan deep sea channel.

Magnetic zoning and seismic structure of the South China Sea ocean basin

We made a systematic investigation on major structures and tectonic units in the South China Sea basin based on a large magnetic and seismic data set. For enhanced magnetic data interpretation, we carried out various data reduction procedures, including upward continuation, reduction to the pole, 3D analytic signal and power spectrum analyses, and magnetic depth estimation. Magnetic data suggest that the South China Sea basin can be divided into five magnetic zones, each with a unique magnetic pattern. Zone A corresponds roughly to the area between Taiwan Island and a relict transform fault, zone B is roughly a circular feature between the relict transform fault and the northwest sub-basin, and zones C, D, and E are the northwest sub-basin, the east sub-basin, and the southwest sub-basin, respectively. This complexity in basement magnetization suggests that the South China Sea evolved from multiple stages of opening under different tectonic settings. Magnetic reduction also fosters improved interpretation on continental margin structures, such as Mesozoic and Cenozoic sedimentary basins and the offshore south China magnetic anomaly. We also present, for the first time, interpretations of three new 2D reflection seismic traverses, which are of ~2,000 km in total length and across all five magnetic zones. Integration of magnetic and seismic data enables us to gain a better 3D mapping on the basin structures. It is shown that the transition from the southwest sub-basin to the east sub-basin is characterized by a major ridge formed probably along a pre-existing fracture zone, and by a group of primarily west-dipping faults forming an exact magnetic boundary between zones D and E. The northwest sub-basin has the deepest basement among the three main sub-basins (i.e., the northwest sub-basin, the southwest sub-basin, and the east sub-basin). Our seismic data also reveal a strongly faulted continent–ocean transition zone of about 100 km wide, which may become wider and dominated with magmatism or transit to an oceanic crust further to the northeast.

Effects of forest fire and drought on acidity of a base-poor boreal forest stream: similarities between climatic warming and acidic precipitation

In a boreal forest catchment in the Experimental Lakes Area in northwestern Ontario, wildfire caused an increase in the concentrations of strong acid anions and base cations of the stream. In the naturally base-poor Northwest (NW) Subbasin, a 1980 wildfire caused exports of strong acid anions to increase more than export of base cations, causing a 2.5 fold increase in the acidity of the stream. Mean annual stream pH declined from 5.15 prior to fire to 4.76 two years after fire. Acid-neutralizing capacity (ANC), calculated as the difference between total base cations and strong acid anions, decreased to 20% of pre-fire values. Sulfate and chloride were the strong acid anions responsible for the decline in ANC, increasing four-fold. While nitrate increased eleven-fold, concentrations were too low to significantly affect ANC. There was a significant correlation between weekly sulfate concentration and base cation concentration (r 2 = 0.83) in the two years after fire. Recovery of ANC was caused by the more rapid decline in concentration of sulfate than by changes in base cations. Drought produced a similar but weaker response than fire, with increased sulfate concentrations and decreased stream pH. Climatic warming that increases drought and fire frequency would have effects that mimic the impacts of acidic precipitation (i.e. higher sulfate concentrations and acidic stream waters). Areas which have higher concentrations of stored S from past acid precipitation or have large areas of peatlands in the watershed may have aggravated losses of S and H+ after drought and fire.

Complexities in the tectonic evolution of the West Philippine Basin

The oceanic West Philippine Basin can be subdivided into a main basin, a northwest sub-basin and a southern sub-basin. These basins have undergone different tectonic histories. Magnetic anomalies 21(?)-17 are present in the main basin and indicate that it formed by two-limbed seafloor spreading. The now extinct spreading center is located within a zone of rough topography associated with the northwest-trending Central Basin Fault (CBF). The morphology of the CBF is primarily the result of post-spreading tectonism. The magnetic anomaly pattern of the main basin does not extend into the southern sub-basin or the northwest sub-basin. Geometrical constraints indicate that the southern sub-basin did not form as part of the southern limb of the main basin. The southern sub-basin may have formed immediately before a ridge jump to the main basin spreading axis. Alternatively, possible shallower crustal depth in the southern sub-basin may indicate that the sub-basin is younger than the main basin and formed separately from it. In the northwest sub-basin, which originated as part of the main basin, the dominant topographic trends are perpendicular to those in the main basin. This difference as well as higher heat flow and shallower depth in the northwest sub-basin indicate that it has undergone a deformational event which did not affect the main basin. This deformation may be related to subduction along the east Luzon margin in mid-Tertiary time. The Gagua Ridge, an uplifted sliver of oceanic crust along the western edge of the northwest sub-basin, may have originated during this event.