Pattern Of Seafloor Spreading Research Articles

Cenozoic tectonic subsidence in the Qiongdongnan Basin, northern South China Sea

AbstractA number of major controversies exist in the South China Sea, including the timing and pattern of seafloor spreading, the anomalous alternating strike‐slip movement on the Red River Fault, the existence of anomalous post‐rift subsidence and how major submarine canyons have developed. The Qiongdongnan Basin is located in the intersection of the northern South China Sea margin and the strike‐slip Red River fault zone. Analysing the subsidence of the Qiongdongnan Basin is critical in understanding these controversies. The basin‐wide unloaded tectonic subsidence is computed through 1D backstripping constrained by the reconstruction of palaeo‐water depths and the interpretation of dense seismic profiles and wells. Results show that discrete subsidence sags began to form in the central depression during the middle and late Eocene (45–31.5 Ma). Subsequently in the Oligocene (31.5–23 Ma), more faults with intense activity formed, leading to rapid extension with high subsidence (40–90 m Myr−1). This extension is also inferred to be affected by the sinistral movement of the offshore Red River Fault as new subsidence sags progressively formed adjacent to this structure. Evidence from faults, subsidence, magmatic intrusions and strata erosion suggests that the breakup unconformity formed at ca. 23 Ma, coeval with the initial seafloor spreading in the southwestern subbasin of the South China Sea, demonstrating that the breakup unconformity in the Qiongdongnan Basin is younger than that observed in the Pearl River Mouth Basin (ca. 32–28 Ma) and Taiwan region (ca. 39–33 Ma), which implies that the seafloor spreading in the South China Sea began diachronously from east to west. The post‐rift subsidence was extremely slow during the early and middle Miocene (16 m Myr−1, 23–11.6 Ma), probably caused by the transient dynamic support induced by mantle convection during seafloor spreading. Subsequently, rapid post‐rift subsidence occurred during the late Miocene (144 m Myr−1, 11.6–5.5 Ma) possibly as the dynamic support disappeared. The post‐rift subsidence slowed again from the Pliocene to the Quaternary (24 m Myr−1, 5.5–0 Ma), but a subsidence centre formed in the west with the maximum subsidence of ca. 450 m, which coincided with a basin with the sediment thickness exceeding 5500 m and is inferred to be caused by sediment‐induced ductile crust flow. Anomalous post‐rift subsidence in the Qiongdongnan Basin increased from ca. 300 m in the northwest to ca. 1200 m in the southeast, and the post‐rift vertical movement of the basement was probably the most important factor to facilitate the development of the central submarine canyon.

Ultrahigh resolution marine magnetic anomaly profiles: A record of continuous paleointensity variations?

A distinctive pattern of small‐scale marine magnetic anomalies (25–100 nT amplitude, 8–25 km wavelength: tiny wiggles) is superimposed on the more generally recognized seafloor spreading pattern between anomalies 24 and 27 in the Indian Ocean. By normalizing and stacking multiple profiles, it is demonstrated that this pattern of tiny wiggles is a high‐resolution recording of paleodipole field behavior between chrons C24 and C27. The pattern of tiny wiggles between anomalies 26 and 27 is compared to an ultrafast spreading (82 mm/yr half rate) profile from the southeast Pacific where a similar signal is observed, confirming the paleodipole field origin of the anomalies. Two basic models are considered in which the tiny wiggles are attributed either to short polarity intervals or to paleointensity fluctuations. We conclude that tiny wiggles are most likely caused by paleointensity fluctuations of the dipole field and are a ubiquitous background signal to most fast spreading magnetic profiles. The implications of this study are that (1) tiny wiggles may provide information on the temporal evolution of the geomagnetic dynamo; (2) the small‐scale anomalies observed in the Jurassic quiet zones may be due to paleointensity fluctuations; (3) tiny wiggles are potential time markers in large regions of uniform crustal polarity such as the Cretaceous quiet zones; and (4) much of the variance in anomaly profiles normally attributed to crustal emplacement processes, particularly at fast and ultrafast spreading rates, is actually due to intensity variations in the paleomagnetic field.

Evolution of the Southwest Indian Ridge from the Late Cretaceous (anomaly 34) to the Middle Eocene (anomaly 20)

The determination of the motion of Antarctica relative to Africa is particularly important when considering the breakup of Gondwana. Two models have been proposed that describe the pattern of seafloor spreading between Africa and Antarctica during the Late Cretaceous (starting at chron 34, 84 Ma) through to the Middle Eocene (chron 20, 46 Ma). In the first model, the motion of Antarctica relative to Africa can be simply described by a rotation about a single pole of rotation. In the second model, which we favor, the relative motion of Antarctica and Africa is more complex, and a major change in spreading direction between chron 32 (74 Ma) and chron 24 (56 Ma) times is required. In this paper we present ten plate tectonic reconstructions of the Southwest Indian Ridge that were produced using a new compilation of magnetic, bathymetric and satellite altimetry data, in combination with interactive computer graphics. These reconstructions illustrate that spreading directions started to change at chron 32 (74 Ma). Between chrons 31 and 28 (69-64 Ma), spreading was very slow (< 1 cm/yr) and the direction of spreading changed from NE-SW to a more N-S direction. Between chrons 26 and 24 (61-56 Ma) the direction of spreading shifted back to a N E-SW orientation. These changes in spreading direction suggest that the present-day fracture zones in the area of the Prince Edward Fracture Zone are younger features (Eocene) than their lengths might imply. Our results also provide important constraints concerning the Mesozoic reconstructions of the Indian Ocean and the motion of South America relative to Antarctica prior to the Eocene.

Geologic constraints on the Cenozoic Antarctica-Australia-Pacific relative plate motion circuit

In a revised interpretation of the sea-floor spreading pattern in the southwest Pacific Ocean, Stock and Molnar predicted that the modern Australia-Pacific plate boundary through New Zealand formed about 35 Ma and has accommodated about 800 km of relative motion. Geologic constraints onshore in New Zealand, however, suggest that the modern plate boundary formed about 23 Ma and has accommodated only 500 km of relative motion. The 300 km of additional displacement must have been accommodated elsewhere in the Antarctica-Australia-Pacific plate circuit. Lithospheric extension between East and West Antarctica, which formed the Transantarctic Rift system, accounts for at least 138 km of the additional displacement before the modern plate boundary formed in New Zealand.

Oceanic crust next to passive margins: A structural origin for magnetic zones

Sea-surface 3-D magnetic intensity and resolution threshold curves have been completed for various patterns of spreading-generated magnetic source blocks on the sea-floor. These curves confirm that the earliest magnetic anomaly source blocks to form between oblique-to-spreading continental margins are too small to be individually resolved at the sea surface. This result suggests that a magnetically bland area (quiet zone) will develop during the early stages of spreading between oblique margins, regardless of the behaviour of the Earth's magnetic field at the time that the sea-floor formed. Sea-surface 3-D magnetic anomaly patterns have been computed for a possible sea floor spreading pattern formed between oblique-to-spreading continental margins during a time when regular magnetic reversals were occurring. This computation not only generates both the slope anomalies and quiet zones that characterize such margins, but also generates a diagnostic set of notched and bifurcated anomaly pattern elements, which we have subsequently recognized in marginal magnetic quiet zones adjacent to eastern Canada.

A revised fit of East and West Gondwanaland

Recently available evidence provides the basis for a revised fit between East and West Gondwanaland before break-up in the Late Jurassic: deep-sea drilling shows that the entire Falkland Plateau is probably underlain by continental crust, marine geophysical studies off southeast Africa indicate large areas of thinned continental or transitional crust; palaeomagnetic studies show that the western side of Madagascar lay alongside equatorial East Africa; and from the pattern of sea-floor spreading between Madagascar and India we deduce that the southern half of the western margin of India cannot have lain, as customarily shown, alongside the eastern margin of Madagascar, but must have lain farther south. This information about Madagascar provides the crucial link between East (Antarctica, Australia, India) and West (South America, Africa) Gondwanaland. The rest of East and West Gondwanaland is brought into contact so that the Falkland Plateau opposes the margin of Antarctica between 10° and 15°E and the southern part of South America fits without deformation into the Weddell Sea re-entrant of Antarctica. In terms of the continuity of geological features and the cluster of pre-break-up palaeomagnetic poles, the revised fit is at least as favourable as that of Smith and Hallam (1970). In its close match of the continental outlines and its harmony with the pattern of subsequent sea-floor spreading, the revised fit is superior to previous reconstructions.

Seafloor spreading history of the Agulhas Basin

Data gathered by recent “Islas Orcadas” cruises reveal the seafloor spreading pattern for a region south of the Agulhas/Falkland fracture zone system. The presence of a magnetic anomaly bight about the Agulhas Plateau indicates that the Agulhas Plateau may have developed at the site of a tectonic plate triple junction during the Late Cretaceous. A westward jump in the seafloor spreading center during the Late Maestrichtian (anomaly 34−31) reduced the offset across the Falkland/Agulhas fracture zone system and resulted in the formation of two conjugate aseismic ridges here described as the Meteor and Islas Orcadas Rises. The magnetic lineation pattern in the Agulhas Basin suggests that a tectonic plate (Malvinas Plate) existed during Campanian to Maestrichtian times. Relative rates of motion are calculated for Antarctica, South America, and Africa for the Late Cretaceous.

Origin and structure of the New England Seamount Chain

The New England Seamount Chain is located along a fracture zone formed by a discontinuity in sea-floor spreading pattern. This discontinuity resulted from either a simple transform fault which opened progressively along its length as it was formed, or the entire fault line opened at one time due to a later change in spreading direction. The opening of the fault may have been initiated during earliest Mesozoic and probably continued to the Cretaceous. Differences in rates and/or duration of spreading on either side of the fault may have caused the offset of a previous continuous basin and ridge system beneath the shelf.

The evolution of the Lau Ridge, Fiji Islands

The Lau Ridge lies about 150 miles west of the Tonga Ridge and is sub-parallel to it. It may be an example of a “dead” island arc, activity having transferred to the Tonga-Kermadec region after an Upper Miocene change in the pattern of sea floor spreading in the Pacific. As a consequence of this change the Fiji Plateau has become detached from the main Pacific lithospheric plate. The continental-type Fiji Islands are seen as having evolved inside a sharp bend in the former arc.

Geologic Structure of Baffin Bay and Davis Strait Determined by Various Geophysical Techniques: ABSTRACT

Between 1963 and 1966, approximately 10,000 mi of sea-magnetometer and bathymetry data was collected in the Baffin Bay, Davis Strait, and Labrador Sea areas. The analysis of these data has shown no clear northward continuation of the linear magnetic anomalies which are associated with the Mid-Labrador Sea End_Page 2494------------------------------ Ridge. Previous geologic investigations of the terrestrial Tertiary basalts of the Davis Strait area have shown however, that the lavas are similar enough in chemical composition to be petrogenetically related. The planned 1970 field work, consisting of sea magnetometer, gravity, bathymetry, seismic reflection profiling, seismic refraction measurements, and dredging, should enable the seaward extent and possible chemical evolution of the aseismic ridge of Davis Strait and the crustal type, structure, and sedimentary thickness of Baffin Bay to be determined. The analysis of these data may enable more substantial comment to be given on the pattern of sea-floor spreading between Greenland and Canada. End_of_Article - Last_Page 2495------------

Mineralogy of Cenozoic Deep-Sea Sediments from the Equatorial Pacific Ocean

Sediments ranging in age from Eocene (50 m.y.) to Holocene have been sampled at or near the sea floor in the equatorial Pacific. The mineralogy of fine fractions of these deposits shows that a component derived in large part from tholeiitic debris is the principal constituent of all middle Eocene (50-m.y.-old) to late Miocene (10-m.y.-old) deposits. This component is characterized by abundant montmorillonite, plagioclase of intermediate composition, and the zeolites phillipsite and clinoptilolite-heulandite. It persists as a major constituent of many late Cenozoic southern equatorial deposits but forms a minor portion of Quaternary sediments from the North Pacific. Southern and western equatorial sediments younger than late Miocene (10 m.y.) are rich in pyroxene (including orthopyroxene) and chlorite and have high pyroxene:plagioclase ratios (typically 1.4:1) in 2- to 20-μ fractions. The abundance of pyroxene may in part reflect late Cenozoic changes in the tenor of oceanic volcanism. Most of the pyroxene-rich debris is derived from the island arcs that form the western margin of the Pacific basin, however, and to a much lesser extent from Andean volcanoes. The appearance of the “island-arc” material coincides with a period of heightened tectonism in many marginal areas of the Pacific, and also with an interval when it has been suggested that the rate or pattern of sea-floor spreading was undergoing a marked change. Northern equatorial deposits of Quaternary age are dominated by windborne continental debris rich in quartz and illite. Fine fractions of periglacial loess deposits associated with Pleistocene ice masses in the northern hemisphere probably constitute the main source of such debris. Tertiary equatorial sediments (with the exception of a few northwestern equatorial occurrences of Pliocene age) consist of less than 20 percent “continental” material. Of the minerals studied, only chlorite appears to be modified by prolonged exposure to pore water. The crystallinity of chlorite deteriorates over a period of 5 to 10 m.y. but then changes little over additional periods of tens of millions of years. The only other observed pronounced diagenetic effect is a systematic increase in the grain size of older sediments due to dissolution of opaline tests and reprecipitation of the silica as cement.