Coring/logging data and physical property measurements from International Ocean Discovery Program Expedition 349 are integrated with, and correlated to, reflection seismic data to map seismic ...sequence boundaries and facies of the central basin and neighboring regions of the South China Sea. First‐order sequence boundaries are interpreted, which are Oligocene/Miocene, middle Miocene/late Miocene, Miocene/Pliocene, and Pliocene/Pleistocene boundaries. A characteristic early Pleistocene strong reflector is also identified, which marks the top of extensive carbonate‐rich deposition in the southern East and Southwest Subbasins. The fossil spreading ridge and the boundary between the East and Southwest Subbasins acted as major sedimentary barriers, across which seismic facies changes sharply and cannot be easily correlated. The sharp seismic facies change along the Miocene‐Pliocene boundary indicates that a dramatic regional tectonostratigraphic event occurred at about 5 Ma, coeval with the onsets of uplift of Taiwan and accelerated subsidence and transgression in the northern margin. The depocenter or the area of the highest sedimentation rate switched from the northern East Subbasin during the Miocene to the Southwest Subbasin and the area close to the fossil ridge in the southern East Subbasin in the Pleistocene. The most active faulting and vertical uplifting now occur in the southern East Subbasin, caused most likely by the active and fastest subduction/obduction in the southern segment of the Manila Trench and the collision between the northeast Palawan and the Luzon arc. Timing of magmatic intrusions and seamounts constrained by seismic stratigraphy in the central basin varies and does not show temporal pulsing in their activities.
Key Points
First core‐log‐seismic correlation in the central South China Sea basin
The fossil ridge and subbasin boundary acted as major sedimentary barriers
Seismic facies and migration of depocenter responded to tectonic events
Bathymetric, gravity, magnetic and backscattering strength data have been used to characterise the segmentation of an 800 km long portion of the ultraslow-spreading Southwest Indian Ridge (SWIR, full ...rate 14 mm/yr) between 49°15′E and 57°E. This analysis reveals that the segmentation defined by along-axis variations of depth and by occurrence of axial offsets does not systematically correspond to the segmentation determined by the along-axis variations of backscattering strength, mantle Bouguer anomaly (MBA) and amplitude of the central magnetic anomalies (CMA). At axial discontinuities with offsets larger than 15 km, thin crust and reduced volcanic production are suggested by the occurrence of MBA highs, almost non-existent CMA and 50% lower backscattering strength relative to the segment centres. By contrast, smaller non-transform discontinuities, with offsets smaller than 15 km, correspond to very weak variations or to no variation of the MBA, the CMA or the reflectivity of the seafloor, suggesting that there is little or no variation of volcanic production and crustal thickness associated with those small discontinuities. These small axial discontinuities bound low-relief bathymetric segments (500–700 m), corresponding to weak or no MBA lows (amplitude <11 mGal), and robust high-relief segments (>1000 m), corresponding to large MBA lows (amplitude >30 mGal). We suggest that the magma supply to these low-relief segments is controlled by near-surface processes such as melt migration and/or crustal plumbing from adjacent high-relief segments. Pronounced MBA lows at high-relief segments are thought to correspond to spreading cells where magma supply is focused in the mantle. These spreading cells are spaced by about 100 km along the SWIR axis. We suggest that the spacing of spreading cells on slow-spreading ridges is primarily controlled by the spreading rate with larger spacing between spreading cells on ultraslow-spreading ridges than on slow-spreading ridges.
A recent survey of the Mid-Atlantic Ridge over the southern edge of the Azores Platform shows that two anomalously shallow regions located off-axis on both sides of the ridge are the two flanks of a ...single rifted volcanic plateau. Crustal thickness over this plateau is up to twice that of surrounding oceanic areas, and original axial depths were near sealevel. The lack of a coherent magnetic anomaly pattern, and the near absence of fault scarps over the plateau suggest that its formation involved outpouring of lava over large distances off-axis. This volcanic plateau formed in Miocene times during an episode of greatly enhanced ridge magmatism caused, as proposed by P.R. Vogt Geology 7 (1979) 93–98, by the southward propagation of a melting anomaly originated within the Azores hotspot. This melting anomaly could reflect excess temperatures of ∼70°C in the mantle beneath the ridge. It propagated at rates of ∼60 mm/yr and lasted no more than a few million years at any given location along the ridge. Enhanced magmatism due to this melting anomaly played a significant role, some 10 Ma ago, in the construction of the Azores Platform.
Temporal fluctuations of magmatic processes during the last 800 kyr have been investigated for the slow spreading Central Indian Ridge. The fluctuations are recorded by variations in lava chemistry ...along a 40 km long profile across the ridge. The temporal relations of the basalts were accurately restored using magnetic microanomalies. We report on the occurrence of ancient lavas enriched in incompatible elements whereas on‐axis samples are typical normal mid‐ocean ridge basalts. The enriched lavas are symmetrically distributed on either side of the ridge, implying that enriched melts reached the seafloor at intervals of about 150–200 kyr. This periodicity is viewed as a characteristic time scale in the aggregation processes of the melts produced from a heterogeneous mantle source. Geochemical variations of zero‐age mid‐ocean ridge basalts may primarily reflect such periodic processes rather than the spatial distribution of mantle heterogeneities.
Non–hot spot, intraplate volcanism is a common feature near the East Pacific Rise or Pacific‐Antarctic ridge. Volcanic ridges and seamount chains, tens to hundreds of kilometers long, are ...asymmetrically distributed about the ridge axis, with most volcanic features occurring on the Pacific plate. Their origins remain controversial. We have analyzed off‐axis volcanic ridges near the Pacific‐Antarctic ridge from bathymetry, backscatter, gravity, and geochemistry data of the Pacantarctic 2 cruise. K/Ar dating of samples dredged on these structures reveals a contrast of up to 3 Ma between the volcanoes and the underlying crust. The volcanic activity, as suggested by the strong backscatter in sonar images, appears to be limited to areas of seafloor younger than about 3 Ma. All surveyed ridges north of the Menard transform fault (TF) show recent activity close to the ridge axis and are not affected by faults. The off‐axis volcanic ridges south of the Menard TF show recent volcanic flows in their center and are affected by N‐S extension. Two different types of volcanoes can be characterized: conical, flat‐topped ones and rough, elongated ones associated with narrow, E‐W trending volcanic ridges, some of them showing strong backscattering on the EM12 imagery. From the morphology of the seamount chains and the ages of the lava samples, we infer that the magma source for the volcanic ridges is related to the feeding of the ridge axis through three‐dimensional mantle convective circulation. We suggest that a change in the relative plate motion since about 5 Ma might have induced an offset of the mantle upwelling circulation under the ridge axis so that anomalously hot mantle rises under the Pacific ridge flank. The kinematic change is also likely responsible for the tectonic deformation in the young lithosphere south of Menard TF.
Accretionary processes at mid‐ocean ridge segments with low magma input have seldom been investigated over the long term. The evolution of such magma‐starved segments over time is still largely ...unknown. We present a study on the structure and evolution of the southernmost intra‐transform ridge segment of the St. Paul Transform Fault System in the Equatorial Mid‐Atlantic Ridge, based on new bathymetry, gravity, and rock sampling data. We show that this area evolves differently from previously described tectonics along ridge segments of similar spreading rate. On the flanks of the axial ridge segment, we observe a succession of structures exhumed by detachment faulting, evolving from east‐facing, long‐lived, corrugated oceanic core complexes (∼6 Ma ago), to short‐lived detachment faults exposing lower crust and mantle rocks and facing alternatively east and west in the more recent part of the segment. The oldest detachment faults have been repeatedly split and partially transferred to the opposite flank through the formation of new detachments into the footwall. The terminations of three old, east‐facing detachments are observed on the east flank of the segment. The westward relocations of the plate boundary appear to compensate for the asymmetry of accretion through detachment faulting, overall creating the same amount of lithosphere on both flanks of the ridge. We interpret the observed changes in the time of the accretionary processes to reflect a decrease of the melt supply over the last ∼6 Myr.
Plain Language Summary
The generation of new seafloor at mid‐ocean ridges where cold underlying mantle delivers low magma supply has not been investigated over the long term. Here we present the analysis of new bathymetry, gravity, and rock sampling data over such a ridge segment located within the St. Paul Transform Fault system in the Equatorial Mid‐Atlantic Ridge, which allowed us to bring constraints on its structure and evolution over the last ∼6 Myr. We show that this area evolves differently from previously described ridge segments of similar spreading rate. We observe the remnants of very large normal faults called detachment faults, which have been active for very long times, forming domes called oceanic core complexes. The fault surfaces have been dissected by further extensional deformation, until the plate motion became accommodated along a new detachment fault formed west of the previous plate boundary. The emergence lines of the detachment faults are observed on the eastern flank of the ridge segment. We also observe a change in time of the structures, from typical oceanic core complexes to shorter ridges formed by the exhumation of mantle rocks. We interpret these changes to possibly reflect a decrease in the melt supply in the last 6 Myr.
Key Points
The southern intra‐transform ridge segment in the St. Paul transform fault system, over the last ∼6 Myr is dominated by detachment faulting
Several detachments have been split and partly transferred to the opposite ridge flank through ridge axis relocation
The change in the accretionary processes, from Oceanic Core Complexes to mantle exhumation, suggests a decrease in the time of the melt supply
The high‐resolution geoid and gravity maps derived from ERS‐1 and Geosat satellite geodetic missions reveal a set of small‐scale lineations on the flanks of slow to intermediate spreading mid‐ocean ...ridges. Assuming that these lineations reflect the variations in crustal structure induced by mid‐ocean ridge axial discontinuities, we use them to investigate how the discontinuities, and the segments they bound, appear, migrate, and disappear. We provide a synoptic description of the main characteristics of the crustal structure variations, as well as their evolution in time, over the flanks of the Mid‐Atlantic, Indian, and Pacific‐Antarctic Ridges. The second‐order segment length does not appear to vary with the spreading rate for the slow to intermediate spreading ridges investigated here. The amplitude of the gravity signal associated with off‐axis discontinuity traces increases with the obliquity of the ridge to spreading and decreases with spreading rate and with the proximity of a ridge section to a hot spot. The patterns of the gravity lineations appear to be very homogeneous over 500‐ to 1000‐km‐large corridors bounded by large fracture zones. Far from hot spots, corridors are characterized either by segments bounded by discontinuities migrating back and forth along the axis, implying a lifetime of 10–30 Myr for the segments, or by segments and discontinuities very stable in space and time, surviving for 40–50 Myr. Closer to hot spots, the segmentation is affected in two ways. First, segments tend to migrate along axis away from hot spots, or toward cold spots. Second, asymmetric spreading tends to keep sections of ridges closer to hot spots than normal spreading would. These observations support the hypothesis that ridge segmentation and its evolution are controlled by mantle dynamics. Our analysis provides observational constraints for further models of crustal production along ridges, which are presented in the companion paper by Rabinowicz and Briais 2002.
Combined analyses of deep tow magnetic anomalies and International Ocean Discovery Program Expedition 349 cores show that initial seafloor spreading started around 33 Ma in the northeastern South ...China Sea (SCS), but varied slightly by 1–2 Myr along the northern continent‐ocean boundary (COB). A southward ridge jump of ∼20 km occurred around 23.6 Ma in the East Subbasin; this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the Southwest Subbasin, which propagated for about 400 km southwestward from ∼23.6 to ∼21.5 Ma. The terminal age of seafloor spreading is ∼15 Ma in the East Subbasin and ∼16 Ma in the Southwest Subbasin. The full spreading rate in the East Subbasin varied largely from ∼20 to ∼80 km/Myr, but mostly decreased with time except for the period between ∼26.0 Ma and the ridge jump (∼23.6 Ma), within which the rate was the fastest at ∼70 km/Myr on average. The spreading rates are not correlated, in most cases, to magnetic anomaly amplitudes that reflect basement magnetization contrasts. Shipboard magnetic measurements reveal at least one magnetic reversal in the top 100 m of basaltic layers, in addition to large vertical intensity variations. These complexities are caused by late‐stage lava flows that are magnetized in a different polarity from the primary basaltic layer emplaced during the main phase of crustal accretion. Deep tow magnetic modeling also reveals this smearing in basement magnetizations by incorporating a contamination coefficient of 0.5, which partly alleviates the problem of assuming a magnetic blocking model of constant thickness and uniform magnetization. The primary contribution to magnetic anomalies of the SCS is not in the top 100 m of the igneous basement.
Key Points:
Deep tow magnetics and IODP Expedition 349 constrained opening history
Variations are observed in spreading rate and onset of drifting and ridge jump
Magnetic anomalies are not primarily sourced from the top 100 m of the basement
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