Thomas Lüdmann

Neotectonic regime on the passive continental margin of the northern South China Sea

Abstract Between 1989 and 1994, more than 6600 km of reflection seismic profiles were obtained in the South China Sea off Hong Kong with the German research vessel Sonne during cruises SO-50B, SO-72A and SO-95. A seismo-stratigraphic interpretation of this data set leads to a new age assignment of the unconformity T0 which we place within the Pleistocene. Both Neogene unconformities T1 and T0 are generated by uplift of the Dongsha Rise and truncation of their overlying strata. This uplift is caused by intrusion of magma into the upper crust. Our seismic profiles show plutons which have penetrated the sedimentary cover, whereby their original stratification in the contact zone is eliminated. These magmato–tectonic events may be correlated to the two main collision phases between Taiwan and the continental margin of East China 5–3 and 3–0 ma ago. The collisional events subsequent to the NNW to WNW drift of Taiwan transformed the compression into strike–slip movements along the continental margin of Southeastern China. The accompanying stress regime is transtensional, with subsidence of the cooling oceanic crust since the cessation of rifting and its consumption beneath the Manila Trench providing the extensional stress. The strike–slip movements remobilized many of the rift and drift faults providing pathways for magma ascent. The tectonic framework of the northern South China Sea is characterized by Miocene faults trending NE–SW. These faults are scarce but are distributed throughout the study area. Pliocene faults striking ENE–WSW to NE–SW are concentrated west of the Dongsha Islands and are mostly strike–slip in character. Recent faults are generally oriented NE–SW subparallel to the synrift faults. They result in part from local uplifts where they are normal in character, but strike–slip motion also occurs. Most of the faults involve the basement and represent reactivated zones of weakness of the rift and drift phases.

THE SEA OF MARMARA: A PLATE BOUNDARY SEA IN AN ESCAPE TECTONIC REGIME

Abstract The Sea of Marmara is dissected by two major fault systems. The first consists of two east-west-striking, transtensional boundary faults and a number of secondary faults subparallel to them. The second is made up of NE-SW-trending, subvertical strike-slips and their conjugates oriented NW-SE that offset the first system. These fault systems segment the deep Marmara Sea into 5 blocks that are either rhomboidal, lazy-Z or wedge-shaped. Three of these blocks may be interpreted as pull-apart basins characterized by transtension, while the other two (sill areas) which separate the basins are transpressional push-up structures. The blocks are subjected to rapid, episodic subsidence, but they also undergo pervasive vertical motions and possible rotations relative to one another. The sedimentary column above the acoustic basement mapped within the Sea of Marmara can be divided into 3 seismic units: a folded and truncated pre-transform unit and two syn-transform units. The lower, thick syn-transform unit exhibits 3 distinct seismic facies: a well-stratified basin-fill facies cut by subvertical growth faults, a push-up facies with a contorted or chaotic internal configuration and a slump facies. The observed neotectonic and sedimentary regimes result from compressional movement between Eurasia and Africa, which has led to major dextral transform movements along the North Anatolian Fault Zone (“escape” tectonics), and from the fact that this fault zone splinters into two overlapping, right-stepping, oblique master faults at the eastern and western border of the Sea of Marmara respectively.

Monsoon-induced partial carbonate platform drowning (Maldives, Indian Ocean)

Multibeam maps and high-resolution seismic images from the Maldives reveal that a late Miocene to early Pliocene partial drowning of the platform was linked to strong sea-bottom currents. In the upper Miocene to Holocene, currents shaped the drowned banks, the current moats along the bank edges, and the submarine dune fields. Bottom currents in the Maldives are driven by the monsoon. It is proposed that the onset and the intensification of the monsoon during the Neogene provoked platform drowning through injection of nutrients into surface waters. Since the late Miocene, topographically triggered nutrient upwelling and vigorous currents switched the Maldives atolls into an aggradational to backstepping mode, which is a growth pattern usually attributed to episodes of rising sea level.

Characteristics of gas hydrate occurrences associated with mud diapirism and gas escape structures in the northwestern Sea of Okhotsk

Abstract An interpretation of 8450 line-km of seismic reflection data acquired during four cruises in the northwestern Sea of Okhotsk shows widespread occurrence of free gas or gas hydrates in the sediment. This occurrence is documented seismically by gas escape structures, acoustic blanking, and by a bottom simulating reflector (BSR). Gas escape structures and vents are concentrated on the northern Sakhalin continental margin. Here, they are associated with local tectonic movements along a N–S trending dextral shear system (the Inessa Shear Zone). Mud diapirism is common in the Derugin Basin where compression dominates. This diapirism is probably closely related to the gas accumulations in the sediment below the base of the gas hydrate stability zone (BGHSZ). Conductive heat flow derived from the BSR depth distribution averages about 30±8 mW m −2 . Only adjacent to basement highs and around the Inessa Shear Zone do values higher than 80±22 mW m −2 occur. These high values are attributed to an elevated geothermal gradient in the tectonically active zone where fluid venting takes place. Areas of drift sedimentation and mass wasting, where sedimentation rates are high, are characterised by computed heat flow values below 30±8 mW m −2 . However, these values must be corrected upwards by 3–20% depending on the sedimentation rate applicable (3.8–100 cm/kyr). The total amount of methane preserved in the hydrate stability zone (HSZ) and trapped as free gas beneath the BSR is estimated at 17±14×10 12 m 3 for the northwestern Sea of Okhotsk and 15±12×10 13 m 3 for the entire Sea of Okhotsk. The latter figure represents about 0.8% of the global reservoir of methane gas from hydrates. Our study documents that the semi-enclosed Sea of Okhotsk offers favourable conditions for the accumulation of gas in its sediments on account of its subarctic climate and the prevailing hydrologic regime. These conditions include high primary productivity, low bottom water temperatures and high sedimentation rates.

Bottom current-controlled sedimentation and mass wasting in the northwestern Sea of Okhotsk

Abstract Quaternary sedimentation in the northwestern Sea of Okhotsk, where tidal and thermohaline currents are active, was studied using 8443 km of high-resolution air gun profiles from four cruises. It is characterized by: (1) bottom current-controlled processes, which lead to widespread deposition of contourite drifts and sediment waves on the North Okhotsk continental margin and the northernmost Sakhalin slope, as well as to erosion and sediment reworking on the northern Sakhalin shelf; (2) mass wasting triggered probably by shallow earthquakes, by gas hydrate instability during sea-level lowstands leading to slumps and debris flows in the western Derugin Basin; (3) deposition of the fluvial load of the River Amur, which results in sediment drift bodies and prograding lowstand wedges during glacial periods, and to contourite drifts and a ‘fan’ during interglacial times; and (4) ice-rafted detritus and hemipelagic sedimentation interrupted by episodic turbidity current activity, especially in the Derugin Basin and its northern, eastern and southern flanks.

Periplatform drift: The combined result of contour current and off-bank transport along carbonate platforms

Hydroacoustic and sedimentological data from the western leeward flank of the Great Bahama Bank document the interplay of off-bank sediment export, along-slope transport, and erosion, which together shape facies and thickness distribution of slope carbonates. The integrated data set depicts the combined product of these processes and allows formulation of a comprehensive model of a periplatform drift that significantly amends established models of carbonate platform slope facies distribution and geometry. The basinward-thinning wedge of the periplatform drift at the foot of the bank escarpment displays along-slope and downslope variations in sedimentary architecture. Sediments are muddy carbonate sands that coarsen basinward. The drift wedge has a pervasive cover of cyclic steps. In zones of lower contour current speed, depth-related facies belts develop, whereas strike-discontinuous sediment lobes, scarps, and gullies characterize areas with higher current speed. This understanding of the impact of currents on carbonate-slope sedimentation has wider implications for seismic and sequence stratigraphic interpretation of carbonate platforms and for applied aspects such as hydrocarbon exploration.

Tectonic contrast between the conjugate margins of the South China Sea and the implication for the differential extensional model

The extensional model of the South China Sea (SCS) has been widely studied, but remains under debate. Based on the latest high-quality multi-channel seismic data, bathymetric data, and other obtained seismic profiles, the asymmetric characteristics between the conjugate margins of the SCS are revealed and extensional model of the SCS margin is discussed further. Spatial variation of morphology, basement structure, and marginal faults are discovered among the SCS margin profiles. As for the NS-trending variation, the basement of northern margin displays in the shape of step downwards to the sea, while the basement of southern margin is composed of wide rotated and tilted blocks, without any obvious bathymetric change. The variation also exists in the development of marginal faults between the conjugate margins, and detachment fault system is identified on the southern margin. Along the southern margin from east to west, the Eastern and Southwestern Basins developed different structural units. Based on the tectonic contrast of the conjugate margins, differential extensional model is proposed to explain the spatial variation of the SCS structure, which introduces detachment faults controlling the evolution of the SCS. The upper crust above the detachment fault was deformed by simple shear, while the lower crust and upper mantle below the detachment fault was deformed by pure shear. Because of the different lateral transfer between the upper brittle faulting and the lower ductile extensional regions, there developed marginal plateau (Liyue basin) and outer rise (Zhenghe massif) on the lower plate margin of the Eastern Basin and the Southwestern Basin, respectively. The evolution of the present SCS may be influenced by the diachronous close of the paleo-SCS.

Lowstand wedges in carbonate platform slopes (Quaternary, Maldives, Indian Ocean)

Seismic, hydroacoustic and sedimentological data were used to analyse the response of atoll‐slope sedimentation in the Maldives to the late Quaternary sea‐level change. The slope deposits, as imaged in multichannel seismic profiles, are arranged into stacked aggrading to backstepping basinward thinning wedges. In a piston core recovered at the lower slope of one of the atolls, the sediment texture ranges from packstone to rudstone. Major components are blackened bioclasts, the large benthic foraminifers Operculina and Amphistegina, together with Halimeda debris and red algae. Radiocarbon dating at a core depth of 66 cm indicates that the wedge sedimentation stopped or was largely reduced after 16 ka BP. Therefore, the atoll‐slope deposits largely consist of sediment formed in situ and deposited during the last glacial lowstand in sea‐level. This is in apparent contradiction to the concept of highstand shedding of tropical carbonate platforms, which requires slope sedimentation during sea‐level highstands, when the platform is flooded. Rather than intrinsic factors, such as sediment bypass along the steep slope, the extrinsic process of current winnowing of the slope appears to be a major controlling factor in the production of this feature. This process may be relevant for other case studies of carbonate platforms, as currents may be accelerated around such edifices, leading to slope winnowing and sediment deposition in more current‐protected zones. The study results also have consequences for the interpretation of outcrop and seismic subsurface data of carbonate platform slope series, because such slope sediment wedges are not necessarily formed during sea‐level highstands, but can consist of lowstand wedges only.

Mixed Carbonate-Siliciclastic Deposits in a Channel Complex in the Northern South China Sea

New high-resolution 3D seismic data image a submarine channel complex in the northern slope of the South China Sea. The channel complex stretches hundreds of kilometers across the slope and flows into the deepsea from the siliciclastic shelf margin, linking neritic environment to the pelagic plain. The evolution of the channel complex developed two sedimentary stages, stage I (19.1–18.5 Ma) and stage II (18.5–17.5 Ma), separated by erosional surfaces. In the first stage, the complex was filled with pure siliciclastic sediments, forming thick-massive sandstone intercalated by thin layers of mudstone. During the stage II, the channel complex was deposited five carbonate-siliciclastic cycles. The unexpected channel-fill carbonate deposits present allochthonous characteristics, suggesting the siliciclastic channel was surprisingly used to transport carbonate sediment from the adjacent neritic carbonate platform. By analyzing the periodical carbonate sedimentary process in the siliciclastic channel complex, we infer that it was related to the in situ carbonate production of the neritic carbonate platform and was most likely to be controlled by the relative sea-level changes. Unlike line-source carbonate slope aprons or small-sized carbonate channels, the large-sized siliciclastic channel complex links directly neritic carbonate platform to deepwater basin and can transport large volumes of neritic carbonates to the pelagic environment in a short period. The new findings help to estimate the contributions of neritic siliciclastic shelf and carbonate platform to deepwater slope more accurately. This study suggests that channel systems are more complex than expected and have significant implications on the conceptual models describing the deepwater sedimentary theory.