How Kin Wong

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.

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.

Multiple Pleistocene ice advances into the Skagerrak: A detailed seismic stratigraphy from high resolution seismic profiles

Abstract A detailed Pleistocene seismic stratigraphy is proposed for the Skagerrak, an over 700 m deep, elongated depression in the northeastern North Sea. By comparing our high resolution seismic results with those from glacial environments of Canada, Alaska and Antarctica, we conclude that at least during the Weichselian (if not also during the Saalian) a glacier grounded in the eastern Skagerrak and advanced well into its western part as indicated by a thick layer of basal till. The surging glacier truncated the basinal flank of a ridge-like “delta moraine” that was presumably deposited under uniform glacial conditions in an ice-distal environment. The deposition of the Hirtshals Moraine in shallow waters of Hirtshals might originate from a glacier that advanced from Sweden into North Jutland. Soon after the ice withdrawal from the Skagerrak, a layer of well-stratified glaciomarine sediments was deposited over the basal till suggesting floating ice conditions in a fjord-like environment. Due to the rising sea level in the Late Pleistocene, fluvioglacial and shallow marine deposits soon dominated over the glaciomarine sediments, indicating an end of glacial conditions in this region. These post-glacial sediments were first deposited in the western Skagerrak, but as the transgression came to an end in the Mid-Holocene, the depocenter shifted to the east. Today the highest sedimentation rates are found in this eastern part.

Seismic stratigraphy and Quaternary sedimentation in the Skagerrak (northeastern North Sea)

Abstract Ice movement, sea-level fluctuations and currents are the controlling factors of the Late Pleistocene and Holocene sedimentation processes in the Skagerrak. The erosional truncation at the top of a Mesozoic depositional sequence defines the extent of the Norwegian Channel and functions as the initial depositional surface for Quaternary sedimentation. On this surface in shallow waters off the Danish coast, Weichselian glacial sediments are deposited. An end-morain ridge marks a temporary stillstand of the retreating ice front. At the beginning of ice-free conditions some 15,000 yrs ago when the sea was about 70 m below the present level, the Skagerrak was a fjord-like forebasin of the Northern Atlantic, and constituted a favorable outlet for rivers draining the subaerial glacial deposits of the southern Skagerrak and the present southern North Sea. Fluvial erosion released large amounts of detrital material which was laid down in the form of a progradational delta-prodelta complex. Slumping and mass-flow processes led to high sedimentation rates and a rapid infilling of the Norwegian Channel in front of the delta complex. With the rapid transgression of the southern North Sea at the beginning of the Holocene, a dramatic change in the depoenvironment of the Skagerrak took place. The modern current pattern became established, whereby sediment transport and redeposition became the rule along the Danish coast. Sandwaves and megaripples with an apparent northeasterly migration direction were formed as a result of the wind-forced Jutland Current. Fine suspended particles bypass the coastal zones to settle out in the deep Norwegian Channel.