Sedimentation processes and new age constraints on rifting stages in Lake Baikal: results of deep-water drilling

 With this paper we present a first attempt to combine the direct results on lithology, composition and age dating in the boreholes BDP-93, BDP-96 and BDP-97 with geological and seismic data from the areas where those sections were drilled. The sedimentary environments represented by the BDP boreholes are markedly different and possess characteristic lithological features. The results of the deep drilling provide the essential means for testing numerous age models used in geological reconstructions of the Lake Baikal rifting dynamics. Neither the basin-wide unconformity interpreted from seismic data, nor the interpreted change from shallow-water to deep-water facies at the boundary of the seismic stratigraphic complexes were found in the BDP-96 boreholes on Academician Ridge. Also, lithology does not support the proposed reconstructions of intense lake level fluctuations and transgressions during the Pliocene at Academician Ridge. The continuous deep-water hemipelagic sedimentation at Academician Ridge has existed for the past 5 Ma. The beginning of an intense rifting phase of the Neobaikalian sub-stage and related drastic changes in sedimentation processes were interpreted on seismic sections as the basin-wide unconformity B10. Different age estimates for this boundary ranged from Late Pliocene (3.5 Ma) to Plio-Pleistocene boundary. As shown by BDP-96 borehole, B10 is associated with a lithological change from diatomaceous ooze to dense silty clay and not with an erosional contact. The new age for this boundary in BDP-96 is approximately 2.5 Ma. This new age constraint suggests that the upper sedimentary strata of Northern Baikal (1.5–1.7 km thick) have formed during the past 2.5 Ma with average sedimentation rates of 60–70 cm/ka. The BDP-93 boreholes at Buguldeika suggest that uplift in Primorsky Range took place prior to 1.07–1.31 Ma, a date which exceeds the age of previous geological models. Received: 12 March 1999 / Accepted: 10 February 2000     

The basins of Sundaland (SE Asia): Evolution and boundary conditions

Most of the basins developed in the continental core of SE Asia (Sundaland) evolved since the Late Cretaceous in a manner that may be correlated to the conditions of the subduction in the Sunda Trench. By the end of Mesozoic times Sundaland was an elevated area composed of granite and metamorphic basement on the rims; which suffered collapse and incipient extension, whereas the central part was stable. This promontory was surrounded by a large subduction zone, except in the north and was a free boundary in the Early Cenozoic. Starting from the Palaeogene and following fractures initiated during the India Eurasia collision, rifting began along large faults (mostly N–S and NNW–SSE strike-slip), which crosscut the whole region. The basins remained in a continental fluvio-lacustrine or shallow marine environment for a long time and some are marked by extremely stretched crust (Phu Khanh, Natuna, N. Makassar) or even reached the ocean floor spreading stage (Celebes, Flores). Western Sundaland was a combination of basin opening and strike-slip transpressional deformation. The configuration suggests a free boundary particularly to the east (trench pull associated with the Proto-South China Sea subduction; Java–Sulawesi trench subduction rollback). In the Early Miocene, Australian blocks reached the Sunda subduction zone and imposed local shortening in the south and southeast, whereas the western part was free from compression after the Indian continent had moved away to the north. This suggests an important coupling of the Sunda Plate with the Indo-Australian Plate both to SE and NW, possibly further west rollback had ceased in the Java–Sumatra subduction zone, and compressional stress was being transferred northwards across the plate boundary. The internal compression is expressed to the south by shortening which is transmitted as far as the Malay basin. In the Late Miocene, most of the Sunda Plate was under compression, except the tectonically isolated Andaman Sea and the Damar basins. In the Pliocene, collision north of Australia propagated toward the north and west causing subduction reversal and compression in the short-lived Damar Basin. Docking of the Philippine Plate confined the eastern side of Sundaland and created local compression and uplift such as in NW Borneo, Palawan and Taiwan. Transpressional deformation created extensive folding, strike-slip faulting and uplift of the Central Basin and Arakan Yoma in Myanmar. Minor inversion affected many Thailand rift basins. All the other basins record subsidence. The uplift is responsible for gravity tectonics where thick sediments were accumulated (Sarawak, NE Luconia, Bangladesh wedge).     

西秦岭海西-印支期裂陷活动及其与古特提斯的关系

海西-印支期，西秦岭地区受古特提斯洋的影响在前海西期变质基底之上发育三期裂陷活动（１．泥盆纪；２．石炭纪；３．二叠至三叠纪），造成该区地层展布呈条带状．深水沉积与浅水沉积相间，以及活动型海槽沉积与稳定型浅海沉积相间的格局．是加里东期之后秦岭构造带东西分异的主要原因。     

6710铀矿区火成岩的地球化学特征及其构造和成矿意义

火山岩型铀矿床通常与酸性至中性的火山岩类有关 ,而 6 710铀矿区的铀矿化与双峰式火山岩和基底花岗岩有紧密的空间关系。研究表明该套双峰式火山岩中玄武岩的Rb Sr等时线年龄为173± 9 7Ma ,流纹岩为 16 4 8± 0 57Ma。前者的不相容元素地球化学特征与板内拉斑玄武岩相一致 ;后者为准铝质 (ACNK =0 88- 0 96 ) ,稀土元素 (ΣREE :(2 50 33- 2 95 99)× 10 - 6 )和高场强元素 (Y ,Zr,Nb等 )的含量较高 ,与A型花岗岩的地球化学特征相似。基底花岗岩的Rb Sr等时线年龄为 2 4 9 9± 5 5Ma ,岩石为过铝质 (ANKC =1 18- 1 2 6 ) ,富集LREE((La/Lu) N=2 8 51-55 0 9)和大离子亲石元素 (Rb ,Th ,U等 ) ,与同造山S型花岗岩的地球化学特征一致。双峰式火山岩表明该区在燕山早期具有拉张裂解的地球动力学背景 ,基底花岗岩代表了海西晚期的挤压造山环境。它们分别为本区铀成矿提供了矿化剂 (CO2 - 3)和铀源。     

Shear wave attenuation characteristics over the Central India Tectonic Zone and its surroundings

The study area lies between latitude 18–26°N and longitude 73–83°E, and mainly covers the Central India Tectonic Zone (CITZ). The frequency-dependent shear wave quality factor (Q_s) has been estimated over the CITZ and its surroundings using Double Spectral Ratio (DSR) method. We have considered 25 local earthquakes with magnitude (M_L) varies from 3.0 to 4.7 recorded at 11 stations running under national seismic network. The Fast Fourier Transformed (FFT) spectra were computed from the recorded waveform having time-window from onset of S-phase to 1.0 s and for a frequency-band of 0.1–10 Hz. Three different shear wave velocities (i.e., 3.87, 3.39 and 3.96 km/s) were obtained over the study area based on a pair of earthquakes recorded at a pair of stations. The low Q_s values of 51–96 at 1 Hz (i.e., Q_s = 51f^0.49; Q_s = 90f^0.488 and Q_s = 96f^0.53) were found in the area covering the Son–Narmada–Tapti (SONATA) lineament, CITZ, eastern part of the Satpura fold belt, Vindhyan and Gondwana basins, Godavari and Mahanadi grabens, and southern part of Gangetic plain. Intermediate Q_s values of the order of 204–277 (i.e., Q_s = 204f^0.56 and Q_s = 277f^0.55) were noted in the cartonic areas, namely, Bundelkhand, Dharwar-Bhandara and Bastar. While the higher Q_s values of 391–628 at 1 Hz (i.e., Q_s = 391f^0.49, Q_s = 409f^0.48, Q_s = 417f^0.48, Q_s = 500f^0.66, Q_s = 585f^0.65 and Q_s = 628f^0.69) were found in the eastern part of the SONATA, CITZ, and the northeastern part of the Satpura fold belt. The low Q_s values might be attributing to the more heterogeneous SONATA rift system. Low Q_s values further may presumably be associated with lower-level of seismicity and apparently account for higher tectonic stress accumulation over long duration. The long-term accumulated stress is generally released through occasional triggering of moderate magnitude earthquakes in the SONATA zone. Surrounding the SONATA region, the higher Q_s values possibly accounts for a more homogeneous subsurface structure along the SONATA zone.     

The Oman Exotics: a key to the understanding of the Neotethyan geodynamic evolution

Abstract

The study of the exotic blocks of the Hawasina Nappes (Sultanate of Oman) leads to give apposit data that allow us to propose a new paleogeographic evolution of the Oman margin in time and space. A revised classification of exotic blocks into different paleogeographical units is presented. Two newly introduced stratigraphic groups, the Ramaq Group (Ordovician to Triassic) and the Al Buda’ah Group (upper Permian to Jurassic) are interpreted as tilted blocks related to the Oman continental margin. The Kawr Group (middle Triassic to Cretaceous) is redefined and interpreted as an atoll-type seamount. The paleogeography and paleoenvironments of these units are integrated into a new scheme of the Neotethyan rifting history. Brecciae and olistoliths of the Hawasina series are interpreted to have originated from tectonic movements affecting the Oman margin and the Neotethyan ocean floor. The breccias of late Permian age were generated by the extension processes affecting the margin, and by the creation of the Neotethyan oceanic floor. The breccias of mid-late Triassic age coincide in time with the collision of the Cimmerian continents with Eurasia. In constrast, the breccias of late Jurassic and Cretaceous age are interpreted as resulting to the creation of a new oceanic crust (Semail) off the Oman margin.     

CO_2 degassing and melting of metasomatized mantle lithosphere during rifting-Numerical study

:Reactivation of metasomatized mantle lithosphere may occur during continental extension,which is an important component of plate tectonics.The lower most part of the metasomatized domains in the subcontinental mantle lithosphere can be locally enriched in CO2.Therefore,partial melting of these metasomatized domains may play a crucial role in the global carbon cycle.However,little is known about this process and up until now few numerical constraints are available.Here we address this knowledge gap and use a 2-D high resolution petrological-thermomechanical model to assess lithospheric rifting.CO2 degassing and melting.We test 4 lithospheric thicknesses:90,110,130 and 200 km with a 10 km thick metasomatized layer at the base using CO2 of 2 wt.%in the bulk composition.The carbonate enriched layer is stable below^3 GPa(>110 km)for a temperature of 1300℃;therefore,we only observe degassing patterns for lithospheric models that are 130 km and 200 km thick.The metasomatized layer for the 130 km thick lithosphere mostly comprises carbonatite melting,whereas in the 200 km thick scenario propagation of melt development from kimberlites to carbonatites occurs as the metasomatic mantle is exhumed during extension.The numerical models fit well into natural rifting zones of the European Cenozoic Rift System for young(shallow)and of the North Atlantic Rift for old(thick)lithosphere.     

Preservation of a Paleoproterozoic rifted margin in the Himalaya:Insight from the Ulleri-Phaplu-Melung orthogneiss

Orthogneiss within the Paleoproterozoic strata of Lesser Himalayan sequence across the Himalaya has been variably linked to development in a continental arc setting, Indian basement, or a continental rift.New whole rock and trace element geochemical data and U/Pb zircon geochronology indicate that the granitoid protoliths to these rocks were derived from upper crustal sources in the Paleoproterozoic and have within-plate, A-type affinities. This is consistent with their generation in a rifted margin and is compatible with paleogeographic reconstructions that indicate an open boundary for present-day northern India in the Paleoproterozoic.     

Okinawa Trough genesis: structure and evolution of a backarc basin developed in a continent

The Okinawa marginal basin was opened by crustal extension into the Asian continent, north of the Taiwan collision zone. It is located behind the Ryukyu Trench subduction zone and the Ryukyu active volcanic arc. If we except the Andaman Sea, the Okinawa Trough is the only example of marginal backarc basin type, opened into a continent at an early stage of evolution. Active rifting and spreading can be observed. Synthesis of siesmic reflection, seismic refraction, drilling, dredging and geological field data has resulted in interpretative geological cross sections and a structural map of the Ryukyu-Okinawa area. The main conclusions of the reconstruction of this backarc basin/volcanic arc evolution are. (1) Backarc rifting was initiated in the volcanic arc and propagated along it during the Neogene. It is still active at both ends of the basin. Remnants of volcanic arc are found on the continental side of the basin. (2) There was synchronism between opening and subsidence of the Okinawa Trough and tilting and subsidence of the forearc terrace. The late Miocene erosional surface is now 4000 m below sea-level in the forearc terrace, above the trench slope. Retreat and subsidence of the Ryukyu trench line relative to the Asian continental plate, could be one of the causes of tilting of the forearc and extension in the backarc area. (3) A major phase of crustal spreading occurred in Pliocene times 1.9 My ago in the south and central Okinawa Trough. (4) En échelon rifting and spreading structures of the central axes of the Okinawa Trough are oblique to the general trend of the arc and trench. The Ryukyu arc sub-plate cannot be considered as a rigid plate. Rotation of 45° to 50° of the southern Ryukyu arc, since the late Miocene, is inferred. The timing and kinematic evolution of the Taiwan collision and the south Okinawa Trough opening suggest a connection between these two events. The indentation process due to the collision of the north Luzon Arc with the China margin could have provoked: lateral extrusion; clockwise rotation (45° to 50° according to palaeomagnetic data) and buckling of the south Ryukyu non-volcanic arc; tension in the weak crustal zone constituted by the south Ryukyu volcanic arc and opening of the south Okinawa Trough. Similar lateral extrusions, rotations, buckling and tensional gaps have been observed in indentation experiments. Additional phenomena such as: thermal convection, retreating trench model or anchored slab model could maintain extension in the backarc basin. Such a hypothetical collision-lateral backarc opening model could explain the initiation of opening of backarc basins such as the Mariana Trough, Bonin Trough, Parece Vela — Shikoku Basin and Sea of Japan. A new late Cenozoic palaeogeographic evolution model of the Philippine Sea plate and surrounding areas is proposed.