摘要:

AbstractWe present here the initial results of a high‐resolution (sparker) reflection seismic survey in Northern Lake Tanganyika, East African Rift system. We have combined these results with data from earlier multichannel reflection seismic and 5‐kHz echosounding surveys. The combination of the three complementary seismic investigation methods has allowed us to propose a new scenario for the late Aliocene to Recent sedimentary evolution of the North Tanganyika Basin. Seismic sequences and regional tectonic information permit us to deduce the palaeotopography at the end of each stratigraphic sequence. The basin history comprises six phases interpreted to be responses to variations in regional tectonism and/or climate. Using the reflection seismic‐radiocarbon method (RSRM), the minimum ages for the start of each phase (above each sequence boundary) are estimated to be: ?7.4 Ma, ? 1.1 Ma, ?393–363 ka, ?295–262 ka, ? 193–169 ka, ?40–35 ka. Corresponding lowstand lake elevations below present lake level for the last five phases are estimated to have been: ?650–700 m, ?350 m, ?350 m, ?250 m and ? 160 m, respectively. The latest phase from ?40–35 ka until the present can be subdivided into three subphases separated by two lowstand periods, dated at ?23 ka and ? 18 ka. From the late Miocene until the mid Pleistocene, large‐scale patterns of sedimentation within the basin were primarily controlled by tectonism. In contrast, from the mid Pleistocene to the present, sedimentation in Lake Tanganyika seems to have responded dramatically to climatic changes as suggested by repeated patterns of lake level fluctuations. During this period, the basin infill history is characterized by the recurrent association of three types of deposits: ‘basin fill’ accumulations; lens‐shaped ‘deep lacustrine fans’; and ‘sheet drape’ deposits. The successive low‐lake‐level fluctuations decreased in intensity with time as a consequence of rapid sedimentary filling under conditions of declining tectonic subsidence. The climate signal has thus been more pronounced in recent sedimentary

ISSN:0950-091X    

DOI:10.1111/j.1365-2117.1996.tb00112.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractStraight sections of many actively uplifting mountain belts have simple patterns of drainage, transverse to their main structural trend. Streams rising near or beyond the topographic ridgepole of these sections are spaced at seemingly regular intervals. To test whether this regularity exists, morphometric aspects of drainage networks were measured in 11 mountain belts. The spacing of drainage basins can be expressed using a spacing ratio, which in effect is the ratio of the length and the width of the catchments under consideration. Average spacing ratios for most linear mountain belts are within a narrow range of values between 1.91 and 2.23. A linear relationship exists between the spacing of catchment outlets and the distance between the main divide and the front of the mountain belt in which they have developed. The Nepalese Himalaya form an exception to this regular pattern. In this mountain belt drainage is blocked and diverted by structures that have developed in relation to the Main Boundary Thrust. Structural complications cause drainage patterns to become less regular, introducing important non trans verse components.The linear relationship between spacing of catchment outlets and half‐width of the mountain belt may be expressed in an equation of the same general form as Hack's law of stream length and drainage basin area. It seems likely that the mechanism underlying Hack's law also explains the consistent regularity of drainage spacing in active mountain belts. However, no generally accepted explanation for Hack's law has been offered. The narrow range of spacing ratios found for drainage networks in active orogens may represent an optimal catchment geometry that embodies a ‘most probable state’ in the uplift‐erosion system of a linear mountain belt.The linear relationship between the half‐width of a mountain belt and spacing of catchment outlets has profound implications for the modelling of erosion of orogenic topography, and for the formation and filling of forela

ISSN:0950-091X    

DOI:10.1111/j.1365-2117.1996.tb00113.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractWe use a quantitative model of apatite fission track (AFT) annealing to constrain the thermal evolution of a sedimentary basin and its margin. Apatites deposited in a basin contain several types of information. Provided temperatures remained below ≅70°C, they retain much of their provenance thermal signatures and mainly record the thermal evolution of their source area. Above 70°C, the fission tracks in apatite rapidly fade, reflecting the thermal evolution of the basin. Therefore, downhole AFT dates in a well section can in principle be used to assess both the provenance detail (from shallow/cool samples) and the subsequent thermal history in the basin (from the deeper samples).We apply this concept to the south Norwegian offshore and onshore using AFT and ZFT (zircon fission track) data; the latter constrain maximum palaeotemperatures and provide additional provenance information. AFT and ZFT data from three offshore wells in the northern North Sea are shown to contain a record of palaeogeographical and tectonic evolution, closely associated with the Norwegian basement. ZFT data from Middle Triassic sediments suggest a Permian volcanic source. Modelling of AFT data from Jurassic sediments presently residing at temperatures below 70°C indicate rapid cooling during the Late Triassic to Early Jurassic, similar to onshore AFT data. During the Cretaceous minor sediment supply was derived from the Norwegian basement, as evidenced by ZFT ages that do not correlate to the onshore, suggesting that parts of southern Norway were covered with sediments at this time. At the end of the Palaeogene and during the Neogene, the south Norwegian basement again became a major source of elastics.AFT and ZFT data indicate that all wells are presently at maximum temperatures. No significant (>500 m) erosion events are indicated in the three wells since the Jurassic. AFT data have not yet effectively equilibrated to present‐day temperatures as nonzero fission track ages are maintained in sediments currently at temperatures of>120°C. This implies that the present‐day thermal regime has only recently been installed. Probable causes include rapid subsidence and an increase in the geothermal gradient during the l

ISSN:0950-091X    

DOI:10.1111/j.1365-2117.1996.tb00114.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractRifted margin architecture along part of the southern Gabonese margin is interpreted from four deep‐penetration, multichannel seismic reflection (MCS) profiles. A series of synthetically faulted crustal blocks are identified, separated by dominantly seaward‐dipping fault zones formed during Cretaceous rifting between Africa and South America. Extensional strain ratios are ≅ 1.5. These faults appear either to transect the entire crustal section or are interrupted by discontinuous zones of midcrustal reflections which may represent detachments.Outer acoustic basement highs are situated just seaward of the continental slope. On the combined basis of seismic geometry, an associated positive magnetic anomaly and an increase in free‐air gravity, these outer highs are interpreted to mark faulted transitions from rifted continental crust to ‘proto‐oceanic crust’, presumably composed of mafic volcanic rocks and possibly slivers of attenuated continental crustal blocks. The outer edge of Aptian salt lies °165 km south‐west of the edge of the continental shelf. The salt forms an° 1.5‐km‐thick horizon overlying the outer highs, and it may be autochthonous there, suggesting salt was deposited contemporaneously with emplacement of proto‐oceanic crust.Differential subsidence and tilting between continental rift‐blocks during post‐rift margin subsidence has resulted in a sympathetic terrace‐ramp geometry in overlying Aptian salt. Salt terraces form above tops of crustal blocks, where salt tends to rise vertically, creating pillows and diapirs. Ramps connecting terraces tend to form above seaward‐facing fault zones; salt flowage there has been both lateral and vertical, creating triangular diapirs along the footwalls of growth faults. Most of these growth‐faults sole within the salt base, but a few continue into t

ISSN:0950-091X    

DOI:10.1111/j.1365-2117.1996.tb00115.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractSediment accumulation within ocean trenches located at actively accreting convergent margins is determined by an interplay between sediment supply, sediment subduction/accretion at the toe of the overriding accretionary complex and the rate of subduction. Modelling trench sedimentation provides insight into the principal controlling factors, and a means of deriving, from the pattern of sedimentation, how factors, such as the rates of sediment supply and subduction, have varied over the period of accumulation of the trench sediments. Two DSDP‐ODP drill sites within the Nankai Trough reveal a coarsening‐upward megasequence, indicating a progressive facies transition from abyssal muds to outer‐trench silts to inner‐trench sands. The changing geometry of the trench‐wedge over the past 1 Myr has been determined by modelling variations in net sediment flux for two trench‐perpendicular profiles. The models were constrained to fit the stratigraphy at the drill sites, and the simulated present‐day geometries of the trench were matched with those shown on seismic reflection profiles by successive adjustment of the model. Results from both sites confirm a ‘slow’ subduction rate of<20 km Myr‐1. At the south‐western site (582), the width of the trench‐wedge has ranged from 13 to 21 km over the past 1 Myr. To the north‐east, at Site 808, the width has ranged from 7 to 13 km over the past 0.5 Myr. These changes in trench‐wedge width are primarily the result of large changes in sediment supply rate. The subduction of the Shikoku Ridge, a fossil spreading centre adjacent to Site 808, has had a major influence on the style of sedimentation within the trench.The style of accretion from the trench to the toe of the accretionary complex has important implications for geometrical adjustment of the trench‐wedge. Thrust displacement lifts the protothrust region out of the trench, resulting in a decreased width. This is followed by a phase of increasing width as the trench‐wedge adjusts towards a new equilibrium. The cyclical, episodic accretion process results in a periodic second‐order variation in trench‐fill size that is superimposed on primary trends determined by variations in sediment supply r

ISSN:0950-091X    

DOI:10.1111/j.1365-2117.1996.tb00116.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Book reviewed in this article:Editions Technip, Paris, paperback, 294 pp.American Association of Petroleum Geologists Education Course Note 36, Tulsa, OK, paperback, 170 pp.

ISSN:0950-091X    

DOI:10.1111/j.1365-2117.1996.tb00117.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY