M. R. Leeder

Preliminary conclusions of the Royal Society and Academia Sinica 1985 geotraverse of Tibet

The 1985 Chinese/British expedition to the Tibetan Plateau attempted to solve the question of the origin of the very thick crustal rocks in this region. Continuing northwards movement of the Indian plate over the past 38 Myr has given rise to severe folding and thrust faulting, causing crustal thickening by internal deformation. Previous collisions of microplate terranes derived from Gondwanaland occurred during Mesozoic times but the Kun Lun terrane of northern Tibet was already part of Laurasia by the Carboniferous.

Drainage systems and the development of normal faults: an example from Pleasant Valley, Nevada

Abstract Drainage systems in regions of active extension contain information about fault zone structure and development, and particularly about the lateral growth of individual fault segments, that is difficult to obtain by more conventional means such as the dating of sedimentary material. We investigate drainage-fault interaction in Pleasant Valley, Nevada, where a large normal-faulting earthquake occurred in 1915. We find geomorphological evidence for a propagation (increase in length) of several km at both ends of one of the 1915 fault segments. At one end we estimate the propagation rate to be about 50 m per earthquake. This estimate is uncertain, but is within the range predicted by theoretical models of fault growth. Drainage along the axes of half graben in this region may be influenced by the spacing of tilted fault blocks: the highest valleys are also the narrowest. Whether the axial streams succeed in reaching lower neighbouring sinks depends on whether sedimentation rates are able to keep basins filled to the levels of the basement highs that separate half graben of opposite polarity. A knowledge of modern drainage-fault interactions may help predict drainage systems and the position of potential syn-rift reservoir sediments in older extensional terrains where fault configurations (but not facies distributions) are known from seismic reflection surveys.

Sedimentology, palaeoecology and palaeoenvironmental evolution of the 1985 Lhasa to Golmud Geotraverse

Vertical and horizontal measurements of almost 30 km of sections were made along the Geotraverse route at 113 localities ranging in age from Ordovician to Tertiary. Over 280 palaeocurrent measurements were taken and 200 thin sections were studied. Ordovician strata occur only in the Kunlun Terrane, w here thick metamorphosed sequences of elastics and carbonates occur. These are tentatively interpreted as platform margin and slope deposits. During Carboniferous times in the Kunlun Terrane transgressive late-Dinantian marine limestones with tropical to subtropical Eurasian reefoidal faunas overlie fluviatile redbeds derived from an unroofed orogenic belt. The Lhasa Terrane contains shelf basin elastics with low -diversity faunas succeeded by thick late Carboniferous/early Permian glacio-marine mixtites deposited by iceberg meltout. Permian carbonate ramp and shelf facies with reefoidal developments occur over both the Lhasa and Qiangtang Terranes, with coal-bearing elastics and fluviatile redbeds also occurring in the latter.

Rates of tectonic uplift in the Corinth and Megara Basins, central Greece

We present evidence of the rates of late Quaternary tectonic uplift that have affected parts of central Greece during this period of active extension and basin formation. Dual lines of evidence indicate upper Pleistocene to Holocene uplift rates of the order of 0.3 mm yr−1 for the Corinth and Megara basins. First, U-series disequilibrium ages of Acropora sp. corals from uplifted marine sediments are used to derive minimum average rates of vertical displacement since deposition. Second, the geometries of cyclical transgressive sequences and of erosional terraces are consistent with the radiometric evidence and the known history of late Quaternary sea level fluctuations. Two types of uplift are distinguished on the basis of structural relationships: (1) fault block rotation about a horizontal axis (tilting) in response to footwall uplift on the active normal fault bounding the Alkyonides Gulf and (2) a more regional uplift which affects the Corinth Basin and Peloponnesos to the south. The results of U-series dating are discussed in the light of initial 234U/238U activity ratios, derived from the coral samples, which are higher than for average marine waters (1.17–1.31 compared to 1.14). These values may be related to a variable freshwater input to the structurally confined Gulf of Corinth or, alternatively, they may reflect a previously higher global 234U/238U activity ratio in marine waters, or post-mortem enrichment in 234U, although the mechanism of the latter is not yet understood.

Ground penetrating radar: application to sandbody geometry and heterogeneity studies

Abstract Ground penetrating radar (GPR) offers a high-resolution, shallow subsurface profiling technique for use in sedimentological and reservoir analogue studies. GPR is similar to seismic reflection profiling but uses electromagnetic radiation in the 50 to 500 MHz frequency range (in geological applications). By using these relatively high frequencies, high resolution data can be obtained. Short duration pulses of electromagnetic energy are transmitted into the ground, reflected from interfaces across which there are abrupt changes in dielectric properties, and are detected by a receiver. These received signals are displayed in nanoseconds two-way time and may be recorded digitally allowing subsequent processing. Some 1000 m of 2D GPR profiles were collected from a modern point bar on the Madison River, Montana USA and have been interpreted using an approach similar to seismic stratigraphic analysis. This has allowed identification of a number of radar sequences and radar facies. Radar sequence boundaries are identified by reflector terminations (onlap, downlap, toplap and erosional truncation) and represent episodes of erosion during the development of the point bar. In contrast, radar sequences and their component radar facies record phases of accretion of the point bar. Each radar sequence is linked to a discrete accretionary unit that can be mapped on the surface of the point bar. Mapping of the radar sequences and radar facies has allowed quantification of their 3D geometry.

On the periodicity and magnitude of Late Carboniferous glacio-eustatie sea-level changes

Data from sedimentary cycles in basins from northern England, the Appalachians and Midcontinent US afford an insight into periodicities and controls of Late Carboniferous glacio-eustatic sea-level changes. Lithofacies with identifiable palaeobathymetry are used to assess the magnitude of sea-level change in the Pennine Basin of northern England; the method yields a minimum value of 42m. Walsh power spectral analysis is employed to resolve periodicities of about 120ka within the Pennine Basin of northern England, 212, 140 and 93 ka within the Midcontinent US succession and 109, 84, 67, 63 ka within the Appalachian Basin. Within the Pennine Basin periodicities and magnitudes are found to be independent of their position within the basin thus any tectonism can be assumed to have had a negligible effect on the transgressions. A simplified modelled climatic curve is derived from Milankovitch parameters and Pleistocene δ18O data. This is used in a basin fill model that simulates the thermally subsiding Pennine Basin. The resulting distribution of marine deposits is seen to depend upon both the steady regional subsidence and the magnitude and extent of glacio-eustatic transgressions.

High sediment yields and cool, wet winters: Test of last glacial paleoclimates in the northern Mediterranean

Opposing models have been proposed for last glacial paleoclimates in the Mediterranean area. To discriminate between the alternative models, we calculate variations in sediment yield within a high-resolution stratigraphic framework developed for the Alkyonides basin, central Greece. Latest Quaternary highstand deposits are distinguised from lowstand Lake Corinth deposits on the basis of seismic reflection data and micropaleontological and palynological analyses of drop cores. After depth conversion and calculation of solid sediment masses in successive time slices, sediment discharge rates during the last glacial lowstand interval (ca. 70-12 ka) are shown to have exceeded discharge rates during the preceeding interglacial highstand interval (ca. 128-70 ka) by =60%, implying enhanced seasonality during the glacial period, with cool, dry summer and wet winter conditions in the region. Sediment yield calculations can thus provide an empirical test of paleoclimate models and offer input to sequence stratigraphic models.

Sediment deformation structures and the palaeotectonic analysis of sedimentary basins, with a case-study from the Carboniferous of northern England

Summary There is a close link between syn-sedimentary faulting caused by earthquakes of magnitude >5 and accompanying ‘soft’ sediment intrafolial deformation in active neotectonic settings. Certain of these intrafolial structures may also be produced by purely sedimentary processes and are termed ‘autokinetic’. Those caused by earthquake-induced stresses are termed ‘allokinetic’. Although certain structures may be caused by either mechanism, it appears that large amplitude dewatering pipes and recumbent-folded cross stratification are likely to be predominantly allokinetic in origin. These structures may be used to define the Allokinetic Deformation Number which can be systematically mapped out in particular lithofacies over outcrops in a sedimentary basin. Results of this mapping are then combined with data on lithofacies thickening and stacking (tectonically-controlled architecture), hangingwall rollover deformation etc., to establish an interdisciplinary outline of palaeotectonic evolution. This methodology of palaeotectonics is tested in the suspected extensional terrain of the Northumberland Basin with encouraging results.

Normal faulting and crustal deformation, Alkyonides Gulf and Perachora peninsula, eastern Gulf of Corinth rift, Greece

Geophysical, structural, geochronological and geomorphological data indicate that the Psatha, East Alkyonides, Skinos and Pisia faults are Holocene-active structures whereas the status of the West Alkyonides, Strava, Perachora and Loutraki faults is less certain. We see no evidence for significant lateral surface fault growth. New data for late Pleistocene footwall uplift of the Psatha fault are comparable with previously estimated Holocene rates. Pre-Holocene stratigraphic sequences in the Alkyonides Gulf allow calculation of vertical displacement on the Skinos fault of 1.42–1.60 km over a period of >0.6 Ma. Previous palaeoseismological studies indicate comparable displacement rates extrapolated to 0.61–2.20 Ma, whereas extrapolation of previous geodetic data indicate a range of 0.17–0.46 Ma. The latter is too short given the evidence of the stratigraphic record, signifying either that these data may not be representative of longer-term rates, or that significant deformation has taken place elsewhere, for example, on offshore antithetic faults. A case is established for uniform late Quaternary (post-MIS 7) uplift of the Perachora peninsula at rates of c. 0.2–0.3 mm a−1. The lack of regional tilting over Perachora–Corinth–Isthmia is in marked contrast to the situation in the Alkyonides–Megara basins to the east.

Lateral erosion (‘toe-cutting’) of alluvial fans by axial rivers: implications for basin analysis and architecture

Abstract: We document the neglected phenomenon of lateral erosion (‘toe-cutting’) of alluvial fans by non-incising axial river channels. Field examples from the Holocene of the Big Lost River basin, Idaho and the Plio-Pleistocene of the Rio Grande Rift, New Mexico help to establish architectural models with more general application to basin analysis. The process of toe-cutting can lead to complete fan destruction and may be a response to climate change, tectonic tilting, fault propagation or a combination of these variables. It gives rise to: near horizontal erosion surfaces cut in fan sediment; steep fan-margin scarps; progressive up-fan incision from the scarp by a network of channels; soil formation up-fan away from the incised channel network; a deposit of axial alluvium that overlies the erosion surface and onlaps the scarp. Once avulsion occurs to take the axial channel away from the bajada margin, distinctive ‘healing-wedges’ of fan alluvium prograde across abandoned axial river channel and floodplain deposits, gradually onlapping the eroded scarp and its upstream network of incised channels. Toe-cutting has important stratigraphic basin analysis and economic consequences: bajada deposits subject to the process exhibit appreciable extra groundwater and petroleum reservoir potential in the intercalations of more porous and permeable axial fluvial sediments.