排序方式： 共有59条查询结果，搜索用时 234 毫秒
41.
Rory Mortimore 《Proceedings of the Geologists' Association. Geologists' Association》2011,122(2):232-297
Inversion tectonic episodes are identified in the Upper Turonian - Lower Coniacian, Santonian - Lower Campanian and later Lower Campanian Chalk. It is suggested that episodic tectonism created the seabed topography on which sea levels and erosional currents acted. Marked differentiation into linear belts of local basins and swells with a greater variety of sediments is present in the Santonian and Lower Campanian. During this same period the locus of sedimentation shifts westwards from the southern margin of the Weald to Wessex as Weald Basin inversion increases. Tectonic episodes also produced synsedimenary fracturing of the Chalk and evolution of vein networks and stylolytes. Upper Cretaceous tectonic and sea-level events also affected the platform of Europe, the Carpathians and the Syrian Arc where sedimento-tectonic scenarios provide analogues for the Chalk. Linking sea-level oscillations and tectonic episodes with microtectonic studies suggests a frequency of events within the range of 0.35-1.5 Ma. 相似文献
42.
We present early to Mid‐Holocene paleo‐geographic reconstructions for the Ramore Head area (Northern Ireland). This coastal area is characterized by Mesolithic occupation (c. 10–6 ka) and preserved early–Mid‐Holocene peats both on‐ and offshore. This paper improves on previous reconstructions by employing a backstripping methodology, which removes accumulated recent deposits from identified buried paleo‐landsurfaces instead of using modern topography as an analogue to the past landscape. Paleo‐landsurfaces are identified offshore from seismic profiles supplemented by cores, and onshore through legacy borehole records. The paleo‐landsurface can be traced offshore to depths of −2 to −19 m and is buried by <5 m of modern sediment. It extends onshore under the coastal town of Portrush and is buried <2.5–10 m below modern ground level. The identified paleo‐landsurface is combined with sea‐level curves from recent Glacio‐Isostatic‐Adjustment models to reconstruct marine transgression during the early–Mid‐Holocene. Comparison is also made with reconstructions based on modern topography. Together, the identified paleo‐landsurfaces and revised reconstructions can assist future site prospection on‐ and offshore and delimit high‐potential areas for heritage management. Revised reconstructions also allow placement of extant archaeology into a more accurate context of landscape change and help develop insights into local‐scale site location patterns. 相似文献
43.
44.
AbstractA simple model is given that describes the response of the upper ocean to an imposed wind stress. The stress drives both mean and turbulent flow near the surface, which is taken to mix thoroughly a layer of depth h, and to erode the stably stratified fluid below. A marginal stability criterion based on a Froude number is used to close the problem, and it is suggested that the mean momentum has a strong role in the mixing process. The initial deepening is predicted to obeywhere u. is the friction velocity of the imposed stress, N the ambient buoyancy frequency, and t the time.After one-half inertial period the deepening is arrested by rotadeon at a depth h = 22/4 u.{(Nf)+ where f is the Coriolis frequency. The flow is then a “mixed Ekman” layer, with strong inertial oscillations superimposed on it. Three quarters of the mean energy of the deepening layer is found to be kinetic, and only one-quarter potential.Heating and cooling are included in the model, but stress dominates for time-scales of a day or less. Non-uniform stratification and currents existing prior to the onset of the wind are easily included.Agreement between the first formula above and laboratory experiments of Kato and Phillips is very satisfactory; the second formula is consistent with observations of Francis and Stommel, though a more thorough test is needed. Oceanic observations in general support the assumption of slab-like mean profiles and direct response of the fluid to local winds. 相似文献
45.
We present N-body simulations of planetary accretion beginning with 1 km radius planetesimals in orbit about a 1 M⊙ star at 0.4 AU. The initial disk of planetesimals contains too many bodies for any current N-body code to integrate; therefore, we model a sample patch of the disk. Although this greatly reduces the number of bodies, we still track in excess of 105 particles. We consider three initial velocity distributions and monitor the growth of the planetesimals. The masses of some particles increase by more than a factor of 100. Additionally, the escape speed of the largest particle grows considerably faster than the velocity dispersion of the particles, suggesting impending runaway growth, although no particle grows large enough to detach itself from the power law size-frequency distribution. These results are in general agreement with previous statistical and analytical results. We compute rotation rates by assuming conservation of angular momentum around the center of mass at impact and that merged planetesimals relax to spherical shapes. At the end of our simulations, the majority of bodies that have undergone at least one merger are rotating faster than the breakup frequency. This implies that the assumption of completely inelastic collisions (perfect accretion), which is made in most simulations of planetary growth at sizes 1 km and above, is inappropriate. Our simulations reveal that, subsequent to the number of particles in the patch having been decreased by mergers to half its initial value, the presence of larger bodies in neighboring regions of the disk may limit the validity of simulations employing the patch approximation. 相似文献
46.
Dayu Zhang Feng Yuan Noel White Pete Hollings Wenjiao Xiao 《International Geology Review》2017,59(9):1082-1096
ABSTRACTThe Dayinsu area is located in the northern part of the West Junggar district near the border between China and Kazakhstan and is an important component of the Central Asian Orogenic Belt (CAOB). The Dayinsu area hosts numerous granitoid plutons in Devonian–Carboniferous volcano–sedimentary strata. The older Laodayinsu and Kubei (345–330 Ma) plutons are located in the west with the younger Bayimuzha and Qianfeng (330–325 Ma) plutons in the east. The whole-rock SiO2 contents of the four granitoid plutons range from 52.22 to 68.42 wt.% and total alkaline contents (K2O + Na2O) range from 4.94 to 9.16 wt.%. The granites are enriched in large ion lithophile elements and light rare earth elements with depletions in Nb, Ta, Ce, Pr, P, and Ti. The plutons are metaluminous with I-type signatures. The geochemistry of the intrusions suggests that they formed in a subduction zone setting, and subsequently underwent fractional crystallization during emplacement, with higher degrees of fractionation in the eastern sector than in the west. Similarities in the geochronology and geochemical characteristics of the granitoid plutons in Dayinsu to those in the Tabei district (west to Dayinsu area) suggest that both districts are part of the Carboniferous Tarbagatay Mountain intrusive event. The early Carboniferous (345–324 Ma) granitoid intrusions in the Tarbagatay Mountain likely formed in an island arc subduction setting during the evolution of the CAOB. 相似文献
47.
Rory N. Mortimore 《Proceedings of the Geologists' Association. Geologists' Association》2018,129(3):392-420
Late Cretaceous sedimentary history has been strongly influenced by both sea-level fluctuations and inversion tectonics. Evidence for tectonic movements, originally identified in German Late Cretaceous basins, is applied to the UK successions. Two periods of movement are conspicuous: a Middle Turonian episode involving huge loss of section along anticlinal axes in southern England and a Late Santonian-Early Campanian episode also involving section loss on structure and section gain off structure. This pattern is repeated where folds or blocks are underlain by inversion thrust faults (e.g. the Purbeck Fault in Dorset, the Falmer Fault in Sussex, the Portsdown Fault in Hampshire and the Bray Fault in Upper Normandy). Other episodes of inversion in the Late Turonian to Middle Coniacian and the late Early Campanian are investigated and are a probable cause of slump beds and slides. These tecto-sedimentary episodes can be applied to structures in Northern Ireland, Inner Hebrides, North Sea and Yorkshire as well as southern Britain. Beyond NW Europe the Late Santonian – Early Campanian event is widely recognised in the Carpathians, southern Europe, Africa and the Levant and coincides with the end of the Long Cretaceous Quiet Zone (Chron 34N to 33R) perhaps representing a major change in Earth dynamics related to Mid-Ocean Ridge crustal production and intra-continental crust tectonism. 相似文献
48.
Rory N. Mortimore 《Proceedings of the Geologists' Association. Geologists' Association》2019,130(1):27-65
Late Cretaceous Chalk sedimentation history across the British Isles included (i) fault controlled uplift and subsidence in Northern Ireland and the Inner Hebrides and (ii) uplift along the lines of en echelon folds in Southern Britain and northern France. Synsedimentary slump folds and downslope displacement structures are compared with penecontemporaneous interbed slides and later tectonic folds and faults. Compressional strike-slip tectonic processes at Flamborough Head, Yorkshire, illustrate intra-Chalk slump beds in a half-graben setting. Progressive ‘growth’ of structures characterises early downslope slump folding, interbed sliding and some listric faulting. Sheet-flints replacing slide shear planes and early fractures provide evidence for early movements. Availability of open-slopes or the depth of burial under which the range of structures developed is reflected in the degree of disruption and fragmentation of chalk and flint. Fragmentation provides clues to the timing of events and origin of the Late Campanian Altachuile Breccia (Northern Ireland) and the Coniacian Hope Gap slides (Sussex). Fragmentation and formation of sheet flints together help distinguish intra-Chalk tectonics from Quaternary glacitectonic structures.The role of marl seams, high porosity chalk beds and hardgrounds on bed-sliding, décollement zones and disruption of chalk blocks from bedrock in glacitectonics is discussed. Chalk formations with marl seams develop a special style of fracturing related to early interbed sliding and pore-fluid escape structures. Marl-seams are shown to be primary sedimentary features and not the products of post depositional pressure-solution. More than any other formation the Late Santonian – Early Campanian Newhaven Chalk contains extensive sheet-flints and shows great lateral variation in thickness and lithology across the fold belts of southern England and northern France. 相似文献
49.
50.
Rory Quinn Wes Forsythe Colin Breen Martin Dean Mark Lawrence Steve Liscoe 《Geoarchaeology》2002,17(5):441-451
Rathlin Island, off the north coast of Ireland, has a history of settlement and seafaring from the Late Mesolithic period to the present day. The maritime Sites and Monuments Record (SMR) for Rathlin indicates many wrecking incidents. In 1999, a reconnaissance side‐scan sonar survey confirmed the presence of 46 targets of possible archaeological potential around Rathlin Island. Thirteen of these anomalies were positively identified as shipwrecks. Of the remaining 33 targets, nine were dived on in order to ground‐truth the geophysical data. A successful and rapid methodology of ground‐truthing side‐scan sonar data for archaeological purposes was developed. The results confirmed the presence of a Danforth Anchor at one site, while the remaining anomalies were identified as geological features. The results from the side‐scan survey and diver‐truthing exercise enhanced the existing maritime SMR. © 2002 Wiley Periodicals, Inc. 相似文献