Plio‐Pleistocene fault reactivation within the Crag Basin, easternUK : implications for structural controls of landscape development within an intraplate setting
Jonathan R. LeeRichard HaslamMark A. WoodsJames RoseR. GrahamJonathan R. FordDavid SchofieldTimothy KearseyC. Williams
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Abstract:
This study examines the long‐term neotectonic evolution of the Crag Basin of eastern England during the Plio‐Pleistocene ( c . 4.0–0.48 Ma) and the influence of neotectonics on coastal and drainage development. The Crag Basin was situated within the western margins of the southern North Sea with palaeogeography influenced by changes in global sea‐level and longer‐term regional‐scale neotectonic uplift and subsidence. This study identifies an additional local‐scale neotectonic control on basin development with localized crustal displacement occurring along normal faults. Plio‐Pleistocene movement along these faults was accommodated by partial dip‐slip (normal) reactivation of an Oligocene‐age (Pyrenean) dextral strike‐slip shear zone, which in turn was inherited from much older Caledonian orogenic crustal structure. Fault displacement was driven by sediment‐loading reflecting enhanced landscape denudation under progressively deteriorating climates and increased rates of erosion/sedimentation. Faulting acted to regulate accommodation space, controlling sedimentation patterns and the courses of major preglacial drainage systems including the Kesgrave Thames and Bytham rivers. The lower reaches of both river systems are considered to have been confluent in the Crag Basin during much of the Early Pleistocene with their lower reaches structurally controlled. Divergence occurred at c . 0.9 Ma with the lower reaches of the Bytham utilizing the former Bytham‐Thames valley and the Kesgrave Thames adopting progressively more southern routes, aligned to the axis of subsidence within the London Basin. The study highlights the significance of tectonic inheritance in driving recent neotectonic crustal deformation and its influence on sedimentation patterns and drainage development within an intraplate setting.Keywords:
Neotectonics
Plio-Pleistocene
Tectonic uplift
Tectonic uplift
Bedrock
Landform
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River terraces
Terrace (agriculture)
Tectonic uplift
Early Pleistocene
Neotectonics
Alluvial fan
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Tectonic uplift
Forcing (mathematics)
Neotectonics
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Anticline
Foothills
Neotectonics
Thrust fault
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Neotectonics
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The large-scale morphology of the eastern margin of the Tibetan plateau can be divided into three zones, the Tibetan Plateau, Longmenshan and Sichuan Basin. To assess the large-scale erosional thickness and erosional rate of the region, we use digital elevation models (DEM) and fission-track thermorchronology to calculate the erosional thickness across the margin. We calculated the erosional rate of Longmenshan in the last 3. 6 Myr (ESR) , and the results indicate that the erosional thickness of Longmenshan is 1. 91-2. 16 km, and the erosional rate of Longmenshan is 0. 53-0. 60 mm/a. Based on results of simulation by flexural deflection, we inferred that the mountain building model of Longmenshan would have been constrained to both erosionally-driven uplift and tectonic shortening-driven uplift. Before 3. 6 Ma, the uplift of Longmenshan is driven by tectonic shortening related to the India-Asia collision, and after 3. 6 Ma the uplift of Longmenshan is driven by erosional unloading.
Tectonic uplift
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Neotectonics
Tectonic uplift
Lithology
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We have used the NEXTMap Britain digital terrain model (DTM) to determine the lithospheric response to erosional unloading and the contribution of tectonics, in the form of elastic plate flexure, to the Cotswold ‘scarp and vale’ landscape. The calculations take into account lithology variations and along-strike changes in escarpment retreat. We show that flexural rock uplift as a result of erosional unloading varies spatially and may contribute up to 50% of the relief in the Cotswold region. This is supported by morphometric analysis, of concavity and steepness, for 66 longitudinal river profiles that drain the scarp and dip slope of the escarpment. Viscoelastic plate models suggest that the uplift is initially rapid (up to 8 m ka −1 ) and essentially complete within 50 ka. These initial rates are compatible with an early post-Anglian incision rate inferred from the Thames terraces. The ‘staircase’ terrace pattern suggests, however, that there have been a number of denudational isostatic events, each associated with a climate cycle. Finally, the analysis reveals an inherited ‘proto-landscape’ that has a subdued relief when compared with the modern DTM. Such a relief is consistent with an early extension of the River Thames, through the Vale of Moreton, to the north of the present-day Cotswold Hills.
Escarpment
Isostasy
Tectonic uplift
Landform
River terraces
Neotectonics
Terrace (agriculture)
Lithology
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Mountains are topographic features caused by erosion after vertical uplift or mountain building. Mountain building is often confused with orogeny, which today means the formation of structures in fold belts. The common assumption that folding and mountain building go together is generally untrue. Many mountains occur in unfolded rocks, granites and volcanic rocks, so there is no direct association of folding and mountain building. In those places where mountains are underlain by folded rocks the folding pre-dates planation and uplift. The age of mountains is therefore not the age of the last folding (if any) but the age of vertical uplift. Since mountains are not restricted to folded rocks, lateral compression is not required to explain the uplift.
A compilation of times of uplift of mountains around the world shows that a major phase of tectonic uplift started about 6 Ma, and much uplift occurred in the last 2 Ma. This period is known as the Neotectonic Period. It is a global phenomenon including mountains on passive continental margins, and those in deep continental interiors. Several hypotheses of mountain building have problems with this timing. Some fail by being only able to make mountains out of folded rock at continental margins. Many translate the vertical uplift into lateral compression, but vertical uplift alone can create mountains.
The Neotectonic Period has important implications for geomorphology, climate and global tectonics. In geomorphology it does not fit into conventional theories of geomorphology such as Davisian or King cycles of erosion. Neotectonic uplift might initiate several cycles of erosion, but most planation surfaces are much older than the Neotectonic Period. The increasing relief associated with Neotectonic uplift affected rates of erosion and sedimentation, and also late Cenozoic climate.
The Neotectonic Period does not fit within plate tectonics theory, in which mountains are explained as a result of compression at active margins: mountains in other locations are said to have been caused by the same process but further back in time. This is disproved by the young age of uplift of mountains in intercontinental and passive margin positions. Subduction is supposed to have been continuous for hundreds of millions of years, so fails to explain the world-wide uplifts in just a few million years.
Geomorphologists should be guided by their own findings, and refrain from theory-driven hypotheses of plate collision or landscape evolution.
Tectonic uplift
Mountain formation
Neotectonics
Orogeny
Post-glacial rebound
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