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- Volume 9, Issue 3, 1997
Basin Research - Volume 9, Issue 3, 1997
Volume 9, Issue 3, 1997
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Quantitative analysis of Miocene to Recent forearc basin evolution along the Colombian convergent margin
Authors Nigel P. Mountney and Graham K. WestbrookThe Colombian accretionary complex forms the active convergent margin of the North Andes block of South America beneath which the east Panama Basin of the Nazca plate is subducted at a rate of 50–64 km Myr−1. Multichannel seismic reflection data, collected as part of RRS Charles Darwin cruise CD40, image a series of well‐developed forearc basins along the length of the margin, bounded on their oceanward side by an active accretionary complex and on their landward side by oceanward‐dipping continental basement. Sedimentary sequences within the forearc basins indicate successive landward migration of the basin depocentre as the structural high bounding its oceanward edge is forced upward and landward by continued growth of the accretionary complex. Uplift beneath the oceanward side of the basins has resulted in progressive landward rotation of the older sedimentary sequences. Prominent seismic reflectors across the basins show a complex onlap–offlap relationship between successive sequences that reflects the interplay between tectonic uplift, sediment supply, differential sediment compaction and basement subsidence due to loading. A numerical model has been devised to investigate how Miocene to Recent forearc basin stratigraphy is controlled by progressive growth of the accretionary complex resulting in elevation of the outer‐arc high and landward motion of the rear of the complex up the seaward‐dipping backstop formed by the leading edge of the continental lithosphere. The effects of sediment accretion are modelled by treating the accretionary complex as a doubly vergent, noncohesive Coulomb wedge, where forces exerted by the proto‐ and retro‐wedges must be balanced for the system to be in equilibrium. The model generates synthetic basin‐fill architecture over a series of steps, each of which represents the deposition of individual sedimentary sequences and their subsequent deformation due to wedge growth. The model accounts for differential sediment compaction and the flexural response of the underlying lithosphere to changes in load distribution over time. Forearc basin evolution is simulated for various rates of sediment supply to the forearc and accretionary complex growth until the synthetic basin‐fill geometry matches the observed geometry. The model enables either the rate of accretionary wedge growth or the rate of sediment supply to the forearc basin to be established. The technique is generally applicable to those convergent margins with forearc basins that have developed between an actively accreting wedge and a seaward‐dipping backstop. Other examples include Peru, S. Chile, Sumatra and Barbados.
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Architecture and controls on Bathonian–Kimmeridgian shallow‐marine synrift wedges of the Oseberg–Brage area, northern North Sea
Authors R. Ravnås and K. BondevikThe Oseberg Fault‐Block, situated along the eastern flank of the northern Viking Graben in the North Sea, was affected by Middle–Late Jurassic rifting initiated in Bajocian–Bathonian times. Temporal variations in stretching rates exerted the major control on the depositional infill patterns of the Bathonian–Kimmeridgian Heather Formation and its intercalated Middle Callovian to Early Oxfordian Fensfjord and Late Oxfordian to Kimmeridgian Sognefjord Formations. Three shallow‐marine, regressive–transgressive synrift wedges are recognized, and are interpreted in terms of discrete rift phases. The lower, regressive segments of the synrift wedges were deposited during periods of relatively low tectonic activity, whereas the upper, overall transgressive segments correspond to extensional pulses or stages during which significant fault‐related subsidence and fault‐block rotation occurred. These rotational tilt stages are further subdivided into an early, a climax and a late synrotational substage.
The lower, regressive segments consist of stacked, shallowing‐upward units, which reflect the advance of wide shallow‐marine, rift‐marginal shorelines during the tectonically quiescent periods. During the intervening rotational tilt stages renewed basin floor tilting and increased basinal subsidence led to retreat of the rift‐marginal depositional systems, renewal of the half‐graben topography, formation of intrabasinal sediment sources (footwall islands) and the re‐establishment of localized footwall, hangingwall and axial depositional systems. These localized depositional systems generally have an overall forestepping‐to‐backstepping character superimposed on the larger‐scale transgressive trend. There was an associated shift from a wave‐ and storm‐dominated environment during deposition of the lower, regressive segment to a more protected, partly current‐(?tidally) influenced environment in the upper, transgressive segment. This reflects a shift from a broad open basin in tectonically quiescent periods to smaller subbasins (embayments or estuaries) during periods with increased rates of rifting.
The footwall highs which formed intrabasinal sediment sources were of limited size compared with the volume of the adjacent depositional sinks. As a consequence, complete infilling of individual half‐grabens were not achieved during the synrotational stages, leaving the subbasins underfilled at the end of each successive rift phase. Mudstone drapes represent periods with deprivation of clastic material and basinal condensation during the latest synrotational to early tectonic quiescence substages, when footwall islands were small or completely submerged and there was a large distance to the (then progradational) rift‐marginal shoreline.
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Insufficiency of compaction disequilibrium as the sole cause of high pore fluid pressures in pre‐Cenozoic sediments
By Henk KooiThe method of indirect demonstration is used to investigate if compaction disequilibrium can account for high overpressures that occur in Mesozoic and older basin formations. First the equations governing compaction disequilibrium are analysed for the factors controlling overpressure levels. Then limiting values of these control parameters are sought which favour high fluid pressures. The analysis shows why ‘close‐to‐lithostatic fluid pressures’ in pre‐Cenozoic basin units are difficult to attain by compaction disequilibrium alone. Subsequently, the limiting favourable conditions are used in a series of generic numerical model experiments. The experiments serve as templates to construct the upper bounds of overpressures due to sediment loading for most geological settings including those where shale seals have developed. Two regional examples are studied in some detail. It is shown that observed overpressures in Mesozoic strata on the Scotian Shelf can be explained by compaction disequilibrium, but require the limiting values assigned to the properties of shale. For the Central North Sea Graben these limiting conditions are not sufficient, providing evidence for an active role of other pressure‐generating mechanisms.
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Tectonic and thermal evolution of Queen Charlotte Basin: lithospheric deformation and subsidence models
Authors S. A. Dehler, C. E. Keen and K. M. M. RohrA two‐layer lithospheric stretching model that includes the effects of decompression melting was used to estimate the deformation and thermal evolution of the Queen Charlotte Basin, British Columbia. The basin contains up to 6 km of Tertiary fill and is postulated to have been formed during a transtensional stage of Cenozoic plate motion between the Pacific and North American plates. Several models of basin formation have been proposed to explain the sediment distribution, contemporaneous volcanism and high present‐day heat flow.
We used bathymetry, Tertiary sediment thickness and crustal thickness to calculate the amount of stretching in the crust and lower lithosphere, and the volume of melt generated during advection of mantle rocks. A second set of calculations traced the thermal evolution of the sediments and lithosphere, and we show maps of estimated present‐day heat flow and sediment maturity. This study differs significantly from previous work in the use of gridded data that provide coverage over a large region and permit lateral variations in lithospheric deformation and thermal properties to be clearly defined, a difficult quest in studies based on single‐point or profile data. In addition, the use of crustal thickness, derived from a regional interpretation of gravity data and constrained by seismic refraction results, as an input allows reliable estimates of extension to be made despite recent deformation of sedimentary strata in Hecate Strait.
We present results for a model which used a prerift crustal thickness of ≈34 km and a short rifting period from 25 to 20 Ma. This model infers that significant thinning occurred beneath south‐western Hecate Strait and southern Queen Charlotte Sound, and several kilometres of igneous crust were added at these sites, without requiring elevated asthenospheric temperatures prior to extension. Net lithospheric extension is surprisingly uniform within the basin and averages 76%, or ≈50 km, across the margin. This amount is consistent with other estimates of extension and may provide information useful in refining models of plate motion along this margin.
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Volumes & issues
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Volume 36 (2024)
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Volume 35 (2023)
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Volume 34 (2022)
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Volume 33 (2021)
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Volume 32 (2020)
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Volume 31 (2019)
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Volume 30 (2018)
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Volume 29 (2017)
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Volume 28 (2016)
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Volume 27 (2015)
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Volume 26 (2014)
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Volume 25 (2013)
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Volume 24 (2012)
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Volume 23 (2011)
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Volume 22 (2010)
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Volume 21 (2009)
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Volume 20 (2008)
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Volume 19 (2007)
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Volume 18 (2006)
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Volume 17 (2005)
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Volume 16 (2004)
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Volume 15 (2003)
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Volume 14 (2002)
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Volume 13 (2001)
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Volume 12 (2000)
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Volume 11 (1999)
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Volume 10 (1998)
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Volume 9 (1997)
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Volume 8 (1996)
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Volume 7 (1994)
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Volume 6 (1994)
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Volume 5 (1993)
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Volume 4 (1992)
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Volume 3 (1991)
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Volume 2 (1989)
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Volume 1 (1988)