Crustal trace of a hot convective sheet

Holford

2005

JGS

The western UK basins of the Irish Sea have provided one of the best natural laboratories for investigating the causes and consequences of intracratonic uplift and erosion (exhumation). To date, the emphasis has been on igneous underplating as the chief process driving their exhumation. In this paper, we demonstrate that tectonic inversion -the shortening of formerly extensional basins and reactivation of their constituent faults -dominated the exhumation of the St. George's Channel basin, offshore Wales. Based on mapping of an extensive 2D seismic grid, evidence is presented for at least two major inversion episodes in the Late Cretaceous and the Neogene, plus minor shortening during the Eocene. Inversion was distinctly noncoaxial, especially during the Neogene when coeval transpression and transtension was focused at discrete bends and stepovers on the basin-bounding St.George's, Bala and Northwest Flank faults. That the principal mechanism driving these uplift episodes was inversion (as opposed to igneous underplating) is corroborated by analysis of thermal history data (apatite fission track and vitrinite reflectance). They reveal late Cretaceous and Neogene geothermal gradients that were comparable with the present day i.e. no significant increase in basal heatflow. Sonic velocity profiles logged in hydrocarbon boreholes constrain the minimum thickness of the eroded section which varies between c.1000m in the centre and c.2240m at the margins of the basin. Given the strength of evidence for tectonic inversion in the St. George's Channel basin, our favoured model invokes superimposition of the effects of inversion and igneous underplating to account for the complex exhumation history of the St. George's Channel basin in particular, and the western UK basins in general. KEYWORDS:ABBREVIATED TITLE: Inv. & Exh. of the SGCB 2 2 Intracratonic basinsadjacent to 'passive' ocean margins undergo uplift in response to a diverse range of processes related chiefly to i) epeirogeny (i.e uplift of broad regions of continental interiors), and ii) intraplate shortening and tectonic inversion. Sedimentary basin inversion, the compressional or transpressional reactivation and shortening of formerly extensional basins, can have a marked influence on the structure and evolution of the basin fill. It follows that inversion has major implications for the exhumation of petroliferous basins, where the attendant cooling and lithostatic pressure release affects hydrocarbon generation and retention directly (e.g. Dore et al. 2002).Reconstructing the tectonic history of the western UK basins is hampered by the difficulty in recognizing and discriminating the superimposed effects of epeirogeny and inversion. This paper reports on the results of recent mapping of an extensive seismic dataset imaging the SE margin of the St. George's Channel basin (SGCB; Fig. 1). The focus of this mapping has been to determine the geometry, kinematics and timing of deformation associated with the Cretaceous-Cenozoic inversion of the NW European 'A...

Section: Contribution Of Inversion To Regional Exhumationmentioning

confidence: 99%

Inversion and exhumation of the St. George's Channel basin, offshore Wales, UK

Williams

Holford

2005

JGS

“…Normal faults are predominant in these basins and there is little or no evidence for upper crustal shortening within them White, 1994, 1995]. Inversion has thus been dismissed as a mechanism for exhumation, leading many workers to ascribe the reported $3 km of exhumation in these basins to epeirogenic processes (i.e., without folding and faulting [Summerfield, 1991]) such as lower crustal igneous underplating associated with Paleocene-Eocene mantle plume activity [White and Lovell, 1997;Jones et al, 2002;Al-Kindi et al, 2003]. The best known of these basins is the East Irish Sea Basin (EISB), the most northeasterly depocenter of the NE-SW trending Irish-Celtic Sea Basin system, a $700 km long chain of exhumed MesozoicCenozoic extensional basins [Tappin et al, 1994;Jackson et al, 1995].…”

Section: Introductionmentioning

confidence: 99%

“…The absence of such structures in the EISB has led some workers to dismiss [Brodie and White, 1995] compressional shortening as a mechanism for exhumation in the EISB. Instead, Paleocene igneous underplating of the lower crust has commonly been invoked as the sole driver Al-Kindi et al, 2003]. However, this mechanism cannot explain the major exhumation episodes during the Early Cretaceous and Neogene that affected this region as revealed by thermochronological studies [Green et al, 1997;Holford et al, 2005].…”

Section: Introductionmentioning

confidence: 99%

Signature of cryptic sedimentary basin inversion revealed by shale compaction data in the Irish Sea, western British Isles

Holford

Green³

et al. 2009

Tectonics

Inversion is one of the most common modes of intraplate deformation and a primary mechanism of sedimentary basin exhumation. The key criterion for identification of basin inversion is the contractional reactivation of extensional faults. Reverse displacement associated with inversion is focused along the upper segments of faults, so the evidence for inversion may be removed in deeply exhumed basins. We believe this is the case in the Irish Sea, western British Isles. Previous authors have attributed the severe exhumation of this basin to plume‐related uplift and have ruled out inversion as a cause of uplift. We present a method for identifying tectonic inversion in a basin where exhumation has removed evidence of reverse displacement on the upper segments of faults. We utilize compaction‐based mapping of erosion patterns recorded by anomalously high sonic velocities in Upper Triassic shales. We show that the amount of section removed during exhumation in the Irish Sea varies between 1.3 and 3.3 km and that exhumation patterns are markedly heterogeneous. Neither the heterogeneity of exhumation nor its focusing over hanging wall blocks of major faults is consistent with solely plume‐related uplift, and we believe that our results record cryptic basin inversion that is superimposed on epeirogenic exhumation (some of which may be plume related). The direct evidence for this inversion has been removed because erosion has proceeded beneath the null lines of reactivated faults, and we propose erosion of reactivated fault segments as one of the major reasons why shortening is frequently underestimated in inverted basins.

“…A lens of high-velocity material within the lower crust, which is several kilometers thick and extends for a distance of 550 km SW-NE beneath northern Britain, has been imaged using a wide-angle seismic experiment. One interpretation of this zone is that it is the Paleogene igneous underplate [Al-Kindi et al, 2003]. …”

Section: Evidence For Paleocene Upliftmentioning

confidence: 99%

Cenozoic vertical motions in the Moray Firth Basin associated with initiation of the Iceland Plume

Mackay¹,

Jones

et al. 2005

Tectonics

It is likely that the Iceland mantle plume generated transient uplift across the North Atlantic region when it initiated in earliest Cenozoic time. However, transient uplift recorded in sedimentary basins fringing the region can be overprinted by the effects of permanent uplift. Identifying and quantifying transient uplift can only be achieved in areas which have a well‐constrained stratigraphic record and across which the relative importance of permanent and transient uplift varies (e.g., the Moray Firth Basin, North Sea). By analyzing the subsidence of 50 boreholes from the Moray Firth Basin (MFB), residual vertical motions unrelated to rifting have been isolated. Transient uplift of 180–425 m occurred during Paleocene times. The western MFB has also been affected by permanent Cenozoic uplift, with denudation decreasing from 1.3 ± 0.1 km in the west of the basin to zero denudation east of 1°W. Dynamic support above the Iceland Plume led to transient uplift of the entire MFB in early Paleocene times, peaking in latest Paleocene times. In early Eocene times the effect of the plume waned, and subsidence occurred. Paleocene permanent uplift of the NW British Isles is generally accepted to have been due to magmatic underplating of the crust emplaced during the British Tertiary Igneous Province (61–58.5 Ma). The cause of Neogene uplift events is poorly understood, but it could also be associated with the Iceland Plume.