Lithospheric necking and regional isostasy at extensional basins 1. Subsidence and gravity modeling with an application to the Gulf of Lions Margin (SE France)

Abstract

We present a numerical pure‐shear stretching model to study the effect of the “depth of lithospheric necking” on the state of flexure at extensional basins. This model avoids the need for low flexural rigidities during synrift basin subsidence. The model also accounts for long‐standing rift flank uplifts in the absence of significant thermal anomalies or underplating beneath rift shoulders, and it predicts low β factors beneath basin centers. Numerical modeling of synthetic continental rift zones and rifted continental margins together with their gravity characteristics demonstrates that isostatic residual anomalies are sensitive indicators of the state of flexure at extensional basins, particularly at basin margins. Isostatic residual anomalies are particularly useful to study the state of lithospheric flexure because the results are not biased by assumptions on the loads that have to be made in a forward modeling approach. Previous studies of lithospheric flexure and gravity modeling might have underestimated flexural rigidities at extensional basins. Comparison of predicted gravity anomaly signatures with gravity data suggests that at narrow rift basins and young rifted continental margins, upward flexure occurs more commonly than does downward flexure. This can be explained by intermediate or deep levels of necking (>15 km). For rifted continental margins, the apparent transition in the state of flexure during basin evolution from upward to downward flexure points to intermediate levels of necking (15–20 km). The occurrence of intermediate to deep levels of necking at narrow rift basins and intermediate levels of necking at rifted margins is in accordance with brittle‐ductile rheological models of the lithosphere. An intermediate depth range for the level of lithospheric necking at rifted continental margins is consistent with a scenario in which rifting preferentially occurs in an area where the lithosphère is weakened by a thickened crust Rifting in areas of stronger lithosphere, as for example cratons, is probably more often associated with a deep level of necking, explaining the widespread occurrence of rift shoulders at failed rift basins. We have tested several depths of necking and a model of local isostasy for the Gulf of Lions margin in the northwestern Mediterranean. At positions of deep synrift grabens, models with depths of necking of 25–35 km predict crustal thicknesses that are more in accordance with observations than are models with a shallower level of necking. It appears that local isostasy cannot account for the present‐day basin configuration and at the same time reproduce the observed strongly laterally varying bathymetry at the end of rifting, supporting a significant postrift flexural rigidity.

Stress-induced late-stage subsidence anomalies in the Pannonian basin

“…The basin formation process is described in terms of the mechanism of lithospheric necking (Braun and Beaumont, 1989;Kooi et al, 1992). The level of necking can be connected with the zone of maximum lithospheric strength , or alternatively with the depth of intra-lithosphere detachment ( Van der Beek et al, 1994).…”

The effect of rift shoulder erosion on stratal patterns at passive margins: Implications for sequence stratigraphy

“…The influence of the depth of necking is more dramatic. A deeper level of necking not only produces more flexural uplift but also considerably alters the flexural state of the lithosphere underlying the remainder of the basin [39]. The resulting stratigraphy in this model displays more offlap (20 km), which also lasts longer than in loading.…”

The effect of riftshoulder erosion on stratal patterns at passive margins - Implications for sequence stratigraphy

“…Because there are several lines of evidence indicating that the continental lithosphere may behave as a relatively strong elastic plate [15,38], permanent uplift can be explained by retention of flexural strength of the lithosphere during rifting, causing uplift in response to tectonic unloading or plastic necking . This seems to be the mechanism which best explains the general occurrence of uplifted rift flanks [3,39]. We model the effect of rift shoulder erosion on offshore stratigraphic patterns by means of a two-dimensional model for lithospheric evolution.…”

“…In the pureshear model model, lithospheric cooling after thinning causes differential vertical motions to take place at the passive margin. We have extended the pureshear model in order to take into account flexural compensation of lithospheric and sedimentary loads, two-dimensional heat flow, finite duration of rifting, and necking of the lithosphere during rifting around its strongest part(s) [39,41]. When a basin is overdeepened due to the necking process, the flexural isostatic response to necking causes uplift of the rift shoulder (Fig.…”

“…Estimates of lithospheric necking depths by forward modelling of rift shoulder uplift and erosion by Van der Beek et al [3] have resulted in necking depths of 10 and 30 km for the Saudi Arabian and Transantarctic flank uplifts respectively. Tectonostratigraphic forward modelling of Neogene Mediterranean extensional basins yields necking depths ranging from 8 to 25 km [39,.…”