Long-term stratigraphic evolution of Atlantic-type passive margins: A numerical approach of interactions between surface processes, flexural isostasy and 3D thermal subsidence
Highlights
► 3D flexure of a lithosphere submitted to thermal conduction and surface processes ► Impact of surface processes vs. lithosphere deformation ► The initial flexural rift-shoulder is eroded away within 10 to 20 Myr. ► Accumulation and subsidence rates decrease exponentially with time.
Introduction
The basins of passive margins preserve the terrigeneous sediments resulting from the erosion of the adjacent continental areas contained within their contributing drainage areas (Fig. 1). The thermal evolution and flexural isostasy of these highly stretched lithosphere are impacted by the (un)loading effects of erosion/sedimentation processes that, in return, affect the relief evolution along the margin (e.g. Burov and Cloetingh, 1997, Gilchrist and Summerfield, 1994, Kooi and Beaumont, 1994, Van der Beek et al., 1994, Van der Beek et al., 1995, Watts, 1989, Watts et al., 1982). These complex couplings are recorded by the geometries of the sedimentary wedges preserved in passive margin basins. They also affect their long-term stratigraphic trends controlled by the balance between the sediment accumulation, basement subsidence and eustasy.
In the early phases of the study of Atlantic-type passive margins, the long-term regressive trends (seaward migration of the shoreline) often characterizing their post-rift evolution was generally attributed to the combination of (i) the exponentially decreasing rate of the thermal subsidence of the basement (the smoothing out of the lithosphere thermal structure by conduction; Bally, 1981) and (ii) a sedimentary supply that was always much larger than the space available for sedimentation (i.e. accommodation, the sum of subsidence and eustasy; Jervey, 1988). This configuration results in a long-term regressive trend of the sedimentary wedge (Bally, 1981) modulated by higher frequency stratigraphic regression/transgression sequences controlled by eustatic sea-level variations (e.g. Vail et al., 1977). This simple view of the evolution of passive margin accommodation/accumulation balance did not include the flexural behavior of the underlying stretched lithosphere. Many authors have then underlined the impact of the flexural response to the isostasy of the lithosphere on the uplift of the rift flanks (e.g. Gilchrist and Summerfield, 1994, Kooi and Beaumont, 1996, ten Brink and Stern, 1992, Van der Beek et al., 1994, Van der Beek et al., 1995). Other authors have highlighted the (un)loading effects by denudation/accumulation resulting from surface processes on the flexure (e.g. Braun and Beaumont, 1989, Reynolds et al., 1991, Van Balen et al., 1995, Watts, 1989). For example, the sedimentary supply resulting from the rift-related relief erosion is expected to decrease exponentially with time (e.g. Kooi and Beaumont, 1996); in other words, it will not provide a constant supply to the passive margin basins. Also, the flexural component of the subsidence may migrate seaward under the weight of a prograding sedimentary wedge (e.g. Watts, 1989). These coupled effects between erosion/accumulation and flexure thus have a complex impact on the distribution, in time and in space, of both accumulation and subsidence, i.e. on the long-term stratigraphic trend of the passive margin basins. However, no comprehensive synthesis, predicting the impact of the flexural behavior of the lithosphere on the long-term stratigraphic trend of passive margin basins, is currently available. Indeed, modeling coupling lithosphere deformation and surface processes usually address large-scale deformation processes, i.e. they cannot resolve the stratigraphic trend of the simulated basins (e.g. Burov and Cloetingh, 1997, Huismans and Beaumont, 2008). On the other hand, models dedicated to stratigraphic simulation (e.g. Granjeon, 1997, Kaufman et al., 1991, Kenyon and Turcotte, 1985, Reynolds et al., 1991) do not include these feedbacks of erosion/sedimentation on deformation processes. The recent development of a numerical modeling tool, coupling the thermal and flexural evolution of the lithosphere to surface processes in 3D (Flex3D; Braun et al., 2013), has however provided a tool to investigate these effects.
The aim of this study is thus to revise the early view of long-term stratigraphic trend of the Atlantic-type passive margins to include the impact of the flexural component of the isostatic response of the stretched lithosphere. We performed a parametric analysis of Atlantic-type passive margins designed to constrain the relative contribution of both the evolution of a stretched lithosphere (initial geometry, thermal state and stretching profile) and the (un)load effects of surface processes on: (i) the accumulation and subsidence histories of the basin (as determined by backstripping), (ii) the denudation and uplift histories of adjacent continental areas (as determined by cooling histories measurements) and (iii) the long-term stratigraphic trends of the sedimentary wedge (as determined by seismic and sequence stratigraphy methods). The novel aspect of our approach is to define a predictive framework integrating the evolution of both domains in erosion and in sedimentation. We use state of the art modeling of the lithospheric flexure combined with a surface processes model and a quantitative estimate of the thermal evolution of the lithosphere (Braun et al., 2013) and sequence stratigraphy concepts (e.g. Jervey, 1988, Posamentier et al., 1988; Posamentier and Vail, 1988) to interpret the resulting simulations. This work, along with two companion papers (Braun et al., 2013, Dauteuil et al., 2013), constitutes a first step into a broader perspective, which is to provide basin geologists with quantitative tools to use the stratigraphic architectures of passive margin basins to track variations in the continental relief triggered by either climatic or deformation processes.
To achieve this, we simulated the long-term (> 100 Myr) evolution of passive margins, testing various initial geometries, thermal states and thinning profiles of the lithosphere as well as various efficiencies of surface processes. We first calibrated the coefficients of the surface transfer law using accumulation histories, relief lengths and sedimentary slopes as measured on several Atlantic-type passive margins. We then discuss the relative contribution of surface vs. deformation process and draw general implications for natural examples that might help basin geologists to assess the primary controls on the evolution of a given passive margin basin.
Section snippets
Principle
We used the numerical Flex3D model of Braun et al. (2013) to simulate the flexural response of the continental lithosphere subjected to an instantaneous stretching. Flex3D calculates the surface deflection of a thin, yet of variable thickness, elastic plate. It is coupled to (i) a three dimensional thermal model incorporating the effects of conduction, advection and production (Pecube; Braun, 2003), which allows to take into account the thermal evolution resulting from the stretching-induced
Approach
In order to calibrate the diffusion coefficient of the transport law (Kdw and Kdc) of the surface process model, we used sediment accumulation histories available 18 basins located along nine passive margins (Table 1 and Fig. 3). We assumed that the terrigeneous rate sediment accumulation in a marginal basin is directly linked to the sediment production (erosion) in the associated drainage areas: we neglected the influence of the sediment transit lag between the erosion and the sedimentation
Parametric analysis
We performed a set of 30 experiments (Table 4) to test the impact of varying the parameters characterizing both the surface processes (diffusion coefficients Kdc and Kdw) and the lithosphere state. For the former, we used the range of values determined in Section 3. For the latter, we compiled the data available in the literature on Atlantic-type margins (e.g. Fig. 8) to determine the range of variability of their crustal thickness (h0c), density (sediments ρs and mantle ρm), stretching (margin
Discussion
Our modeling quantifies the contribution of 10 parameters on a set of metrics that can be evaluated on many natural passive margins such as: the denudation/accumulation history (initial accumulation rate and total volume of accumulated sediments), the uplift/subsidence history (initial relief and maximum basement depth), and the long-term stratigraphic trend (durations of the regressive/transgressive phases). The purpose of this parametric analysis is not to simulate a specific margin but
Conclusions
- 1.
After calibrating diffusion coefficients of the surface transfer law using several Atlantic-type margin accumulation patterns, we performed a parametric analysis to quantify the relative contribution of 10 parameters characterizing the lithosphere (initial geometry, thermal state and stretching profile) and the efficiency of surface processes on a set of metrics that can be evaluated on many natural passive margins: the denudation/accumulation history (initial accumulation rate and total volume
Acknowledgment
This work has been funded by the CNRS/INSU program “Actions Marges”, ANR programs “chaire d'excellence” (attributed to Jean Braun) and “TOPOAfrica” (coordinated by F. Guillocheau). We thank Dr. Sara Mullin for post-editing the English style.
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