Lithospheric Strength and Rift Migration Controls on Synrift Stratigraphy and Breakup Unconformities at Rifted Margins: Examples From Numerical Models, the Atlantic and South China Sea Margins

Abstract

Synrift stratigraphy and the distribution of breakup-related erosional unconformities vary vastly between passive margins and cannot be explained by classical rifting models. Here we use numerical modeling to predict their spatiotemporal distribution. We show that synrift stratigraphy mimics rift architecture, which is controlled by lithospheric strength. Basinward rift migration during extension produces (1) oceanward younging, syntectonic and posttectonic sequences, (2) rift migration unconformities, RMUs, predating breakup, and (3) a breakup unconformity, BU, that only extends over the outermost margins, since breakup is not linked with a sudden stress drop. With small synrift sedimentation, the RMUs and BU laterally merge to form a margin-wide unconformity. In symmetric, wide conjugate margins, which arise for weak lithospheres such as the South China Sea, a long phase of distributed deformation with little subsidence results in early synrift sediment over most of the margins. RMUs merge into a single event that marks the subsequent focusing of deformation into a narrow breakup area, which experiences short-lived intense thinning and subsidence. In asymmetric conjugate margins, lateral rift migration transports shallowly deposited, early synrift sediments from the narrow to the wide, hyperextended margin, leading to a condensed syntectonic sequence and a single BU in the narrow margin and a series of RMUs in the wide one. For very weak lower crusts, lateral rift migration generates large synrift sag basins in the wide margin, as in Angola and Congo margins. Our models resemble the observed margins tectonic diversity and may be used as templates for interpreting their distal, unexplored areas.

The stratigraphic record of continental breakup, offshore NW Australia

“…Overall, our work supports previous findings that continental breakup processes are variable in time and space, and can involve multiple episodes of uplift and unconformity development (e.g., Soares et al 2012;Alves & Cunha 2018;Gong et al 2019;Monteleone et al 2019;Xie et al 2019). We also demonstrate that migration of rift axes probably plays an important role in controlling the occurrence, distribution, and magnitude of breakup unconformities (Pérez-Gussinyé et al 2020). Stratigraphic successions on continental rifted margins provide a critical record of these complex breakup processes, but unlocking these archives can be difficult and requires integrating geological and geophysical analyses (e.g., Soares et al 2012;Gong et al 2019;Monteleone et al 2019;Peron-Pinvidic et al 2019;Pérez-Gussinyé et al 2020).…”

“…Regardless of whether the Cuvier Abyssal Plain comprises oceanic crust or marks a COTZ (see Reeve et al 2021 and references therein), our results indicate the IVU coincided with: (i) the localisation of extension along narrow continental rift zones (Gascoyne and possibly Cuvier margin), which can become COTZs (e.g., as inferred by Bridges et al 2012 in the onshore Gulf of Aden rift, Ethiopia), and perhaps a seafloor spreading centre within the Cuvier Abyssal Plain; and (ii) an increase in magmatism focused along the axis of extension, which produced the igneous rocks that carry the magnetic chron signature (e.g., as inferred by Collier et al 2017 along the South Atlantic rifted margin). Similar migration (from inboard to outboard positions) and localisation of extension through time has been recognised from both active rifts and ancient rifted margins, where such narrow zones of extension play an important role in the late-stages of rifting and transition to seafloor spreading (e.g., Ebinger & Casey 2001;Geoffroy 2005;Corti 2009;Bastow & Keir 2011;Bastow et al 2018;Peron-Pinvidic et al 2019;Pérez-Gussinyé et al 2020).…”

“…10A). We thus suggest unconformity development likely reflects localised rift flank uplift (Pérez-Gussinyé et al 2020), perhaps driven by depth-dependent extension following strain localisation along rift zones in the Gascoyne Margin COTZ and Cuvier Abyssal Plain; i.e. lower crustal and/or lithospheric material flowed landward away from rift axis and subsiding COTZ to under the rift flanks (Fig.…”

“…1), which developed sub-aerially during the transition from continental rifting to seafloor spreading (e.g., Falvey 1974;Driscoll et al 1995;Lavin 1997;Soares et al 2012;Mohriak & Leroy 2013;Morley 2016;Gong et al 2019;Xie et al 2019). These breakup unconformities broadly separate faulted, syn-rift rocks from overlying, largely unfaulted post-rift rocks, indicating subsidence was punctuated by a period of margin-wide uplift and/or erosion (e.g., Falvey 1974;Driscoll et al 1995;Alves & Cunha 2018;Pérez-Gussinyé et al 2020). Such syn-breakup uplift has variously been attributed to: (i) a thermal response to mantle upwelling (e.g., Falvey 1974;Morley 2016); (ii) rift flank uplift caused either by convective heat transfer from deeper parts of a rifted basin (e.g., Cochran 1983), or an isostatic response to depth-dependent extension (e.g., White & McKenzie 1988;Issler et al 1989); or (iii) isostatic rebound of over-deepened sedimentary basins (e.g., Braun & Beaumont 1989).…”

“…The stratigraphic architecture and formation of these unconformities and their encasing strata, i.e. the breakup sequence, thus provides an important record of the tectonic and geodynamic evolution of continental margins (e.g., Soares et al 2012;Alves & Cunha 2018;Gong et al 2019;Monteleone et al 2019;Peron-Pinvidic et al 2019;Pérez-Gussinyé et al 2020).…”