Post-rift development of passive continental margins: Source areas that act as sinks and sinks that act as sources

It is a common assumption that landscapes along elevated, passive continental margins (EPCMs) are directly related to the processes of rifting and crustal separation. The elevated plateaux along such margins are widely believed to have remained high since continental separation and remained flat by continuous erosion to a perched base level (no surface uplift) or alternatively represent a breakup surface (no rock uplift), despite continental-stretching theory predicting deposition of a thick post-rift sequence overlying both the rift and its margins. The absence of a post-rift section from many EPCMs is taken as evidence that none was ever deposited, consistent with these margins being permanently elevated since rifting.

However, our recent studies in Greenland and Brazil show that typical EPCM landscapes formed many tens of millions of years after break-up along these margins, and that the present-day high-level plateaus are the remnants of post-breakup erosion surfaces (peneplains) that were uplifted in the Neogene. Similar characteristics of EPCMs around the world, with elevated plateaux cut by deeply incised valleys terminated by areas of rapid decline in elevation to a coastal plain, suggest that similar processes controlled their development. We thus infer that EPCM landscapes in general are unrelated to the processes of rifting and continental separation.

We present geological, geomorphological and thermochronological evidence from EPCMs around the world in support of this hypothesis, and we argue that the absence of a post-rift section at many EPCMs does not mean that none was deposited. Instead, the post-rift section along many EPCMs was removed by erosion during post-breakup uplift events that are unrelated to formation of the margin but relate in some way to the presence of the margin. Since EPCMs develop through episodes of burial and exhumation, sources and sinks along the margin vary through time and in space. Erosion of basins along uplifted margins may thus transform basins into source areas whereas presently uplifted margins may have acted as sinks during previous burial episodes, both prior to, during and after break-up (Fig. 1). Consequently, for the source-to-sink concept to work, it is important to compare erosion events with the ultimate sink from that event.

Figure 1. Cartoon to illustrate the post-rift development of an EPCM based on observations in Greenland and Brazil. (A) A rift margin in continental crust approximately 30–50 Myr after cessation of rifting: Accumulation of syn-rift sediments (blue) and post-rift sediments (green), due to cooling of the rift and its margins. (B) After one phase of uplift of the rift and its margin: Erosion has formed a peneplain, governed by the base level. The surface of the peneplain is uniform regardless of the resistance of the underlying rocks. Subsidence continued offshore and the accommodation space filled with sediment (orange). (C) After renewed subsidence: Sediments (beige) cover the erosion surface forming an erosional unconformity in the basin and burying the peneplain across basement rocks to some extent. (D) After a second phase of uplift: The peneplain that formed after the first phase of uplift is now an uplifted high-level surface. Sediments that were deposited horizontally in the post-rift section now dip seaward and are truncated by the new erosion surface. Subsidence still continued offshore and the accommodation space filled with sediment (yellow). From Japsen et al. (2012a).