Formation, uplift and dissection of planation surfaces at passive continental margins - a new approach

The usefulness of large-scale, low-relief, high-level landscapes as markers of uplift events has become a subject of disagreement among geomorphologists. We argue that the formation of low-relief surfaces over areas of large extent and cutting across bedrock of different age and resistance must have been guided by distinct base levels. In the absence of other options the most likely base level is sea level. We have analysed West Greenland landscapes in a recent study by combining the cooling history from apatite fission-track analysis (AFTA) data with the denudation history from landscape analysis and the stratigraphic record. An important difference between our approach and that of classical geomorphology is that we now have the ability to document when thick sections of rocks have been deposited and then removed. The present-day high-level plateau in West Greenland is the remnant of a planation surface that was formed by denudation that lasted c. 20 million years during which up to 1 km of cover was removed after maximum burial at the Eocene-Oligocene transition. Here we present additional AFTA data to show that the planation surface is the end-product of Cenozoic denudation even in basement areas and argue that Phanerozoic sediments - most likely of Cretaceous-Palaeogene age - must have been present prior to denudation. The planation surface was offset by reactivated faults and uplifted to present-day altitudes of up to 2 km. The uplift occurred in two late Neogene phases that caused incision of valleys below the planation surface and their subsequent uplift. We therefore find that the elevated and deeply dissected plateau is evidence of episodic post-rift uplift that took place millions of years after cessation of sea-floor spreading west of Greenland. We suggest that other margins with similar morphology may also be characterized by episodic post-rift uplift unrelated to the processes of rifting and continental separation, rather than being permanently uplifted since the time of rifting, as is commonly assumed.