Models for the Ellesmerian mountain front in North Greenland: a basin margin inverted by basement uplift

Abstract

In North Greenland an early Palaeozoic trough sequence was compressed against a carbonate shelf which flanked it to the south during the mid-Palaeozoic Ellesmerian orogeny. Excellent N-S fiord sections reveal the frontal structures and good stratigraphic control permits their interpretation to depth. In the east, the structures are thin-skinned, with S-vergent folds and thrusts. Traced westwards these structures pass into a mountain front monocline which attains a maximum amplitude of 7 km. Attempts to interpret the structure in terms of familiar thin-skinned mountain front models do not lead to credible deep sections. It is necessary to invoke basement uplift, with an early Cambrian basin margin extensional basement ramp becoming reactivated in compression during the Ellesmerian orogeny. Four restorable deep sections are presented to illustrate this new mountain front model. If realistic, these show that horizontal displacements were modest, of the order of tens of km. The thin-skinned zone to the east has a hitherto unexplained southward tilt towards the foreland; this may be due to weak reactivation of a gently inclined segment of the basement ramp. The ramp geometry at depth along the four sections is modelled from the shape of the deformed basement-cover interface; the amplitude of the monocline is seen to be controlled by the inclination of the ramp and thrust displacement on it. There are also implications for the amount of Cenozoic displacement on the Nares Strait lineament between Greenland and Ellesmere Island, a topic of long-standing controversy. The monocline ends at the strait and the Ellesmerian structures revert to thin-skinned on the Canadian side. It is inferred that a transfer fault existed there during the early Cambrian extensional phase, terminating the basement ramp, and which on Ellesmerian reactivation produced the swing in strike and apparent offset of geological markers which some workers believe to be due to post-Ellesmerian strike-slip. If correct, this means that Cenozoic displacement along the strait (Wegener Fault) was limited to a few tens of km.