Extensional evolution of the central East Greenland Caledonides

Abstract

This thesis addresses the complexity of both syn- and post-orogenic extension in the overriding plate during Caledonian continental collision through field and laboratory investigations in the central East Greenland Caledonides. During the course of this work, attempts were made to answer some of the outstanding regional and local questions in East Greenland geology. Structural, U-Pb and ⁴⁰Ar/³⁹Ar geochronologic, petrographic and thermobarometric data were combined to constrain and reconstruct a portion of the tectonic history of this orogen. Most extension was accommodated along a system of orogen-parallel, N-S striking normal faults known as the Fjord Region Detachment (FRD) system. The FRD system comprises two temporally distinct, but overlapping, splays just south of 73⁰ N. The lowermost splay is called the Hogedal detachment was active from ca. 417 to 380 Ma, and was active for a second time as recently as ca. 357 Ma. The uppermost splay is the Tindern detachment. This fault was active from ca. 425-423 Ma, exhuming material at rates as fast as 6.5 mm/year. Continued extension in the hanging-wall of this fault accounts for additional denudation at much slower rates over a 25 my time-period. In-between activity on these faults, there is evidence to suggest that middle-crustal thickening continued to occur. Thus, the East Greenland Caledonides preserve evidence for crustal thickening (minimum -16 km) and orogen parallel shear, followed by rapid upper-middle crustal thinning (-13 km), followed by coeval middle-crustal thickening (unknown amount) and upper-crustal thinning (5 km), and ending with crustal collapse (-16 km thinning).

(cont.) This is the first time that an alternation between thrusting and normal faulting has been observed in an over-riding plate during continent-continent collision, and only the second time that it has ever been documented in a collisional orogen. The data imply that there was a fundamental cyclicity between crustal thickening and thinning, consistent with dynamical models of orogenesis in which plate-forces responsible for contraction and gravitational forces responsible for extension, oscillate between periods where one dominates. Furthermore, given the established relationship between topography and synorogenic extension in active mountain belts, it is likely that activity along the Tindern detachment, the earliest splay of the FRD, was controlled by Caledonian paleotopography that formed during the initial stages of orogenesis. The fact that most middle- and upper-crustal extension was restricted to the FRD implies that a localized inherent crustal weakness may have developed after initial movement along the Tindern detachment. Given that late-stage Devonian activity on the FRD may have played a prominent role in the formation of the Devonian basins, which themselves likely controlled the geometry and location of subsequent Mesozoic extension and formation of the North Sea basins, the implication is that the position of rifting of the north Atlantic ocean was partially [pre-determined] inherited from the initial Caledonian paleotopography.