Structural Control and Stratigraphy of the Mafic and Ultramafic Units between Graah Fjord and Bernstoff Isfjord, South-East Greenland. South-East Greenland Mineral Endowment Task Project, SEGMENT 2009-2014 [BSc thesis]

Dismembered, elongated bands composed of gabbroids, peridotites and pyroxenites are found within a broad othogneiss package in the exposed outcrop area between Graah Fjord and Bernstorff Isfjord, in the Skjoldungen region, South-East Greenland. The bands of mafic and ultramafic rocks are the oldest rock units of the lithostratigraphy mapped and represent slivers of a lower crust-upper mantle environment. The formation of the magic and ultramafic rock units is comparable to the formation of the Mafic Complex of the Ivrea Zone, northwest Italy.

Three phases of ductile deformation in this lower crustal environment provide the controlling influence on the structure of the bands of mafic and ultramafic rocks. Early D₁ deformation was recognized by a SE-NW planar fabric in the gabbroids found locally in hinges of F₂ folds and transposed in the S₂ fabric along the limbs of F₂ structures. The deformation responsible for this fabric production also produced the elongate lens geometry of the mafic and ultramafic rocks. Widespread D₂ deformation is recorded by pervasive SE-NW S2 foliation, SE-NW shear zones and a strongly developed mineral stretching lineation. In addition, kilometer scale F₂ fold structures are developed and fold the S₁ fabric into closed to isoclinal, steeply inclined to the SW, moderately plunging folds to the SE, W and NW. Associated and parallel with the S₂ fabric are shear zones that concentrate on unit contacts locally preserving mylonitic textures. The banded lithostratigraphy observed between the mafic and ultramafic units and the orthogneiss is due to the thinning, stretching and folding of the stratigraphy during this event. The third phase of ductile deformation is recorded by the production of a series of broad, open NE-E trending F₃ fold structures that fold the S₂ fabric into regional scale, steeply inclined, moderately plunging folds. This event results in the refolding of earlier F₂ folds into a Type 2 fold interference model (Grasemann et al., 2004) and exhibits significant control of the structure of the bands of mafic and ultramafic rocks.

The ductile deformation of the bands of mafic and ultramafic rocks was mapped and studied in detail and together with petrology of the mafic and ultramafic units represented the focus of the study.