Unravelling shear zone patterns in high grade terranes using GIS based geological and geophysical datasets: a case study from W Greenland
Sandra Piazolo1,2, Ian Alsop3, Jeroen van Gool1, Bo Møller Nielsen1
1Geological Survey of Denmark and Greenland, Thoravej 8, 2400 Copenhagen NV, Denmark
2
now at: Department of Earth Sciences, 4 Brownlow Hill, Liverpool, L69 3GP3
School of Geography & Geosciences, University of St. Andrews, St. Andrews, Fife, KY16 9AL UK piazolo@liv.ac.ukAlthough, the recognition of large-scale (~50 km) patterns of high strain zones is often essential for the understanding of high grade terrains, it is often difficult to detect such broad and diffuse high strain zones by traditional methods of geological mapping. Our work shows that an effective use of different GIS based datasets from several geoscience disciplines, such as structural geology, metamorphic petrology and geophysics, can help to recognize such patterns and consequently to unravel the complex metamorphic and structural history of a such a terrain.
The case study lies in the transition zone between the Palaeoproterozoic Nagssugtoqidian and Rinkian orogens of West Greenland. Results show that the region can be subdivided into two main domains that are characterized by differences in structure, metamorphic grade and aeromagnetic signature. The southern part (being closest to the core of the Nagssugtoqidian Orogen) is characterized by Archean orthogneisses with partially preserved granulite facies assemblages, predominately S to S>L tectonites and a high amplitude, but irregular magnetic signature. Foliation patterns show large-scale, sub-isoclinal folding. and weak lineations which are refolded. The amphibolite facies grade northern domain lies immediately to the north of the granulite block and is characterized by several roughly parallel NE-SW trending, linear magnetic signatures extending for 40-50 km. These correspond to a pronounced preferred orientation of both lineations and foliations and coincide with a well developed S-L fabric. Kilometric-scale reversals in the direction of lineation plunge suggest original sinuous whale-backing of the lineation. In the northern part of this domain, the linear magnetic features are less pronounced whilst a mixture of S-L and L>S fabrics are observed. The Northern Domain displays belts which exhibit steeper lineations and foliations than typically observed elsewhere and which mimic the linear magnetic signature. These belts are predominately associated with metasedimentary sequences, although a similar feature is also observed in orthogneisses at the boundary between the southern and the northern domains, and within metasedimentary units in the granulite facies southern domain. These high strain belts are interpreted to be the result of strain localization processes in rheological weaker metasedimentary and amphibolite facies units. Subsequent late-stage reactivation of these shear zones may have resulted in the relative uplift of the southern domain, thus juxtaposing higher grade granulite facies block with adjacent amphibolite facies units, and largely overprinting early shear sense criteria.
Thus, granulite facies metamorphism (with amphibolite facies at higher crustal levels), is associated with large scale isoclinal folding and strain localization within mechanically weak zones. The granulite facies block may have acted as a relatively rigid pod or indentor within these weaker units. Subsequent low amplitude folding and reactivation of the high strain zones during extension resulted in the relative uplift and juxtaposition of the presently exposed domains. This works demonstrates that regional patterns of strain localization on scales of 10 to 100 km can be resolved via combined datasets incorporating structural, metamorphic and geophysical signatures within the GIS framework.