The Late- to Post-Caledonian Extensional History of Northwest Hinnoy, North Norway

Abstract

This investigation focuses on the lithologies and late- to post-Caledonian, upper crustal structures exposed in westernmost Hinnoy, north Norway, and the information about the timing and kinematics of faults that they may yield. The main abjective is the mapping of brittle to semi-brittle faults and determining what significance they have for Caledonian and later tectonic evolution of rocks in the region. Two major lithologies are found in the study area, migmatitic gneiss and mangerite, along with an intrusive dolerite and many pseudotachylites. The major rock types have been metamorphosed to granulite facies in the southern part of the study area and to amphibolite facies in the northern part. Important structural features of the study area includes many shear zones and eight major brittle faults. The shear zones are defined by cataclasites, mylonites, and ultramylonites and demonstrate semi-brittle to plastic behavior. They appear to occur in two sets over the study area. One set has a northeast strike, the other a northwest strike. The northeast-striking set appears to have formed first and demonstrates oblique-dextral reverse movement. The younger, northwest-striking set demonstrates oblique-sinistral normal movement. The brittle faults observed have a major plane along which brittle deformation and displacement has occurred. Analysis of data collected along these faults implies that general movement along the faults striking northeast was oblique-sinistral normal and movement along the northwest-striking faults was oblique-dextral normal. Eight samples were selected for 40Ar/39Ar isotopic analysis using both muscovite and potassium feldspar. The muscovite analysis provided a general cooling history beginning sometime after ca. 570 Ma and before ca. 440 Ma. The potassium feldspar results document a pulse, or multiple pulsees, of uplift and cooling between ca. 235 Ma and 185 Ma. Structures in the study area are related to a structural anomaly that is not yet completely understood. Several structural anomalies have been previously documented for this area: 1) a Jurassic basin postulated by Davidsen and others (2001); 2) Heier's Zone of Pseudotachylite (Heier, 1960); 3) the Eidsfjord fault; and 4) an extensional detachment postulated by Steltenpohl and others (2004). Results of this study lend at least some support for all of these anomalies, implying that they are most likely related to one another.