JOINT ORIENTATIONS OF LATE CALEDONIAN GRANITES IN NORTHERN SVALBARD

A suite of late- to post-kinematic granites intruded Caledonian basement rocks in northern Svalbard at ~420-390 Ma. Joint-lineament data were collected from Hornemantoppen, Newtontoppen, and Rijpfjorden granite bodies that span 220 km from W to E. Data were collected via tracing the surface expression of subvertical fractures visible in aerial photography available at the Norsk Polarinstitutt map website. Data were collected at 14 sites, with interpretations reviewed by multiple operators. Distinctive features located on Google Earth were used as reference points to determine the orientation and scale of traced lineaments. Data were then plotted as smoothed circular histograms to determine preferred orientations.

Sites had one to five preferred orientations. In aggregate, the sites may reflect a regional pattern of two orthogonal sets; one oriented approximately N-S and E-W, and the other trending NE-SW and SE-NW. Joint expression varies significantly from site to site, from showing both orthogonal sets to expressing only one direction of an orthogonal pair, with various permutations between. This variance may be a result of the interplay between topography and joint orientation, as it determines which joints form visible lineaments. A correlation between topography and expression is interpreted in the Rijpfjorden sites, where the most strongly expressed joints are those oriented at a high angle to the shoreline. Another potential explanation is that the joint sets show a clustered or domainal pattern, and different clusters are expressed at different sites. These two hypotheses may also work in tandem.

The two orthogonal sets suggest two or more distinct jointing events. Literature suggests that cooling joints in granites are common, and so one of these sets may be connected to original emplacement and cooling. Subsequent deformation, uplift, and erosion associated with post-emplacement events may have then produced the second orthogonal set. A N-S E-W orthogonal joint pair has been identified in Cretaceous intrusive rocks in SW Svalbard. An initial hypothesis is that the orthogonal pair of similar orientation in these granite bodies might be a result of the same stress field, and thus of Cretaceous age. However, the relative ages of each joint set are presently unconstrained.