Unravelling the structure of the ocean-continent transition from high resolution, photo-based 3D reconstructions of onshore dyke complexes

The temporal and spatial partitioning of strain between faulting and magmatism during continental breakup has important implications for the development of the crust- and upper mantle structure at rifted margins, but remains poorly understood. Late Neoproterozoic basaltic dyke complexes emplaced into continental basement and sedimentary cover units in the northern Scandinavian Caledonides represent an onshore-analogue of an ocean-continent transition. The dykes and their host-rocks are largely unaffected by Caledonian deformation and metamorphism, and are excellently exposed in three dimensions owing to a combination of glacial dissection and glacial retreat. Many of these outcrops of potentially high scientific value remain unmapped, mainly because they occur along steep, up to 300 m high ridges of glacier cirques in rugged mountain terrain that is largely inaccessible for traditional field mapping. Combined terrestrial and UAV-based Structure-from-Motion (SfM) photogrammetry provides an accurate and quick method of obtaining high resolution 3D information of such outcrops with minimal logistical effort. SfM-derived point clouds can be processed to identify structural discontinuities, such as faults and lithological contacts, and extract parameters such as strike, dip, thickness, density, and relative sequence of emplacement of the dykes. Based this information, the history of progressive intrusion and tilting can be reconstructed, and the amount of tectonic extension vs. magmatic dilation estimated. To demonstrate the effectiveness of this approach, we present a case study from a quarry in Lusatia, Germany. Here, like in northern Scandinavia, several generations of cross-cutting basalt dikes are exposed along a vertical, rocky cliff, but with the benefit of easy accessibility, permitting direct observation and verification of the digital data with field measurements. To improve accuracy, and to allow the extraction of oriented and scaled data as well as draping of independently acquired spectral data, ground control points are established in the scene using total station surveying. Multi-and hyperspectral data will potentially be used as complementary information to accurately distinguish composite dikes lacking intervening screens of host-rock.