EXPLORING THE TECTONIC AND CLIMATIC DRIVERS OF ASYMMETRIC TOPOGRAPHY AND FLUVIAL INCISION IN A RIFTED MARGIN, SURNA VALLEY, SOUTHWESTERN NORWAY

Morphologic characteristics of a landscape, such as topographic relief and fluvial steepness, result from a balance of rock uplift and climatic denudation, although isolating the respective roles of these drivers is difficult. The southwestern coast of Norway displays high escarpment relief in a post-breakup, post-glacial setting. Here we report on patterns of relief and fluvial incision in a region characterized by glacial sculpting, rapid isostatic uplift, and a well-established brittle template of normal faults.

The Surna valley (Surnadalen) of mid-southern Norway is a SW-NE striking wide, alluvial, U-shaped valley whose SW margin defines part of the Great Escarpment. Surnadalen displays clear morphometric asymmetry, its inland (SE) side defined by high elevation (>1000 m) and well-developed drainage networks that display clear evidence of alpine glacial carving. By contrast its seaward side is lower (~500 m) and has neither developed drainage networks nor evidence for valley glaciers. Inland drainages are marked by a series of knickpoints, most of which are probably the result of valley glacial retreat. However, several display distinct convex-up longitudinal profiles and negative concavity values; this subset of knickpoints form a linear array aligned parallel with and projecting directly into a major strand of the Møre-Trøndelag fault complex. Many brittle fault outcrops that either strike parallel to or cross-cut the limbs of regional-scale MTFC-parallel folds are known to the SW of the knickpoint alignment. More brittle faults were discovered near or adjacent to several of the incised zones during fieldwork. These specific knickpoints, and only these knickpoints, are zones of deep bedrock incision. The depth of incision is highest in the southwest and is reduced along-strike to the northeast; incision depth does not correlate with fluvial process zone, lithology, structure, drainage basin size, climate, or isostatic uplift rates. We propose Surnadalen’s asymmetry is best explained by a model of late Cenozoic normal fault reactivation, including post-glacial activity that differentially partitions post-glacial rebound along important and pervasive structures parallel to the Surna valley axis where fluvial erosion became localized at or near the principal structural boundary.