Title: Structural Evolution of the Hornelen Basin (Devonian, Norway) from Detrital Thermochronology
Description: The Hornelen basin, a tectonically-controlled Devonian extensional basin formed during the late stages of the Caledonian orogeny in SW Norway, provides an opportunity to investigate the geologic and tectonic evolution of the Caledonides through the use of single grain detrital thermochronometers. The Caledonides are an extensively studied orogen, which today expose medium- to high-grade metamorphic rocks from the core of the Siluro-Devonian continent-continent collision between Baltica and Laurentia. Important questions remain unresolved concerning the structural style of orogenic collapse, the geodynamic mechanism for the exhumation of ultra-high pressure metamorphic terranes, and the latest stage evolution of the orogen as a persistent topographic welt 400 million years after the main collisional event. This thesis presents the novel approach of using multiple chronometers on a single, large set of detrital samples from the Hornelen to elicit layers of information about the evolution of the basin source regions (and thus the ancient, regional geology of the now-long-since-eroded surface of the Caledonides).

Using an extensive sample set that spans the basin through both space and time, and a suite of thermochronometers that span the range of closure temperatures from 850ºC (zircon) down to 60ºC (apatite fission-track), I interpret the data in terms of changes in basin provenance that are directly controlled by the structural setting of the basin, thus providing insight into the large-scale structure of an orogenic collapse. The low temperature chronometers (apatite and zircon fission-track) provide information about the post-depositional thermal history of the basin, and insights into the structural controls on this latest uplift history. Overall, these data shed new light on both the late-stage evolution of the Caledonian orogeny as well as the post-Caledonian history of uplift that continues to shape the modern Norwegian landscape. In chapter 1, I employ detrital zircon U/Pb dating on a suite of twenty samples that complement the previous work of Johnston (2006) and Pedersen (2011), and document a spatial pattern that supports Cuthbert’s (2000) observations of distinct N-S asymmetry in basin provenance.

Chapter 2 presents detrital mica ⁴⁰Ar/³⁹Ar dating on twenty four samples from the same suite combined with (unpublished) data from three additional samples obtained by C. Warren at Open University; these data reflect a different dimension of the changes in basin provenance than the zircon data, documenting an up-section younging trend in mica ages. I interpret the detrital mica data to reflect progressive exhumation of the metamorphic core of the Caledonian orogen during basin opening. Chapter 3 delves further into the detrital zircon data using a petrochronological approach, complementing the U/Pb age data from Chapter 1 with trace element analyses from forty three zircons, mainly the youngest Caledonian (Scandian) aged grains, but also including an enigmatic population of Archean and Paleoproterozoic zircons. Single grain trace element data from these zircons (which have no obvious source in the allochthons) points to a provenance in the WGR for both the youngest, and oldest, detrital zircon populations in the Hornelen.

Chapter 4 takes a different approach to detrital thermochronology, focusing on low temperature thermochronometers from the Hornelen and surrounding basement rocks, and interpreting two data sets (apatite and zircon fission-track) in terms of the latest uplift history of SW Norway: a history which is poorly constrained by the lack of onshore sedimentary deposits since the Devonian, and for which low-temperature thermochronology represents one of the most fruitful modern approaches. Based on subtle cooling-age differences around the Hornelen, I conclude that a single low angle detachment fault underlying the basin is unlikely, and instead, a series of reactivated, N-S trending normal faults have determined the structural evolution of the Hornelen area since the Devonian.