Title: Quantifying Cenozoic exhumation on the Irish mainland and offshore using low-temperature thermochronology and seismic studies
Researchers: Remi Rateau, Dr David Chew
Small magnitude cooling and/or reheating events are difficult to detect using low temperature thermochronology as the thermochronological techniques are not sensitive at temperatures <40°C. By employing high-resolution vertical profiles, including the latest U-Th/He apatite approaches that are sensitive at very low temperatures (broken-grain age dispersion and 4He/3He in situ dating), the resulting continuous thermal histories could form a novel case study on constraining small magnitude phases of exhumation on passive margins. The onshore exhumation constraints would have been compared with estimates of the amount of late Mesozoic - Cenozoic sediment deposited in the Irish offshore basins using seismic studies. However, based on my prior petroleum industry background (exploration and seismic interpretation in the offshore Faroe-Shetland Basin, Atlantic Margin) and the industry interest in the Irish Atlantic Margin, the project goals were modified to focus on the offshore Irish Atlantic Margin. Since the early 1990s, numerous studies have recognized that anomalous post-rift subsidence and uplift has affected the so-called ‘passive’ North-East Atlantic Margin (see review in Praeg et al., 2005, Cogné et al., 2016 and Anell et al., 2009). In Ireland, such anomalous subsidence and uplift, in comparison to the expected ‘stretching and cooling’ model of McKenzie (1978), have also been identified in the Irish Atlantic Margin by tectono-stratigraphic (Hall and White, 1994; Clift and Turner, 1998; Jones et al., 2001), thermal modelling (Geotrack industry reports 1991-2008; McCulloch, 1993) or integrated thermal/stratigraphic/compaction studies (Scotchman and Thomas, 1995; Corcoran and Mecklenburgh, 2005). Many studies have also been undertaken onshore Ireland in order to better understand the Mesozoic and Cenozoic exhumation that resulted in the present-day scarcity of post-Variscan outcrops (Keeley et al., 1993, McCulloch, 1993, McCulloch, 1994, Dewey, 2000, Green et al., 2000, Badley, 2001, Allen et al., 2002, Tiley et al., 2004, Bennett, 2005). Recently, the geochronology group at TCD have derived thermal history models from low-temperature thermochronology data (AFT and AHe) of samples along pseudo-vertical profiles in the west of Ireland (Cogné et al., 2014) and around the Irish Sea (Cogné et al., 2016; Döpke, 2017). These studies have demonstrated that the systematic use of AHe integrated with AFT data, combined with modelling several samples that have been affected by the same thermal history (vertical profile approach) allows for the identification of previously unresolved small-scale exhumation events. The project will now aim at better characterizing uplift and erosion events in the offshore basins west of Ireland by using a similar approach to the one used onshore Ireland by the TCD group. The thermal histories will be constrained and contextualized by integrating well and seismic tectono-stratigraphic studies. The new results, associated with previous exhumation studies, will shed a new light on the spatial and temporal distribution of exhumation events in the Irish Atlantic Margin.
The main hypotheses underlying this research project are: a) The combinination of two low-temperature thermochronometers (AFT and AHe) in vertical profiles can, in some cases. quantitatively constrain samll-scale exhumation events; and b) the cuttings available from the PAD core store can lead sufficient good quality apatites for these techniques.
The key research objectives for the project are: a) To obtain reliable AHe data from the offshore IAM for the first time. b) To test the robustness of the integrated approach used by Cogné et al. (2016) onshore Ireland in order to resolve small-scale exhumation events. c) To improve the characterization of the offshore exhumation events: extent, timing, magnitude. d) To provide new data for better understanding the geological processes causing the post-rift exhumation events. e) To quantify the uncertainties associated with the techniques and to evaluate the reliability and suitability of such studies for the IAM.