Ian G. Stimpson

North Chilean forearc tectonics and cenozoic plate kinematics

Abstract The continental forearc of northern Chile has been subjected to contemporaneous extension and compression. Here, cross-sections constructed across the forearc are presented which show that since initial shortening, deformation of the forearc has occurred in two tectonically distinct areas. These inner and outer forearc areas are separated by the strain discontinuity of the Atacama fault system and the tectonically neutral Central Depression. The outer forearc, the Coastal Cordillera, exhibits extensional tectonics, with large (up to 300 m) normal fault scarps preserved. These faults cut the earlier thrusts responsible for the elevation of Jurassic rocks at the coast above their regional elevation. The normal faults have been re-activated, displacing Quaternary salt deposits in the Salar Grande. This re-activation of the basement faults is probably due to the subduction of anomalously thick oceanic crust, producing an isostatic imbalance in the outer forearc. In the inner forearc, cross-sections through the Sierra del Medio and Cordillera de Domeyko show that structures of the Pre-Cordillera are best explained by a thick-skinned thrust system, with localized thin-skinned tectonics controlled by evaporite detachment horizons. Current forearc deformation features indicate a strong degree of correlation between subduction zone geometry and forearc tectonics. The timing of Cenozoic tectonism also fits well with established plate motion parameters, and the spatial and temporal variation in the state of stress of the forearc shows a close relationship throughout the Cenozoic to the plate kinematics and morphology of the subducting Nazca plate.

Training the next generation of near-surface geophysicists: team-based, student-led, problem-solving field exercises, Cumbria, UK

Discussions with employers of graduate applied geophysicists (reinforced by recent literature) indicate a progressive reduction in the numeracy and literacy of graduating students. In particular, there is a perception that problem-solving and quantitative analysis skills are not being gained during university studies, which could be partly attributed to an emphasis on classroom lectures and timetable constraints rather than research-informed and active learning in the field. This paper provides a pedagogic overview of a Masters level, student-led residential field exercise in the Lake District, Cumbria, UK that has run for eight years. The valley has complex glaciated bedrock buried by recent sediments, which poses a challenge for students to recognize, understand and quantify in three dimensions. Participating student ‘companies’ are set a competitive task to win a contract for a full geophysical valley survey to determine the route of a gas pipeline. Students initially complete a desk study, collating available multi-disciplinary (geology, remote sensing and geotechnical) data sets. The student-led field exercise then acts as a geophysical reconnaissance mission, with teams mapping depth to bedrock and estimating extents of any coastal salinity incursions. Full costings are produced to simulate a real work contract and the successful company is awarded the ‘contract’, based on ‘client’ presentations on the final day of the exercise. Comments on the student learning outcomes are provided, including employability skills in team working, problem-solving, quantitative data analysis, project and budget management and client presentation skills. Recent student evaluations are discussed with very positive comments from graduate geophysicists who have entered related employment emphasizing how the exercise has prepared them for the workplace.

Geophysical characterization of derelict coalmine workings and mineshaft detection: a case study from Shrewsbury, United Kingdom

A study site of derelict coalmine workings near Shrewsbury, United Kingdom was the focus for multi-phase, near-surface geophysical investigations. Investigation objectives were: 1) site characterization for remaining relict infrastructure foundations, 2) locate an abandoned coalmine shaft, 3) determine if the shaft was open, filled or partially filled and 4) determine if the shaft was capped (and if possible characterize the capping material). Phase one included a desktop study and 3D microgravity modelling of the relict coalmine shaft thought to be on site. In phase two, electrical and electromagnetic surveys to determine site resistivity and conductivity were acquired together with fluxgate gradiometry and an initial microgravity survey. Phase three targeted the phase two geophysical anomalies and acquired high-resolution self potential and ground penetrating radar datasets. The phased-survey approach minimised site activity and survey costs. Geophysical results were compared and interpreted to characterize the site, the microgravity models were used to validate interpretations. Relict buildings, railway track remains with associated gravel and a partially filled coalmine shaft were located. Microgravity proved optimal to locate the mineshaft with radar profiles showing ‘side-swipe’ effects from the mineshaft that did not directly underlie survey lines. Geophysical interpretations were then verified with subsequent geotechnical intrusive investigations. Comparisons of historical map records with intrusive geotechnical site investigations show care must be taken using map data alone, as the latter mineshaft locations was found to be inaccurate.

Geophysical surveying to determine the structure and origin of the Woore Moraine, Shropshire, UK

Abstract The Woore Moraine, situated within the Cheshire Plain, is one of the largest glacial landforms in Britain. However, its key characteristics, internal structure and glaciological significance are poorly understood due to limited exposure of its key geological features. This paper focuses on the results of a suite of complementary geophysical techniques (ground-penetrating radar, electrical resistivity tomography and seismic refraction) that were used to image the interior structure of the moraine and help determine its origin. Geophysical imaging reveals a two-layer moraine structure comprising a thin and heterogeneous carapace of intercalated sediments overlying a core of subglacial clay-rich diamicton and proglacial sands deformed by a series of fore- and back-thrusts. These features indicate that the moraine is glaciotectonic in origin, involving initial compression of proglacial sediments against a bedrock obstruction and subsequent modification due to overriding.

The geophysical investigation of lake water seepage in the regulated environment of the Bosherston Lily Ponds, South Wales, UK. Part 1: natural, fracture-related pathways

The detection, characterization and assessment of water loss through hydrological pathways is an important aspect of civil, hydrological and environmental engineering. At any site, variations in the form and hydrological integrity of both natural and man-made features can play a critical role in fluid transportation through the presence of enhanced hydraulic conductivity. Traditional hydrological and invasive engineering investigation methods are often unsuitable for ecologically sensitive environments and in this paper, we report the results of a non-invasive hydro-geophysical study at the Bosheston Lily Ponds in Pembroke, South Wales, UK, an area classified a Special Area of Conservation (SAC). For over 30 years, the lake system has experienced unexplained water loss and the geophysical surveys focused on identifying hydraulic conduits/pathways in the underlying carboniferous limestone and assessing the integrity of the man-made dam structure at the outlet of the lake system. The site is managed under tight regulatory regime that limits the scope and extent of any hydrological/geophysical investigations. The main objectives were to determine whether bedrock fracturing is promoting natural water loss and, more pertinently, if structural failures in the modern (and original) dam structures were responsible for significant loss of water at the outlet of the lakes. Ground-penetrating radar (GPR), electrical resistance tomography (ERT) and self-potential (SP) surveys were collected at targeted sites and the information gained used to ascertain the nature of the observed/predicted lake water loss. The results show that both the modern and original dam appear to be intact, structurally sound and show no evidence for significant water flow through its structures. Instead, localized zones of natural, high-density fracturing in the limestone bedrock appear to be the predominant cause of lake water loss.