Structural Inheritance in active and ancient extensional regimes

Overview

Structural inheritance is a characteristic of continental lithosphere deformation controlled by dynamic and long-term weakening of fault zones. Many extensional basins and continental margins show evidence for a strong influence by pre-existing structures and fabrics (structural inheritance) during their early phase that then declines as basin boundary faults grow and interact and rifting matures. How this reorganisation occurs, the controlling mechanism(s) and effects on basin fills and seismicity patterns in active regions is not well under understood and is the topic for this PhD research project.

Structural inheritance in sedimentary systems has been attributed to 1) direct reactivation of pre-existing faults and fabrics, 2) local perturbations of regional stress fields which causes faults to form or reactivate in non-optimal orientations, (transverse or – off-axial trend faults) and 3) segmentation and retardation of normal fault array growth either by basement blocks or magmatic sectors in basins.

The aim of this project is to investigate what controls the transition from inheritance-dominated to far-field stress-dominated deformation in extensional basins using a combined structural, geophysical and modelling approach. The specific focus will be on examples that represent systems in the structural reorganisation phase or where the axis-parallel faults and off-axis basement-influenced structures are superimposed but remain visible. Candidate faults (with off-axis orientations) are located in the West Greenland, East Africa Rift in Tanzania, the Ordos rifts in NW China and the Italian Apennines, which form the main study areas, however these features are common to most margins and basins.

Click on an image to expand

Image Captions

Fig. 1. East Africa Rift, Tanzania showing rift faults, volcanic centres and basement fabrics (pale grey background lines). Inset: Oldyino Lengai active carbonatite with rift. border scarp in the background (view to south).
Fig. 2. The Ordos rift system, NW China, Red symbols are metamorphic basement foliations (commonly in ~1.9 Ga gneisses), black lines are active faults. White circles are major historical earthquakes – the larger ones have dates and M estimates
Fig. 3. Model set-up for West Greenland to study Proterozoic mantle relict subduction scar and its control on inheritance in the Mesozoic – Cenozoic Labrador Sea, Baffin Bay ocean basins (Peace et al 2017, Heron et al 2019)
Fig. 4. 2D model for lithospheric extension using the ASPECT numerical code

Methodology

Desk studies – Initially a comprehensive literature review will be undertaken. This will be followed by desktop GIS mapping project of the main basement structural trends and the identification of active fault structures using available remote sensed data for target areas. You will also compile and analyse GNSS data and InSAR results for active extensional basins to visualise the strain distribution across structures in a variety of orientations in regional and local stress field reference frames.

Fieldwork -You will undertake field studies on specific off-axial trend basinal faults where inheritance is suspected with comparative studies on faults with axis parallel trends that are far-field stress controlled. Contrasting basin margins with basement fabric parallel and cross-cutting structures will also be studied. Following the desk study of the tectonic geomorphology of the identified study area, active faults scarps will be mapped and kinematic data collected in the field. Detailed structural analysis of the brittle and ductile structures (faults and fabrics) in outcrops will be recorded with geometry, kinematic indicators and associated mineralisation. Relative timing of deformation phases will be established to build a chronology of events. Samples will be collected to constrain deformation conditions and if appropriate date the structural events. Digital mapping and 3D model construction using UAV/Lidar will be performed on appropriate structures.

Seismic interpretation – 2D seismic reflection data will be interpreted in Petrel with the objective of mapping in detail the transition from basement-influenced to far-field influenced deformation. From these datasets basin-wide thickness variation maps and age constraints on the sedimentary fills will be used to determine the location and geometry of syn-rift faults in extensional basins and margins to determine inheritance patterns and evolution. In this part of the project you will work with Dr Tom Phillips, a Leverhulme Early Career fellow, who has expertise in recognising and understanding fault reactivation and inheritance in seismic datasets.

Modeling -Using Coulomb 3.0 to model fault interactions and lithosphere-scale models using the Aspect code to simulate crustal and mantle anisotropies, you will test conceptual models that aim to show how strain localisation, stress shadowing and rheological variations control the structural reorganisation in extensional basins. Here you will work with Dr Phil Heron, a Marie Sklodkowska Curie research fellow who models geodynamic processes using the Aspect software. This study will build on existing models that have been set-up for West Greenland (Heron et al 2018 and In prep).

Project Timeline

Year 1

Year 1 Literature review, desktop study, planning for first fieldwork and introduction to modelling, first fieldwork campaign, Iapetus DTP training, attendance at national conference Analysis of results, 9 month confirmation report and interview and preparation of first results for publication.

Year 2

Year 2. Seismic interpretation, sample preparation and laboratory study, first modelling study and 2nd fieldwork campaign. Iapetus DTP training, attendance at national conference. Analysis of results, 21 month progression report and interview and preparation of further results for publication.

Year 3

Year 3-3.5. Further seismic interpretation, sample preparation and laboratory study, further modelling study. Iapetus DTP training, Attendance at international conference. Analysis and synthesis of results,

Year 3.5

33 month progression report and interview and preparation of further results for publication. Thesis preparation and submission.

Training
& Skills

You will become part of the Structural Research Group at Durham, an established research unit of 15 academic, postdoctoral and postgraduate structural geologists. Your project will also be linked into the Geodynanics research group. Your project will also overlap with the Leverhulme Early Career project on structural inheritance being carried out by Dr Tom Phillips. (See Phillips et al 2016)
Training will be provided in aspects of structural geology, including manipulation and analysis of multiple datasets using a GIS platform, mapping and interpretation of active fault zones, interpretation of geodetic datasets, identification of fault rocks and kinematic analysis, sample collection to support laboratory-based analysis including optical analysis and possible geochronology carbonate Durham and Keyworth (if appropriate). Training in desktop seismic interpretation using Petrel will be provided and in use of numerical modelling tools (Coulomb and Aspect). Training GNSS and InSAR processing will be provided by Newcastle University).
You will be expected to present posters and talks at conferences including at least one international conference.
You will also become part of the IAPETUS DTP which offers a multidisciplinary package of training focused around meeting the specific needs and requirements of each student who thus benefit from the combined strengths and expertise that is available across our partner organisations.

References & further reading

R. J. Walters, L. C. Gregory, L. N. J. Wedmore, T. J. Craig, K. McCaffrey, M. Wilkinson, J. Chen, Z. Lie, J. R. Elliott, H. Goodall, F. Iezzi, F. Liviog, A. M. Michetti, G. Roberts, E. Vittori. 2018. Dual control of fault intersections on stop-start rupture in the 2016 Central Italy seismic sequence. Earth Planetary Science Letters, 500, 1-14.

Peace, A.L., McCaffrey, K. Imber, J. van Hunen, J. Hobbs, R. Wilson. R. 2017. The role of pre-existing structures during rifting, continental breakup and transform system development, offshore West Greenland. Basin Research 30, 373-394

Phillips, T.B. Jackson, C.A.L. Bell, R.E. Duffy, O.B. Fossen, H. 2016. Reactivation of intrabasement structures during rifting: A case study from offshore southern Norway. Journal of Structural Geology 91, 54-73.

Heron, P.J., Peace, A.L., McCaffrey, K.J.W., Welford, J.K., Wilson, R., van Hunen, J. and Pysklywec, R.N., 2019. Segmentation of rifts through structural inheritance: Creation of the Davis Strait. Tectonics, 38, 2411-2430.

Foulger, G.R., Dore, T., Emeleus, C.H., Franke, D., Geoffroy, L., Gernigon, L. Hey, R. Holdsworth, R.E., Hole, M., Haskuldsson, A., Julian, B., Kusznir, N., Martinez, F., McCaffrey, K.J.W., Natland, J.H., Peace, A., Kenni Petersen, K.D., Schiffer, C., Stephenson, R., Stoker, M. 2019. A continental Greenland-Iceland-Faroe Ridge, Earth-Science Reviews, https://doi.org/10.1016/ j.earscirev.2019.102926,

Phillips, T.B. & McCaffrey, K.J.W. 2019. Terrane boundary reactivation, barriers to lateral fault propagation and reactivated fabrics – Rifting across the Median Batholith Zone, Great South Basin, New Zealand. Tectonics.

Schiffer, C. Dore, A.G. Foulger, G. Franke, D., Geoffroy, L., Gernigon, L. Holdsworth, R.E., Kusznir, N. Lundin, E. McCaffrey, K.J.W., Peace, A., Petersen, K.D. Phillips, T.B. Stephenson, R. Stoker, M. Welford. J.K. 2019. Structural inheritance in the North Atlantic. Earth Science Reviews, Available Online

Further Information

For further information contact Ken McCaffrey at k.j.w.mccaffrey@durham.ac.uk

Twitter handle @kmccaffrey

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