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Title: Lithosphere extension at magma-poor rifted margins : extensional fault contribution, geometric evolution and orogenic reactivation
Author: Gómez Romeu, J.
ISNI:       0000 0004 7970 515X
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2019
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Magma-poor rifted margins are formed by brittle fault deformation within the upper lithosphere with distributed pure-shear deformation below. In this thesis, I investigate extensional faulting at magma-poor rifted margins and how these faults reactivate during orogeny. An important question is how does the amount of fault extension compares with that of the continental crust and lithosphere at the scale of a whole conjugate pair of rifted margins. To investigate this, I use the Iberia-Newfoundland conjugate margins example to determine; (i) continental crust extension using gravity anomaly inversion, (ii) continental lithosphere extension using subsidence analysis and (iii) extension from observed fault heave summation. Fault population analysis indicates that up to 35% of fault extension may be beneath seismic resolution and, when this is included, extension by brittle faulting is comparable with that measured for continental crust and lithosphere extension which is approximately 181 km. Uncertainties in fault heave summation arise due to ambiguities in fault geometry interpretation and polyphase faulting. The difficulty of interpreting present-day fault geometries is clearer when the geometric evolution of these structures is considered particularly within distal magmapoor rifted margins. To investigate this problem, I use; (i) first-order seismic observations of the distal western Iberia margin and (ii) a lithosphere deformation model (RIFTER) that includes the flexural isostatic response to both extensional and compressional tectonics producing isostatically compensated as well as balanced lithosphere cross-sections. Modelling results show that to understand extensional faulting within distal rifted margins it is important to consider the flexural isostatic rotation of faults. Flexural isostatic rotation produces present-day sub-horizontal inactive faults; (i) beneath the hyperthinned continental crust extended by low offset faults and (ii) at the sea-bed within the exhumed mantle domain due to a rolling-hinge fault overlie by allochthon blocks. Modelling shows that a distal ridge between the transition of the hyperthinned and exhumed mantle domains must be formed by outof-sequence faulting. Understanding extensional faulting at magma-poor rifted margins is important not only to better comprehend lithosphere deformation during extensional processes but also to better understand orogen formation. To examine this, the Western Pyrenees are used as a case-study where evidence exists for a reactivated hyper-extended basin that preserves the earlier rift history. The RIFTER model is used to investigate the formation of this orogen which can only be understood by including both the extensional and compressional histories. The compressional stage is characterized by the inversion of the hyper-extended rift system reactivating extensional structures followed by crustal shortening of the proximal rift domain. Modelling results show how rift-related faults may be reactivated and incorporated into the present-day Western Pyrenees architecture.
Supervisor: Kusznir, Nick Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral