Maxwell, Angus Sean; (1990) Inversion tectonics in the Iberian ranges of Castille-La Mancha, Spain. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The Iberian Ranges of Central Spain represent a contracted complex graben system, formed by reactivation of Hercynian lineaments during the same Mesozoic extension and subsidence which formed the Tethyan ocean to the east. In the thesis, part of the range, between the Serrania de Cuenca and Sierra de Albarracin in Castille, is analysed to investigate basin inversion during the Tertiary and to prove that inversion is a three dimensional phenomenon related to the resolution and partitioning of compressive stresses about intrabasinal heterogeneities. An analysis of the stratigraphy was carried out to reconstruct the basin structure immediately prior to the onset of deformation. It was found that the area is separated into two graben systems by NW-SE horst which concentrated most of the cover (thin skin) thrusting and folding during the Tertiary. Eighteen serial sections drawn at 2.5km intervals were constructed and partially restored to investigate cover strain and to assess the distribution of decollement strata and their effect on deformation. Thin, poorly gypsiferous regions of the decollement promote imbricate thrusting while the converse promotes detachment folding. The evolution of deformation above a buried fault scarp is investigated in a region where Tertiary syn-tectonic molassic sediments are preserved. Two main deformation events are recognised. It is found that the fault scarp strongly effects the distribudon of stresses leading to diapirism in the decollement strata which accommodates the buttressing effect of the scarp and the promodon of cover failure over the scarp where differential stresses are highest. Diapiric walls and minor scarps perpendicular to the strike of the basement fault nucleate zones of strike-slip faulting in cover whose style is dependant on the thickness and gypsum content of the decollement. Shear indicators in these strata are well developed and allow the sense of displacement of cover relative to basement to be determined. In the Serrania de Cuenca their analysis leads to a conceptual model for basementicover interaction and the initiation of décollement. Gravity tectonics is preferred but differential shortening during pinching of the half grabens and slight inversion of the basin bounding faults also occurs. The deformation of basement is analysed using the Sierra de Albarracin as a case study and Alpine structural geology is discriminated from Hercynian. Strike-slip deformation plays an important role in the evolution of basement folds and fractures and severely limits the application of section balancing, however, a partly balanced crustal model is proposed based on a rheologically layered crust. Brittle microstructural analysis leads to the inference of approximate σ1 trajectories and reveals that one principal stress was always close to vertical. This allows the resolution of stress on fault surfaces to be analysed using simple graphical techniques such as ROMSA and YRPROG. The shapes and orientations of stress tensors from sample areas throughout the study area reveals that the pre-existing basin structure has a strong modifying effect on the regional stresses and may cause a variety of structures, normally attributed to different tectonic regimes, to develop in response to the same regional stress event.

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