Late Orogenic Extension In Mountain Belts

Author : Rodrigo Dias Lima
Publisher :
ISBN 13 :
Total Pages : 424 pages
Book Rating : 4.:/5 (15 download)

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Download or read book From Mountain Belts to Continental Margins written by Rodrigo Dias Lima and published by . This book was released on 2018 with total page 424 pages. Available in PDF, EPUB and Kindle. Book excerpt: Continents typically thin and break apart on sites that underwent a previous orogenic history. Geodynamic and conceptual models explain the causal link between orogenesis and extension via the influence of inherited hot geotherms and reactivation of weak structures. In many regions, however, extension post-dates orogeny by 10’s to 100’s million of years implying that the orogen’s thermal structure has been long re-equilibrated, and deformation is not everywhere accommodated by kinematic reactivation. In contrast, preexisting basement fabrics often influence the locus and propagation of deformation, suggesting a mechanical control. It has remained unexplored how preexisting orogenic fabrics affect the crustal rheology that governs extension. This dissertation documents such rheological inheritance from orogenesis, and explores its mechanical consequences on continental extension. This study is focused on the Death Valley region of the North American Basin and Range province, a hallmark example of continental extension preceded by orogenesis. The ranges that border Death Valley expose a basement complex that preserves the geologic record of the Mesozoic Sevier-Laramide deep crustal processes and later overprinting and exhumation by Basin-and-Range extension in the Cenozoic. Here, field observations, microstructural analysis (including EBSD), thermodynamic modeling of metamorphic reaction textures, and geochronology all document rheological inheritance in the region. The thermal evolution of the Mesozoic orogeny is recorded by zircon U-Pb geochronology (Chapter 1); magmatic age modes reveal that crustal heating in the Cordillera interior was sustained for ca. 36 m.y. (Late Cretaceous through Eocene). Such heating reached temperatures in excess to produce crustal melting, enabling deep crustal flow that affected the dynamic of the orogen (e.g., stability of surface topography). Chapter 2 explores how partial melting produced fine-grained fabrics via melt-rock reactions that favored textural softening along shear zones that accommodated syn-convergent extension. Importantly, the orogenic thermal evolution resulted in petrological transformations that included the segregation of melt products into compositional domains, resulting in deep crustal rheological heterogeneities that influenced softening mechanisms during later extension. The mechanical consequences of the inheritance of rheological heterogeneities on continental extension are then explored in thermomechanical experiments (Chapter 3). In particular, the experiments do not include weakening effects in order to evaluate the effect of the mid-crustal heterogeneities on localization. The results show that at the crustal-scale, the inherited heterogeneities favor unstable extensional regime and boudinage, and at the meso-scale nucleate localization by the growth of strain instabilities within the mid-to-lower crust. The linkage of the mid-crustal shear zone with upper crustal faulting results in detachments systems that can either exhume the middle crust into core complexes or concentrate thinning into necking zones, depending on the flowing behavior of the lower crust. The geological observations and thermomechanical experiment detailed in this dissertation led to a conceptual model wherein the heterogeneous distribution of peak, orogenic fabrics (including peak-metamorphic textures and segregation of melt products) impose a mechanical control, the rheological inheritance, on continental extension.