Contractional tectonics : the Himalayan orogen and Perdido fold-thrust belt

Abstract

This dissertation focuses on the kinematic evolution of two major categories of contractional tectonics: collisional orogenic belts and toe structures of passive margins, which are characterized by fold-thrust belts that are hundreds of kilometer-scale and tens of kilometer-scale, respectively. The Himalayan orogen is an excellent example of collisional orogenic belts along convergent plate boundaries. It is commonly structurally defined as three stacked units separated by two fault systems: the Main Central thrust (MCT) and South Tibet detachment (STD). The development and emplacement of the middle unit, the Himalayan crystalline core, has long been debated within the extrusion framework, a process that involves exhumation of the crystalline core to the surface. Recently, the debate has expanded to two end-member regimes: extrusion versus underplating. To determine how the crystalline core evolved, an integrated investigation was conducted, involving structural mapping, microstructural, quartz c-axis fabric, and geochronological analyses across the northern margins of two frontal klippen in the Nepal Himalaya: the Dadeldhura klippe and Kathmandu Nappe. The work suggests that the STD occurs and merges with the MCT in these two klippen. The merging of the MCT and STD requires that the crystalline core was emplaced at depth via tectonic wedging kinematics, incompatible with extrusion models. By synthesizing the Himalayan evolution history from the development and emplacement of the crystalline core to ongoing deformation, a reconstruction shows that Himalayan mountain-building processes are dominated by underplating. The Perdido fold-thrust belt is a gravity-driven toe structure in the passive margin of the Gulf of Mexico. Structural models for the Perdido fold-thrust belt are highly dependent upon the interpretation of seismic images, which commonly display wipe-out zones associated with faults. Fault interpretations in seismic wipe-out zones are commonly non-unique. Trishear, a quantitative fault-propagation folding model, was applied to an anticlinal structure in the Perdido fold-thrust belt and reproduced the fold geometry. Three dimensional kinematic evolution was reconstructed by interpolating the best-fit models of the serial cross sections. The trishear modeling indicates that the Perdido fold-thrust belt underwent ~7.5-12.5 km shortening, which could balance the landward extension of the passive margin during the same period.