Characterization of brittle deformation in geological composite materials (felsic mylonites and layered sedimentary rocks) and estimation of paleostresses

Rheology of polyphase rocks (e.g., polymineralic rocks and layered rock terrains) has become one of the most active frontiers of Earth sciences during the last 25 years. However, the deformation processes of geological composite materials are complicated because competent and incompetent components tend to deform in different manners under the same temperature and pressure conditions. Brittle deformation in competent components is generally controlled by ductile deformation of the incompetent components in which they embedded. Therefore, brittle deformation and resultant microstructures in competent components bear significant information for interpreting the mechanical properties and deformation behavior of their surrounding incompetent materials, as well as information for the stress magnitude. Based on measurements and analysis on characterization of the brittle deformation in felsic gneisses, mylonites and interlayered sedimentary rocks, the present study aims to find out proper piezometers to estimate the differential stresses prevailing in geological composite materials and to explore their applications on calibrating the paleostresses for the rocks deformed in the Earth's crust. This dissertation consists of five chapters. Chapter 1 introduces basic concepts in continuum mechanics and the mechanical models for describing the deformation processes of geological composite materials. Under this framework, each aspect related to deformation of geological composite materials, measurement methods of stress, strain and strain rate, various deformation mechanisms and corresponding structures, fracture mechanics, flow law, and the existing mechanical models, are introduced in detail with critical review. Chapter 2 deals with tourmaline microboudin data measured from felsic gneiss and mylonite samples from the Gaoligong and Chongshan shear zones in Yunnan, Southwest China. After critically reviewing the piezometer built on the mid-point fracturing model, a new microboudinage piezometer is developed based on interfacial shear-induced near-end fracturing stress transfer model. The new microboudinage piezometer has a simple form: τ =0.25C/(L/W), where τ is the shear flow stress of the ductile matrix (e.g., polycrystalline quartz), C is the tensile fracture strength of a columnar mineral (e.g., tourmaline), and L/W is the length/width ratio of the microboudin. Taking tourmaline tensile strength C=145 MPa as estimated, the application of the piezometer on the samples from the Gaoligong shear zones yields the shear flow stresses of 21- 61 MPa for felsic gneisses and mylonites. The earlier formed microboudins have consistently larger mean length/width ratios than the later formed ones, suggesting that the rocks recorded a tectonic history of continuous hardening prior to the cessation of quartz plasticity. Meanwhile, application of the piezometer on samples from the Chongshan shear zone, combined with microstructural observations, independent estimates of deformation temperature, accurate geochronological ages of various isotopic systems and regional geophysical data, provides constraints on strain rates, differential stresses and their correlations with temperature and corresponding depth for the polycrystalline quartz matrix. The differential stresses calibrated by this flow law are bracketed by the lower and upper bounds obtained by Luan and Paterson (1992) and Gleason and Tullis (1995), respectively, and are very close to the geometrical mean of these bounds. Chapter 3 aims to expand the applicability of the microboudinage piezometer established based on tourmaline to other minerals. Feldspar, as one of the most abundant minerals in the crust, commonly exhibits brittle behavior in plastically deformed matrix under the greenschist-facies conditions, which makes it an excellent selection for the expansion of the piezometer. Different from the tourmaline microboudins with regular shape, the feldspar microboudins are treated as a circular-truncated-conical fibers in the mechanical analysis. A feldspar microboudinage piezometer is established with the estimated tensile strength of feldspar. We apply the piezometer on granitic mylonite samples, in which feldspar microboudins embedded in the ductile matrix of quartz, from the Red River, Chongshan and Gaoligong shear zones in Yunnan, Southwest China. The differential stresses estimated from the later formed feldspar microboudins are larger than the earlier formed ones, depicting an evolution during exhumation of the mylonite upwards the brittle-ductile transition. The results from the feldspar microboudinage piezometer and the quartz recrystallized grain-size piezometer are largely consistent within the range of uncertainties. The microboudinage piezometer, combined with results of finite strain measurements, deformation temperature estimates, and thermochronological data, offers valuable information about rheological properties of deformed felsic rocks. Chapter 4 reports the characterization of two types of orthogonal joints (ladder-pattern network and fracture grid-lock system) in layered sedimentary rocks in Sainte-Anne-des-Monts, (Quebec, Canada), Ausable Chasm (NY, USA) and Beauharnois (Quebec, Canada), and proposes a new interpretation for the formation of the grid-lock type orthogonal joints. Joint spacing (s)-bed thickness (t) relationships of the ladder-pattern systematic and cross joints in the sandstone beds of the Tourelle Formation in the region of Sainte-Anne-des-Monts are investigated. Based on analysis with the near-end fracturing stress transfer model, a power law: s=mt1-1/k is proposed for the s-t relationship, where m equals half the ratio of layer tensile strength to interface shear stress, and k is the Weibull modulus. With the m value, the shear stress prevailing in shale can be calculated if the tensile strength of sandstone has been experimentally determined. The m value is significantly larger for cross joints than systematic joints, indicating that the interfacial shear stress normal to systematic joints was higher than the interfacial shear stress normal to cross joints, by a mean factor of 3.1, during their formations. Based on the anatomic investigation on the grid-lock type orthogonal joints in the Potsdam sandstone of Cambrian age at Ausable Chasm (NY, USA) and Beauharnois (Quebec, Canada), a new interpretation is proposed that the vertical orthogonal joints may result from the auxetic effects of quartz-rich sandstone in the absence of local or regional stress rotation. In the terrains of flat-lying sandstone with widely spaced and narrowly spaced vertical, orthogonal joints, both σ2* and σ3* are tensile and perpendicular to widely spaced and narrowly spaced sets, respectively, whereas the vertical stress (overburden pressure) is always compressive (σ1), at the time of fracturing. The aspect-ratio of sandstone blocks bounded by orthogonal joints is correlated with the value of negative Poisson's ratio of the sandstone. Chapter 5 is a general discussion linking results and interpretations of former chapters and finally Chapter 6 briefly outlines the main conclusions of the present study and lists the work that should be conducted in the future.