Most of previous models suggest that the Central Asia Orogenic Belt grew southward in the Phanerozoic. However, in the Bayanhongor region in west-central Mongolia, volcanic arc, accretionary prism, ophiolite, and passive margin complexes accreted northeastward away from the Baydrag micro-continent, and hence the region constitutes the southwestern part of a crustal-scale syntaxis close to the west. The syntaxis should be original, because presumably reorientation due to strike-slip faulting can be ignored. It is reconfirmed that the Baydrag eventually collided with another micro-continent (the Hangai) to the northeast. A thick sedimentary basin developed along the southern passive margin of the Hangai micro-continent. This region is also characterized by an exhumed metamorphosed accretionary complex and a passive margin complex, which are both bounded by detachment faults as well as basal reverse faults which formed simultaneously as extrusion wedges. This part of the Central Asia Orogenic Belt lacks exhumed crystalline rocks as observed in the Himalayas and other major collisional orogenic belts. In addition, we identified two phases of deformation, which occurred at each phase of zonal accretion as D1 through Cambrian and Devonian, and a synchronous phase of final micro-continental collision of Devonian as D2. The pre-collisional ocean was wide enough to be characterized by a mid-ocean ridge and ocean islands. Two different structural trends of D1 and D2 are observed in accretionary complexes formed to the southwest of the late Cambrian mid-ocean ridge. That

is, the relative plate motions on both sides of the mid-ocean ridge were different. Accretionary complexes and passive margin sediments to the northeast of the mid-ocean ridge also experienced two periods of deformation but show the same structural trend. Unmetamorphosed cover sediments on the accretionary prism and on the Hangai micro-continent experienced only the D2 event due to microcontinental collision. These unmetamorphosed sediments form the hanging walls of the detachment faults. Moreover, they were at least partly derived from an active volcanic arc formed at the margin of the Baydrag micro-continent.

Emphasis in this book has been placed on principles, methods, and technique. The structure of specific areas is not discussed, except for the purpose of illustrating principles. The writer has intentionally refrained from a treatment of the more speculative phases of geotectonics because he believes that such subjects can be intelligently studied only by geologists with a broad background in many fields of geology.

The indecisiveness of some of the criteria used in structural geology may cause dismay to some students. It is better, however, for the reader to realize the difficulties of structural geology and to understand at the outset that such a subject can never be treated with mathematical precision. The structural geologist must be proficient in weighing and balancing the evidence.

Much new information on structural geology has been published since the last edition of this book. Obviously the more significant new data should be introduced into an elementary text. But in order to keep the length within reasonable bounds it would be necessary to eliminate some of the older material. This is not easy to do. Some might be eliminated completely and some might be shortened, but presumably the original text was as concise as possible.

Standing on a rocky terrain, one may occasionally notice beautiful structures in rocks. The variety of geometrical shapes of the structures poses great inquisitiveness to an onlooker. What are these structures, how are these formed and what does their occurrence indicate are some of the questions that haunt his/her mind. Answers to these questions are hidden in a discipline of geology called structural geology, which encompasses the study of all aspects of the deformation structures such as their geometry, field relations, geographic distribution, genesis and related aspects. Since the structures in the context of structural geology are formed by deformation, these are also called deformation structures or tectonic structures. They occur on scales ranging from tens of kilometres to those that can be seen only under a microscope. Faults, folds and joints are some common examples of structures. Structural geology is closely related to another discipline of geology called tectonics, which includes deformation on a much larger scale, i.e. from regional to lithospheric. Structural geology and tectonics commonly go together and even overlap when studied on the same scale. Structures are sometimes associated with economic minerals and hydrocarbons. The type and orientation of structures are important in civil engineering work such as dams, tunnels and bridges. Structural geology has implications for academic, economic, societal and environmental aspects.

This chapter presents a panoramic view of the wonderful world of structural geology so that the reader systematically starts gearing up for an in-depth study of this discipline.