Lineation

Abstract: Summary All available ship and aeroplane tracks across the Indian Ocean were searched for identifiable magnetic anomalies and transform faults, and hence the age and direction of motion at the time of formation of about two-thirds of the floor of the ocean established. The magnetic lineations show that India moved away from Antarctica at about 18 cm/y for 20 My in the Early Tertiary. This rapid motion ceased in the Eocene and was followed by a period in which little or no spreading took place west of the Ninety East Ridge. Australia separated from Antarctica during this period. The present spreading episode began about 36 My ago. This detailed study has permitted instantaneous poles of rotation to be obtained, and has established that Africa is now moving northward at 2cm/y relative to Antarctica in the South West Indian Ocean. The evolution of the triple junction between the South East, South West and Central Indian Ridges is clearly reflected in the topography and magnetic lineations. The depth of parts of the ocean formed since the Late Cretaceous increases with age in the manner expected from the temperature structure of a cooling plate, and supports the evolution determined from the magnetic lineations in a most remarkable way. Heat flow observations are more scattered but also consistent with the same thermal model. The proposed evolution agrees with the distribution of known continental fragments and with the Late Cretaceous palaeomagnetic poles from surrounding continents and one obtained from the shape of the magnetic lineations south of India. It is, however, not yet clear how to reconstruct Gondwanaland from the Late Cretaceous reconstructions.

Abstract: FUNDAMENTALS. Nature of Structural Geology. Kinematic Analysis. Dynamic Analysis. Deformation Mechanisms and Microstructures. STRUCTURES. Joints and Shear Fractures. Faults. Folds. Cleavage, Foliation, and Lineation. Shear Zones and Progressive Deformation. Plate Tectonics. Concluding Thoughts. DESCRIPTIVE ANALYSIS. References. Indexes.

Abstract: The largest tract of ultrahigh-pressure rocks, the Dabie-Hong'an area of China, was exhumed from 125 km depth by a combination of normal-sense shear from beneath the hanging wall Sino-Korean craton, southeastward thrusting onto the footwall Yangtze craton, and orogen-parallel eastward extrusion. Prior to exhumation the UHP slab extended into the mantle a downdip distance of 125–200 km at its eastern end, whereas it was subducted perhaps only 20–30 km at its far western end ∼200 km away. Structural reconstructions imply that the slab was >10 km thick. U/Pb zircon and 40Ar/39Ar geochronology indicate that exhumation up to crustal depths occurred diachronously between 240 and ∼225–210 Ma, reflecting a vertical exhumation rate of >2 mm/yr. The upper boundary of the slab is the Huwan shear zone, a normal-sense detachment that reactivated the plate suture. The lower boundary is represented by the Lower Yangtze fold-thrust belt. NW-trending stretching lineations, NE-vergent, WNW-ESE trending folds, dominant top-NW shear, and conjugate, but overall asymmetric, shear band fabrics, document that exhumation was accomplished by updip and orogen-parallel extrusion accompanied by layer-parallel thinning. The orientation and shape of the folds, and a change from SE to SW flow directions, imply that the slab rotated clockwise about a western pivot during exhumation; this rotation was likely caused by the eastward increasing depth of subduction mentioned above, combined with a possible marginal basin and a weak eastern plate boundary. Exhumation of the slab produced considerable shortening in the Lower Yangtze fold-thrust belt, perhaps producing the foreland orocline.

Abstract: Part 1 Fundamentals: introduction primary and nontectonic structures force and stress deformation and strain rheology. Part 2 Brittle structures: brittle deformation processes joints and veins faults and faulting. Ductile structures: ductile deformation processes and microstructures folds and folding foliations and lineations ductile shear zones, textures and transportation deformation, metamorphism and time - an essay. Part 4 Tectonics and regional deformation: whole Earth structure and plate tectonics extensional tectonics convergence and collision fold-thrust belts - an essay strike-slip tectonics perspectives of regional geology. Appendix: geologic time scale (DNAG).