Magnetic Memory of Rocks: the Kazakhstan Orocline and Climatic Record of the Indian Monsoon.

Abstract

The Kazakhstan orocline, a pair of concentric horseshoe-shaped volcanic arcs of Devonian (external) and late Paleozoic (internal) age, is thought to have formed during the amalgamation of Eurasia. Paleomagnetic investigations of several volcanic complexes ranging in age from Silurian to Permian are described in Chapters 2, 3 and 4 of this thesis. These studies have resulted in the construction of a tectonic model for the formation of the Kazakhstan orocline. Our results suggest that in the Middle Devonian, a nearly straight, northwest-southeast trending volcanic arc delineated the northeastern margin of the Kazakhstania landmass. In the Late Devonian, an initial collision with Tarim pinned Kazakhstania’s southern corner, while dextral shear motion and the considerable clockwise rotation of Siberia dragged its northern end. Relative convergence between Siberia and Tarim caused initial buckling of the Kazakhstania continent trapped between them, subdividing the volcanic arc into three (southwestern, middle, and northeastern) segments. Continued subduction under the established limbs of the orocline, with an estimated outward-directed subduction velocity of ~6mm/yr, gradually led to the closure of the intervening Junggar-Balkhash Ocean and tightening of the orocline. By the Late Permian, the Junggar-Balkhash Ocean no longer existed and the Kazakhstan orocline had obtained its present-day strongly curved shape.

The ratio of two pedogenic iron oxides, goethite and hematite, has been demonstrated to be a good proxy for precipitation in soil-forming (terrestrial) environments. A similar interpretation of the mineral ratio has been increasingly applied to studies of marine sediments, in which variation in the goethite to hematite ratio is thought to reflect variation in the precipitation regime at the source area of the sediments. The rock magnetic study of Bengal Fan sediments described in Chapter 5 of this thesis suggests that in some intervals of the studied sedimentary section, variation in the ratio reflects a change in the degree of diagenetic alteration of the initial detrital assemblage rather than a climatic signal. Therefore, when assigning climatic interpretation to changes in the relative abundance of goethite and hematite in marine sediments, the possibility of diagenetic modification should be evaluated.