Mechanisms of isostatic compensation in areas of lithospheric extension : examples from the Aegean.

Abstract

The results of previous studies in the Aegean have greatly influenced our understanding of continental extensional tectonics. However, although lithospheric rheology is known to exert a significant control on tectonic style, it had not been assessed across the province. The effective elastic thickness, Te, provides a good measure of lithospheric rheology. Through analyses of the isostatic mechanism, the spatial variability in rheology across the Aegean is examined. A crucial step in the process is to obtain a good gravity dataset. This study compiles new and existing gravity data across both onshore and offshore regions to produce the best gravity dataset currently available for the Aegean region. Short wavelength components of the Bouguer gravity (less than ~80 km) are seen to correlate with surface geological features. Longer wavelength components are coherent with topographic features, which may be interpreted in terms of the lithosphere's isostatic response to loading. The amplitudes of Bouguer anomaly minima across the sedimentary basins of central Greece are used to estimate the depth to basement. A model is derived for the gravitational signature of buried sedimentary strata whose porosity decreases exponentially with burial depth. The model results are presented in graphical form which may have general application. The resulting depth estimates for the eastern Gulf of Corinth (2.7 to 3.7 km) are used along with a seismic depth conversion and erosional marine terrace geometries to model the generation of topography across an active rift segment. These boundary element elastic models estimate the local rift-flank Te as ~6 km. The primary objective of this thesis - to assess the spatial variability in lithospheric rheology - is achieved through the analysis of gravity coherence. Windowed (short time) Fourier transforms are used, and the observed coherence is modelled with a thin elastic plate to estimate Te. Across the Aegean extensional province, Te is between 10 and 20 km, and values of between 8 and 10 km are observed across the extending Rhodope metamorphic core complex. The broad pattern is similar to that in measured surface heat flow, indicating that geothermal gradient is an important factor in controlling lithospheric rheology. This thesis presents the first comparison between the conventional Fourier transform method and the wavelet transform method in a-coherence analysis. Due to the localisation properties of the continuous wavelet transform, an improved spatial resolution of the variability in coherence is observed. The weakest lithosphere across the Rhodope complex is observed to occur in a narrow zone (~ 100 km wide) trending parallel to normal faulting, where a Te of less than 5 km is predicted. These features are not resolved with the Fourier method. The results of this thesis are then compared to observations across the Basin and Range and the Tibetan Plateau. It is argued that rheological similarities between these collapsing orogens account for similarities in the tectonic style. A scheme is subsequently suggested for the evolution of the Aegean province since approximately 20 Ma