Subsidence, deformation, thermal and mechanical evolution of the Mesozoic South Alpine rifted margin: an analogue for Atlantic-type margins
Giovanni Bertotti, 2001. "Subsidence, deformation, thermal and mechanical evolution of the Mesozoic South Alpine rifted margin: an analogue for Atlantic-type margins", Non-Volcanic Rifting of Continental Margins: A Comparison of Evidence from Land and Sea, R. C. L. Wilson, R. B. Whitmarsh, B. Taylor, N. Froitzheim
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The geological record of the currently exposed South Alpine transect of the Mesozoic passive continental margin provides information on the evolution of Atlantic- type margins. Before the onset of rifting, in Mid-Triassic to Carnian time, strong subsidence affected the central parts of the Southern Alps, in the Lombardian basin. No major fault is documented for this time span. Thermal conditions were strongly perturbed. Continental rifting began in Norian times, and until the beginning of the Liassic period was characterized by overall subsidence of the Lombardian basin with rates up to 200 mm ka-1. The strong subsidence is a result of continuing extension and of generalized crustal cooling. Subsidence rates were still important in Liassic times, although lower than in Late Triassic time. At the end of the Liassic period, the site of extension shifted towards the west, where crustal break-up eventually took place in Mid-Jurassic times. Previously poorly documented features such as the very strong subsidence in the initial rifting stages, the changing geometry and mechanics of normal faults are here associated with the thermal interactions between pre-existing thermal anomalies and rifting.
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Non-volcanic continental margins may form up to 30% all present-day passive margins, and remnants of them are preserved in mountain belts. The papers in this volume demonstrate the benefits of integrating offshore and onshore studies, and illustrate the range of information obtained at different scales when comparing evidence from land and sea. Data sets collected across a range of spatial scales are evaluated: thin sections, cores, outcrops, seismic reflection profiles, and other geophysical data. The outcrop scale is crucial because it enables the spatial gulf to be bridged between DSDP and ODP cores and marine seismic data. There is also the problem that basins on land and beneath the sea inevitably have had different post-rift histories resulting in their contrasting present-day elevation. In mountain belts, portions of continental margins and oceanic crust are superbly exposed, but dismembered by subsequent compressional tectonics. Off present-day passive margins, extensional features have only been slightly deformed, if at all, by compressional movements, but are buried beneath significant thicknesses of post-rift sediments and so can only be sampled by ocean drilling at a small number of points.
The first paper reviews the synergies that have occurred between investigations of the eastern North Atlantic non-volcanic margins and remnants of similar Mesozoic margins preserved in the Alps, and some later papers return to this theme. However, papers describing margins from other parts of the world show that it may be premature to use models based on the Atlantic and the Alps as the paradigm for all non-volcanic margins. The following 25 papers in the book are grouped under the following headings: (1) Margin overviews; (2) Exhumed crust and mantle; (3) Tectonics and stratigraphy; (4)Numerical models of extension and magmatism.