Joint Modelling of the Thermo-Tectonic Evolution in an Extensional Area

Carminati, Eugenio ¹; Cuffaro, Marco ³; Miglio, Edie ²; Doglioni, Carlo ¹; Ruffo, Paolo ⁴
(1) Dipartimento di Scienze della Terra, Sapienza Università di Roma, Rome, Italy. (2) MOX-Dipartimento di Matematica, Politecnico di Milano, Milan, Italy. (3) IGAG, CNR, Rome, Italy. (4) ENI E&P, San Donato Milanese, Italy.

Geometry, sedimentary facies distribution and thermal evolution of extensional basins are controlled by a number of factors and processes, acting at local, regional and global scales: amount and rate of thinning; latitude at which the basin develops; pre-rift thickness and heterogeneities of the lithosphere; amount of clastic sediment supply; number, spacing and dècollement depth of normal faults; sediment supply; sediment compaction; plate motions relative to the mantle.

Using natural examples, we show that, if a reasonable knowledge of the thermal parameters of both covers and basement is available, thermo-kinematic modeling can provide useful first-order estimates in frontier areas of heat flow and temperature evolution through time. We also show how compaction can control the geometry of sedimentary layers and syn-diagenesis strain accumulation. We show, for example, that early fracturing can occur by differential compaction in prograding carbonate platforms and, eventually in sedimentary layers onlapping fault escarpments. Such early fractures could control the flow of diagenetic fluids and eventually of hydrocarbons.

In resource assessment, global scale processes are normally neglected. It is disregarded that the bathymetry of rift zones is generally asymmetric, being the eastern flank in average slightly shallower (100-300 m) than the western one, indicating differential subsidence between the two limbs. Also, based on surface wave tomographic models, shear wave velocities in the upper mantle indicate a difference between the western and eastern flanks of an oceanic basin and related passive margins, thus indicating different temperatures, densities and melt percentage in the mantle. This is explained by the fact that, in mantle reference frames, a net "westward" rotation of the lithosphere relative to the mantle can be observed. This implies that upwelling of mantle beneath ocean ridges is not symmetric with respect to the ridge axis, but rather denser and undepleted mantle floors the western parts of rift zones, while lighter mantle floors the eastern parts.

We show, with ad hoc geological, geophysical and theoretical modeling, that plate motions with respect to the mantle can control regional subsidence (upon which local subsidence associated with active normal faults is superimposed) in extensional basins and subcrustal temperature and heat flow, thus contributing in the definition of the thermal history of sedimentary basins.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.