Thermo‐mechanical modeling of the Tyrrhenian Sea: Lithospheric necking and kinematics of rifting

Abstract

We present the results of quantitative forward modeling of the Sardinian rifted margin of the Tyrrhenian Sea. The purpose of this study is to investigate the thermomechanical structure that affects the thinning of the lithosphere across the margin. The role of lithospheric necking during basin formation, constrained by basement topography, Moho depth, and gravity anomalies, is modeled for different compensation models. Independent constraints are obtained from the analysis of the predicted thermal structure. A deep level of necking (25 km) is required to explain the observed crustal geometries and gravity anomaly signature. The model predicts spatial and temporal variations in rheology during extension with important implications for kinematics of lithospheric thinning. Prerift lithospheric conditions, strain rate, and temperature during extension appear to be the key controls on the style of lithospheric rifting and necking in the Tyrrhenian Sea. A forward model for basin stratigraphy is presented for the polyphase history of the Tyrrhenian rifting. This model provides quantitative estimates of time-space dependent crustal thinning and rates of extension. Research School of Sedimentary Geology. References Banda, E., and S. Cloetingh, Physical properties of the lithosphere, in The European Geotraverse, a Continent Revealed, edited Volcanism of the southern Tyrrhenian Sea and its geodynamics implication, J. Geophys. Res., 78, 5221-5232, 1973. Barrole, R., Seismic evidence of an earlier SPADINI ET AL.: MODELING OF THE TYRRHENLAN RWFING 643 Pliocene erosional surface in the deep part of the Tyrrhenian Sea, in Sedimentary Basins of Mediterranean Margin, edited by F.C. Wezel, pp. 127-14.5, Tecnoprint, Bologna, 1981. Bassi, G., Relative importance of strain rate and rheology for the mode of continental extension. Geophys. J. Int., in press, 1995. Bassi, G., C.E. Keen, and P. Potter, Contrasting styles of rifting: models and examples from the eastern Canadian margin, Tectonics, 12, 639-655, 1993. Bertotti, G., and M. ter Voorde, Thermal effects of normal faulting during rifted basin formation. 2: the Lugano-Val Grande normal t•ult and the role of the pre-existing thermal anomalies, Tectonophysics, 240, 145-157, 1994. Braun, J., and C. Beaumont, A physical explanation of the relation between flank uplifts and the breakup unconformity at rifted continental margins, Geology, 17, 760-764, 1989. Cassinis, R., R. Franciosi, and S. Scarascia, The structures of the Earth's crust in Italy: A preliminary typology based on seismic data.

Role of pre-rift rheology in kinematics of extensional basin formation: constraints from thermomechanical models of Mediterranean and intracratonic basins

“…For this young rifted basin, the available crustal structure and gravity data ( Figure 6) enable us to discriminate between the effects of different modes of extension. As demonstrated by Spadini et al (1995), models invoking local isostasy or a shallow level of necking generate predictions for basement topography which are incompatible with the data. As shown by Figure 6, the basement topography data have the resolving power to discriminate between various assumptions on the depth levels of necking, showing an apparently better fit for a deep level of necking of 25 km.…”

“…In subsequent work (e.g. van der Spadini et al, 1995) we have realized that these relationships might be more complex, particularly in view of the part played by necking with depths for intra-lithospheric detachment (van der .…”

“…For example, intraplate seismicity distributions do not show earthquake loci at the levels of the subcrustal mantle lithosphere, where rheological profiles predict considerable strength (Cloetingh and Banda, 1992). Furthermore, depending on the actual crustal configuration, more than one strong layer might be present in the lithosphere, yielding a situation with several stress guides in the lithosphere which is subsequently described by one parameter; the effective level of necking would then coincide with the weak layer in between the strong layers (Spadini et al, 1995). Braun and Beaumont (1989) Modelling approach…”

Subduction or delamination beneath the Apennines? Evidence from regional tomography

“…The existence of deep seismicity down to ∼ 400 km beneath Calabria and the Tyrrhenian Sea and of the active volcanism of the Eolian Arc suggests that the subduction processes are active here (e.g., Isacks and Molnar, 1971;Selvaggi and Chiarabba, 1995). It was first proposed by Malinverno and Ryan (1986) that the loop-shaped boundary, leading to back-arc extension in the Tyrrhenian sea (Spadini et al, 1995), was formed due to strongly curved subduction occurring in the narrow zone in front of the Apennines. However, such a strong bending of the subducting plate appears to be mechanically not plausible, as was shown by analogue experiments by Faccenna et al (1996).…”

Phantom plumes in Europe and the circum-Mediterranean region

“…Regarding the circum-Mediterranean area, the presence of a mantle plume has been suggested based on some geochemical characteristics of mafic magmas [e. Sea is completely different (Peccerillo, 2005;; 3) the composition of Italian peninsula volcanic rocks (mostly potassic to ultrapotassic) has never been found in oceanic intraplate tectonic settings (i.e., no OIB show potassic to ultrapotassic compositions); 4) the depth of the Tyrrhenian Sea crust is very high compared to the depth of oceanic crust of a similar age (Malinverno, 1981;Spadini et al, 1995), in disagreement with the hypothesis that doming would be induced by a mantle heat anomaly; 5) the calculated T p of the Tyrrhenian Sea exceeds only by a few tens °C that of a mantle normal conditions (i.e., T p ~1320 °C vs. 1280 12°C , respectively; Zito et al, 2003;Cella et al, 2006) ; 6) numerical modelling suggests that the development of the Apennine belt cannot be explained by the occurrence of a mantle plume but requires tectonic forces such as those in subduction settings (e.g., Carminati et al, 1999); 7) sub-crustal earthquakes indicate the existence of a slab at least below the Northern Apennines and the Calabrian Arc down to a depth of 500 km (Amato and Selvaggi, 1991;Giardini and Velonà, 1991;Selvaggi and Chiarabba, 1995;Carminati et al, 2002); 8) the off-scraping of sediments previously deposited on continental lithosphere and their accretion in the Apennines thin-skinned accretionary wedge suggests the subduction of continental crust for some 170 km in the Northern Apennines (Carminati et al, 2005) and some 280 km in the Southern Apennines (Scrocca et al, 2005). These values may be added to previous oceanic lithosphere subduction to obtain a total subduction of more than 200 km and 500 km in the two areas, whereas below Calabria, some 770 km of oceanic lithosphere were subducted since 23 Ma (Gueguen et al, 1998).…”