Eugenio Carminati

Slab Retreat and Active Shortening along the Central-Northern Apennines

The Quaternary geodynamic evolution and the tectonic processes active along the Central and Northern Apennines thrust fronts and in the adjacent Padane-Adriatic foredeep domains are analysed and discussed.

The westward drift of the lithosphere: A rotational drag?

Net westward rotation of the lithosphere relative to the underlying mantle is a controversial phenomenon fi rst attributed to tidal effects, and later to the dynamics of mantle convection. In spite of a number of independent geological and geophysical arguments for westward tectonic drift, this phenomenon has received little recent attention. We suggest that this differential rotation is a combined effect of three processes: (1) tidal torques act on the lithosphere generating a westerly directed torque decelerating Earth’s spin; (2) the downwelling of the denser material toward the bottom of the mantle and in the core slightly decreases the moment of inertia and speeds up Earth’s rotation, only partly counterbalancing the tidal drag; (3) thin (3– 30 km) layers of very low viscosity hydrate channels occur in the asthenosphere. It is suggested that shear heating and the mechanical fatigue self-perpetuate one or more channels of this kind, which provide the necessary decoupling zone of the lithosphere.

Active strike-slip faulting in El Salvador, Central America

Several major earthquakes have affected El Salvador, Central America, during the Past 100 yr as a consequence of oblique subduction of the Cocos plate under the Caribbean plate, which is partitioned between trench-orthogonal compression and strike-slip deformation parallel to the volcanic arc. Focal mechanisms and the distribution of the most destructive earthquakes, together with geomorphologic evidence, suggest that this transcurrent component of motion may be accommodated by a major strike-slip fault (El Salvador fault zone). We present field geological, structural, and geomorphological data collected in central El Salvador that allow the constraint of the kinematics and the Quaternary activity of this major seismogenic strike-slip fault system. Data suggest that the El Salvador fault zone consists of at least two main ∼E-W fault segments (San Vicente and Berlin segments), with associated secondary synthetic (WNW-ESE) and antithetic (NNW-SSE) Riedel shears and NW-SE tensional structures. The two main fault segments overlap in a dextral en echelon style with the formation of an intervening pull-apart basin. Our original geological and geomorphologic data suggest a late Pleistocene–Holocene slip rate of ∼11 mm/yr along the Berlin segment, in contrast with low historical seismicity. The kinematics and rates of deformation suggested by our new data are consistent with models involving slip partitioning during oblique subduction, and support the notion that a trench-parallel component of motion between the Caribbean and Cocos plates is concentrated along E-W dextral strike-slip faults parallel to the volcanic arc.

Jurassic rifting evolution of the Apennines and Southern Alps (Italy): Parallels and differences

Jurassic rifting in the southern Alps and Northern Apennines occurred within a relatively short (2–4 m.y.) interval of tectonic activity (late Hettangian–early Sinemurian) in proximal areas (central-eastern southern Alps and Northern Apennines), and was followed by calm postrift sedimentation. The main axis of the rift jumped in the late Early Jurassic to the previously unaffected, mostly nonsubsiding and/or uplifted distal areas of southwestern Tuscany and the western southern Alps. Synchronous stretching occurred in the Helvetic-Brianconnais domains of the Alps and in the Longobucco-Caloveto area (Calabria), here interpreted as the conjugate margin of western southern Alps and southwestern Tuscany. Major differences occur in the extensional structural style of the southern Alps and Apennines. The Northern Apennines are characterized by diffuse thin-skinned stretching at shallow depths and by localized thick-skinned stretching at deeper crustal levels. This style contrasts with the well-known thick-skinned nature of extensional tectonics in the southern Alps, which are also characterized by a larger spacing between major normal faults. It is suggested that these different extensional styles were controlled by contrasting rheologies induced by the different stratigraphies. The strong strain partitioning and the diffuse shallow thin-skinned tectonics of the Northern Apennines basins are here related to the occurrence of thick (up to 2 km) evaporites that underlie a relatively thin (up to 1.3 km) carbonate platform.

Plio‐Quaternary vertical motion of the Northern Apennines: Insights from dynamic modeling

We test the effects of different geodynamic mechanisms on the Pliocene to present-day dynamics, and in particular on the vertical motions, of the Northern Apennines system by means of two-dimensional finite element models. We show that the Pliocene features of the Northern Apennines (exhumation of deep rocks in western Italy and fast subsidence rates in the Adriatic foredeep) can be adequately reproduced by a model (SLAB) characterized by the passive sinking of a negatively buoyant Adriatic slab and by the upwelling of buoyant asthenosphere beneath western Italy. Model SLAB is, however, not able to account for the Quaternary and present-day pattern of vertical motion of eastern Italy and of the Adriatic basin. A deep configuration of the system characterized by a detached slab (model DETACH) may explain the vertical motions of these areas, constrained by uplift of the eastern portions of the Apenninic chain, by an eastward shift of the foredeep depocenter and by lower subsidence rates in the Adriatic basin. The tectonic and stratigraphic data showing major differences between the Tertiary and the Quaternary evolution of the Northern Apennines are discussed.

Control of differential compaction on the geometry of sediments onlapping paleoescarpments: Insights from field geology (Central Apennines, Italy) and numerical modeling

Basin strata onlapping pelagic carbonate platform margins in the Central Apennines commonly dip considerably (as much as 20°–30°) toward the basin when platform strata (Calcare Massiccio) are retrodeformed to horizontal in order to undo the tilting related to Apenninic compressional tectonics. These angular relationships are commonly ascribed to normal drag associated with synsedimentary faulting. We show that such angular relationships are fully consistent with results of numerical simulations that aim at modeling first-order geometric features associated with sedimentation and compaction of deposits onlapping the escarpment at the margin of basins. Therefore, growth of an underlying normal fault is not generally required to generate them. This conclusion is also supported by field investigations of the contacts at pelagic carbonate platform margins and by paleoescarpment analysis, which strongly suggest post–early Liassic tectonic quiescence.

Decollement depth versus accretionary prism dimension in the Apennines and the Barbados

(1) Along representative cross sections of the Apennines and the Northern Barbados accretionary prisms, we measured the area, the decollement depth, the angle a of the upper envelope and the angle b of the dip of the regional monocline. The continental sections of the Apennines accretionary prism have a deeper decollement than the oceanic sections of the Northern Barbados, 6-10 km depth and <1 km depth, respectively, because the sediment pile is thinner on the incoming Barbados plate and its denser oceanic structure is more easily subducted. Considering the frontal 50 km, the Apennines have an average cross- sectional area of 500 km 2 and the Northern Barbados Ridge of 100 km 2 . The total area is a function of the depth of the decollement plane. Therefore, at a given amount of subduction, the deeper the decollement depth is, the bigger the area of the wedge will result, assuming negligible compaction and erosion. As a consequence, the larger area/volume and higher elevation of the Apennines with respect to the Barbados is determined by the Apenninic deeper decollement. Despite these differences, the geometry of both decollements is, in some cases, comparable, in particular, close to the boundary between the crystalline crust and the sediment pile, where the main density and strength contrasts are concentrated. Variations in depth of the decollement occur moving along strike in both accretionary prisms. The geometry of the prisms is further controlled by the different values of a and b, their sum, and the distance of the accretionary prism relative to the subduction hinge. INDEX TERMS: 8099 Structural Geology: General or miscellaneous; 8150 Tectonophysics: Evolution of the Earth: Plate boundary—general (3040); 8122 Tectonophysics: Dynamics, gravity and tectonics; KEYWORDS: decollement depth, accretionary prism, Apennines, Barbados. Citation: Bigi, S., F. Lenci, C. Doglioni, J. C. Moore, E. Carminati, and D. Scrocca, Decollement depth versus

Thermal and tectonic evolution of the southern Alps (northern Italy) rifting: Coupled organic matter maturity analysis and thermokinematic modeling

The southern Alps were characterized by strong variations, both in space and time, of heat flow during Mesozoic rifting. The regional thermal history was reconstructed using organic matter (OM) maturity data from outcropping sediments. One-dimensional (1-D) thermal modeling performed on selected successions suggests that OM maturity was mainly controlled by high geothermal gradients (heat flow peaks of 85 to 105 mW/m2 in the Middle Jurassic) and differential burial during Norian–Early Jurassic extensional phases. The results of 1-D modeling show an eastward increase of heat flow peak values. These results were compared with those obtained with two-dimensional (2-D) thermokinematic models. The models show a time shift (ca. 10 Ma) in the heat-flow peak (Aalenian-Bajocian for 2-D and Bajocian for 1-D modeling). However, the Bajocian age was a priori imposed on 1-D models. Available geochemical data could be fitted assuming Aalenian-Bajocian peak ages. Consequently, this misfit is not alarming. The eastward increase in heat-flow peak values is tentatively explained with an eastward increase of radiogenic heat production in the crust instead of with differential stretching. The comparison of paleothermal data and numerical modeling was done to gain knowledge on the potentials and limitations of numerical modeling in frontier areas. Although some differences do exist in the results of geochemical and thermokinematic models, we can conclude that if a reasonable knowledge of the thermal parameters of both covers and basement is available, thermokinematic modeling can provide useful first-order estimates in frontier areas of heat flow and temperature evolution through time.

TEMSPOL: A MATLAB thermal model for deep subduction zones including major phase transformations

Abstract TEMSPOL is an open MATLAB code suitable for calculating temperature and lateral anomaly of density distributions in deep subduction zones, taking into account the olivine to spinel phase transformation in a self-consistent manner. The code solves, by means of a finite difference scheme, the heat transfer equation including adiabatic heating, radioactive heat generation, latent heat associated with phase changes and frictional heating. We show, with a few simulations, that TEMSPOL can be a useful tool for researchers studying seismic velocity, stress and seismicity distribution in deep subduction zones. Deep earthquakes in subducting slabs are thought to be caused by shear instabilities associated with the olivine to spinel phase transition in metastable olivine wedges. We investigate the kinematic and thermal conditions of the subducting plate that lead to the formation of metastable olivine wedges. Moreover, TEMSPOL calculates lateral anomalies of density within subducting slabs, which can be used to evaluate buoyancy forces that determine the dynamics of subduction and the stress distribution within the slab. We use TEMSPOL to evaluate the effects of heat sources such as shear heating and latent heat release, which are neglected in commonly used thermal models of subduction. We show that neglecting these heat sources can lead to significant overestimation of the depth reached by the metastable olivine wedge.

Reverse migration of seismicity on thrusts and normal faults

In this work, the control exerted by the stress axes orientation on the evolution of seismic sequences developing in compressive and extensional regimes is analysed. According to the Anderson fault theory, the vertical stress is the minimum principal stress in compressional tectonic regimes, whereas it is the maximum principal stress in extensional regimes. Using Mohr diagrams and discussing the present knowledge about the distribution of vertical and horizontal stress with depth we show that, in absence of localised fluid overpressure, such changes imply that thrust and normal faults become more unstable at shallower and greater depths, respectively. These opposite mechanical behaviours predict, in a rather isotropic body, easier rupture at shallower level in compressional regimes later propagating downward. On the contrary, a first deep rupture propagating upward is expected in extensional regimes. This is consistent with observations from major earthquakes from different areas in the world. We show that the exceptions to downward migration along thrusts occur along steeply inclined faults and probably imply localised supra-hydrostatic fluid pressures. Moreover, we show that the inversion of the meaning of the lithostatic load has consequences also for the role of topography. High topography, increasing the vertical load, should inhibit earthquake development in compressional environments and should favour it in extensional settings. Although several factors, such as geodynamic processes, local tectonic features and rock rheology, are likely to control earthquake locations, stress distribution and tectonic regime, these model predictions are consistent with seismicity distribution in Italy, central Andes and Himalaya. In these areas, large to medium compressional earthquakes occur at the low elevation borders of compressional mountain belts, whereas large extensional earthquakes occur in correspondence to maximum elevations. D 2004 Elsevier B.V. All rights reserved.