N. D'Agostino

Midcrustal shear zones in postorogenic extension: Example from the northern Tyrrhenian Sea

Metamorphic core complexes of the Aegean region have revealed midcrustal, shallow-dipping extensional shear zones. These shear zones display constant kinematic indicators over large regions (100–200 km). We analyze the example of the northern Tyrrhenian Sea and then compare it to the Aegean region. We first summarize our observations on ductile extension and metamorphic evolution in the northern Tyrrhenian Sea from Alpine Corsica to Tuscany. (1) Extension migrated from west to east from the early Miocene in Corsica to the Recent in the Apennines; (2) Extension is accommodated by shallow east dipping extensional shear zones at the depth of the brittle-ductile transition, from the early Miocene to the Pliocene. (3) West dipping normal faults accommodate extension on the eastern side of the volcanic arc. (4) Extension is preceded along the convergence front by the formation of a thrust wedge, where high-pressure and low-temperature conditions are recorded; maximum PT conditions decrease toward the east, and PT paths are systematically very cold, suggesting that a large part of the exhumation occurred during synorogenic extension. We discuss the possible mechanisms that account for constant shear sense over large domains. The model involves retreat of the slab and migration of the volcanic arc. Partially molten lower crust acts as a low strength zone where extensional strain is localized. Eastward motion of the upper mantle as a consequence of the migration of the slab induced a component of shear toward the volcanic arc at the base of the stronger upper crust. In the weak upper mantle and lower crust, to the west of the volcanic arc, extensional stresses are not transmitted; this produces a top-to-the-east sense of shear at the base of the upper crust that migrates eastward, following arc migration.

Active crustal extension in the Central Apennines (Italy) inferred from GPS measurements in the interval 1994–1999

We present the first GPS estimate of crustal extension in the central Apennines (Italy) through the analysis of the deformation of a sub-network of the National GPS Geodetic network IGM95 in the interval 1994–1999. The selected sub-network spans the entire active deformation belt perpendicularly to its axis and allows the evaluation of (1) the total extension rate absorbed in this sector of the Apennines and (2) the seismogenic potential of the normal faults active in the Late Pleistocene-Holocene interval within the network. Results of this reoccupation are consistent with an extensional strain rate of 0.18 × 10−6 yr−1 concentrated in an area of about 35 km width, giving an average extension rate of 6±2 mm/yr across the central Apennines. The pattern of active deformation suggests active elastic strain accumulation on the westernmost of the two fault systems active in the Late Pleistocene-Holocene interval and may also suggest the presence of another active fault system not recognized so far.

Evidence for localized active extension in the central Apennines (Italy) from global positioning system observations

To assess how contemporary crustal extension is accommodated in the central Apennines, we use a new continuous and survey-style global positioning system velocity solution and model the velocity field using a bicubic spline interpolation method. The partitioning of contemporary deformation over the ∼100-km-wide central Apennines belt reveals a pattern of strain accumulation that largely reflects the spatial distribution of historic and recent seismicity. The highest gradients of horizontal velocities are observed across those faults associated with M > 6 historical earthquakes. Dislocation modeling shows that interseismic elastic loading, in which creep occurs below the seismogenic upper crust on the downdip extensions of historically active faults, reproduces the observed velocity gradients. The current resolution level of Quaternary fault slip rates estimates hinders the comparison with past deformation patterns and, in particular, the discrimination between (1) migrating episodes of short-term focused activity, (2) a distributed pattern of simultaneous deformation on parallel fault systems, or (3) long-term localization of active extension. Taking into account geomorphological evidence, we propose that the geodetically observed deformation spatially corresponds with a long-term localization of strain along the long-wavelength (>100 km) topographic bulge caused by its highest gravitation potential energy relative to surrounding lowlands.

Tectonic evolution of fault-bounded continental blocks : Comparison of paleomagnetic and GPS data in the Corinth and Megara basins (Greece)

[1] We report on new paleomagnetic and anisotropy of magnetic susceptibility (AMS) data from Plio-Pleistocene sedimentary units from Corinth and Megara basins (Peloponnesus, Greece). Paleomagnetic results show that Megara basin has undergone vertical axis CW rotation since the Pliocene, while Corinth has rotated CCW during the same period of time. These results indicate that the overall deformation in central Greece has been achieved by complex interactions of mostly rigid, rotating, fault bounded crustal blocks. The comparison of paleomagnetic results and existing GPS data shows that the boundaries of the rigid blocks in central Greece have changed over time, with faulting migrating into the hanging walls, sometimes changing in orientation. The Megara basin belonged to the Beotia-Locris block in the past but has now been incorporated into the Peloponnesus block, possibly because the faulting in the Gulf of Corinth has propagated both north and east. Paleomagnetic and GPS data from Megara and Corinth basins have significant implications for the deformation style of the continental lithosphere. In areas of distributed deformation the continental lithosphere behaves instantaneously like a small number of rigid blocks with well-defined boundaries. This means that these boundaries could be detected with only few years of observations with GPS. However, on a larger time interval the block boundaries change with time as the active fault moves. Paleomagnetic studies distinguishing differential rotational domains provide a useful tool to map how block boundaries change with time. INDEX TERMS: 1208 Geodesy and Gravity: Crustal movements—intraplate (8110); 1518 Geomagnetism and Paleomagnetism: Magnetic fabrics and anisotropy; 1525 Geomagnetism and Paleomagnetism: Paleomagnetism applied to tectonics (regional, global); 8107 Tectonophysics: Continental neotectonics; 9335 Information Related to Geographic Region: Europe;

Interseismic coupling, seismic potential, and earthquake recurrence on the southern front of the Eastern Alps (NE Italy)

Here we use continuous GPS observations to document the geodetic strain accumulation across the South-Eastern Alps (NE Italy). We estimate the interseismic coupling on the intracontinental collision thrust fault and discuss the seismic potential and earthquake recurrence. We invert the GPS velocities using the back slip approach to simultaneously estimate the relative angular velocity and the degree of interseismic coupling on the thrust fault that separates the Eastern Alps and the Venetian-Friulian plain. Comparison between the rigid rotation predicted motion and the shortening observed across the area indicates that the South-Eastern Alpine thrust front absorbs about 70% of the total convergence between the Adria and Eurasia plates. The coupling is computed on a north dipping fault following the continuous external seismogenic thrust front of the South-Eastern Alps. The modeled thrust fault is currently locked from the surface to a depth of ≈10 km. The transition zone between locked and creeping portions of the fault roughly corresponds with the belt of microseismicity parallel and to the north of the mountain front. The estimated moment deficit rate is 1.3 ± 0.4 × 1017 Nm/yr. The comparison between the estimated moment deficit and that released historically by the earthquakes suggests that to account for the moment deficit the following two factors or their combination should be considered: (1) a significant part of the observed interseismic coupling is released aseismically and (2) infrequent “large” events with long return period (> 1000 years) and with magnitudes larger than the value assigned to the largest historical events (Mw≈ 6.7).

Assimilation of GPS-Derived Atmospheric Propagation Delay in DInSAR Data Processing

Microwave radiation is almost insensitive in terms of power attenuation to the presence of atmosphere; the atmosphere is however an error source in repeat pass interferometry due to propagation delay variations. This effect represents a main limitation in the detection and monitoring of weak deformation patterns in differential interferometric Synthetic Aperture Radar (DInSAR), especially in emergency conditions. Due to the wavelength reduction current, X-Band sensors are even more sensitive to such error sources: procedures adopted in classical advanced DInSAR for atmospheric filtering may fail in the presence of higher revisiting rates. In this work, we show such effect on data acquired by the COSMO-SkyMed constellation. The dataset has been acquired with very high revisiting rates during the emergency phase. This feature allows clearly showing the inability of standard filtering adopted in common processing chains in handling seasonal atmospheric delay variations over temporal intervals spanning periods shorter than 1 year. We discuss a procedure for the mitigation of atmospheric propagation delay (APD) that is based on the integration of data of GPS systems which carries out measurements with large observation angles diversity practically in continuous time. The proposed algorithm allows a robust assimilation of the GPS atmospheric delay measurements in the multipass DInSAR processing and found on a linear approximation with the height of the atmospheric delay corresponding to a stratified atmosphere. Achieved results show a significant mitigation of the seasonal atmospheric variations.

GPS aided atmospheric phase delay mitigation in differential SAR interferometry: Experiences from the 2009 L'Aquila earthquake

Atmospheric propagation delay is a key error source in DInSAR processing and represents a main limitation to the use of DInSAR in emergency conditions. Current X-Band sensors, characterized by frequent passes, are even more affected by such problems. In this work we discuss a procedure based on the use of information extracted from GPS data.