Francesco Mazzarini

Cenozoic geodynamics of the Ross Sea region, Antarctica : Crustal extension, intraplate strike-slip faulting, and tectonic inheritance

An integrated study of onshore and offshore geology of the Ross Sea region (namely, Victoria Land, north of Ross Island, and the Ross Sea, Antarctica) has revealed a complex, post-Eocene tectonic framework. Regional NW-SE right-lateral, strike-slip faults are the outstanding feature of this framework and overprint an older Mesozoic extensional event, responsible for formation of N-S basins in the Ross Sea. The Cenozoic framework includes kinematic deformation and reactivation along the NW-SE faults, including formation of pull-apart basins, both positive and negative flower structures, and push-up ridges. N-S extensional faults are well developed between NW-SE faults and indicate E-W extension during the Cenozoic, produced by the NW-SE right-lateral strike-slip motion together with regional crustal extension. NNW-SSE compression, induced by the right-lateral, strike-slip kinematics, is indicated by locally inverted NE-SW faults and basins. The evolution, geometry, and location of the Rennick Graben and the Lanterman Range fit well into this model. Variations in the deformational style across the region can be linked to corresponding variations in the bulk crustal rheology, from brittle behavior in the west, to ductile deformation (at subseismic-scale resolution) near the Eastern Basin. A semibrittle region that favors N-S clustering of Cenozoic magmatic activity lies in between. In this region, Cenozoic volcanoes develop at the intersections of the NW-SE and the major N-S faults. The NW-SE faults cut almost continually from the Ross Sea to East Antarctica through lithospheric sectors with different rheology and thickness. At least two of the NW-SE faults correspond to older Paleozoic terrane boundaries in northern Victoria Land. The NW-SE faults link in the Southern Ocean with major transform faults related to the plate motions of Australia, New Zealand, and Antarctica.

The Yerer-Tullu Wellel volcanotectonic lineament: a transtensional structure in central Ethiopia and the associated magmatic activity

Abstract An east-west trending structure in central Ethiopia, the Yerer Tullu-Wellel volcanotectonic lineament, intersects the Main Ethiopian Rift and lies between latitudes 8°20′N and 9°05′N. Interpretation of Landsat images, calibrated by field study, indicates the relative chronology of the observed fracture systems (northeast-southwest, east-west, north-south and northwest-southeast). Along this structure there are several central volcanoes concentrated along the latitude of 8°45′N. Most of these volcanoes have basic lava flows at their base with acid dome, plug, and/or pyroclastic deposits as their evolved end members. The chemistry of these volcanic rocks shows a progressive decrease in alkalinity and silica undersaturation from western areas towards the rift margin. Radiometric ages of these volcanoes range from 12 Ma to Recent and indicate that the volcanic activity shifted eastward in time. The central volcanoes are usually located at the intersections of the above described fracture systems. The role of the transtensional Yerer Tullu-Wellel volcano-tectonic lineament since Late Miocene times is discussed within the geodynamic evolution of the Main Ethiopian Rift.

Slab window-related magmatism from southernmost South America: the Late Miocene mafic volcanics from the Estancia Glencross Area (∼52°S, Argentina–Chile)

Abstract The Estancia Glencross Area (EGA) volcanic rocks form a series of five isolated buttes located at the southern end (∼52°S) of the discontinuous belt of Cenozoic basaltic lava formations occurring in the extra-Andean Patagonia. EGA volcanics are subalkaline basalts and basaltic andesites erupted at 8.0–8.5 Ma in a region closely behind the Andean Cordillera. EGA volcanism predated by about 4–5 my the onset of the volcanism in the nearby Pali Aike Volcanic Field, which produced highly primitive, alkaline lavas. Incompatible trace-element distributions and Sr–Nd isotope compositions of EGA rocks are those typical of within-plate OIB-type basalts and are indicative of minimal interaction of sub-lithospheric magmas with enriched reservoirs. The geochemical characteristics of EGA volcanics, as well as their age and location are consistent with a model of slab window opening beneath this region. The high silica content and the garnet signature of the estimated EGA primary magma are explained by a two-stage process involving the initial production of melts from a garnet lherzolite source followed by the reaction of these melts with harzburgite country rocks during their ascent through the mantle lithosphere. The melt/harzburgite reaction, favoured by a slow melt ascent rate, as well as the low magma production at EGA, are likely related to the dominantly compressive stress regime operating in this area during Late Miocene.

The changing face of Mount Etna's summit area documented with Lidar technology

1986–2007 period amounts to 112 ± 12 10 6 m 3 ,a t a mean annual rate of 5.3 10 6 m 3 . The comparison of the various surveys furthermore emphasizes the levels of accuracy and resolution of the different techniques applied. The Lidar technology used in 2007 allows production of high-precision maps in near-real-time, facilitating work concerning environmental hazards such as numerical simulations of, e.g., lava flows. Citation: Neri, M., F. Mazzarini, S. Tarquini, M. Bisson, I. Isola, B. Behncke, and M. T. Pareschi (2008), The changing face of Mount Etna’s summit area documented with Lidar technology, Geophys. Res. Lett., 35, L09305, doi:10.1029/2008GL033740.

Spatial distribution of cones and satellite-detected lineaments in the Pali Aike Volcanic Field (southernmost Patagonia): insights into the tectonic setting of a Neogene rift system

Abstract The relationships between the distribution and morphometric features of eruptive structures (scoria and spatter cones, maar, tuff rings) and the fracture network were investigated in the Pliocene–Quaternary Pali Aike Volcanic Field (southernmost Patagonia, Argentina–Chile). The alkali basaltic/basanitic magmas which erupted in this area have nearly primary magma compositions and often bear mantle xenoliths; hence magma ascent from deep-seated reservoirs was probably very fast, with no significant stagnation at crustal levels. Field surveys and satellite image analysis led to the identification of up to 467 eruptive structures and four main NW–SE, NE–SW, E–W and N–S fracture systems. The spatial distribution of eruptive cones and fractures was investigated through the computation of power-law exponents ( Df ) for self-similar clustering. The self-similarity of cones and fractures was defined between lower and upper cut-offs which were in turn related to the thickness of the fractured mechanical layer. The fractal character of cones and fracture distribution (clustering) in the Pali Aike Volcanic Field area was thus correlated with crustal thickness. The self-similarity of fractures was used to establish the relative chronology of the detected fracture systems. The self-similar clustering exponent is highest in the E–W and NW–SE fracture systems ( Df =1.78 and 1.77, respectively), and lowest in the N–S system ( Df =1.65). The self-similar clustering of eruptive structures is well defined ( Df =1.45). The intense volcano-tectonic activity in the Pali Aike area marks a major Pliocene–Quaternary phase in the development of the Magellan Neogene Rift System.

Evolution of an active lava flow field using a multitemporal LIDAR acquisition

This work was partially funded by the Italian 930 Dipartimento della Protezione Civile in the frame of the 2007–2009 Agree- 931 ment with Istituto Nazionale di Geofisica e Vulcanologia–INGV. A.F. 932 benefited from the MIUR‐FIRB project “Piattaforma di ricerca multi‐disci- 933 plinare su terremoti e vulcani (AIRPLANE)” n. RBPR05B2ZJ. S.T. 934 benefited from the project FIRB “Sviluppo di nuove tecnologie per la prote- 935 zione e difesa del territorio dai rischi naturali (FUMO)” funded by the Italian 936 Ministero dell’Istruzione, dell’Universita e della Ricerca.

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.

The origin of along-rift variations in faulting and magmatism in the Ethiopian Rift

The geological record at rifts and margins worldwide often reveals considerable along-strike variations in volumes of extruded and intruded igneous rocks. These variations may be the result of asthenospheric heterogeneity, variations in rate and timing of extension; alternatively, pre-existing plate architecture and/or the evolving kinematics of extension during breakup may exert first order control on magmatism. The Main Ethiopian Rift (MER) in East Africa provides an excellent opportunity to address this dichotomy: it exposes, along-strike, several sectors of asynchronous rift development from continental rifting in the south to incipient oceanic spreading in the north. Here we perform studies of volcanic cone density and rift obliquity along strike in the MER. By synthesizing these new data in light of existing geophysical, geochemical and petrological constraints on magma generation and emplacement, we are able to discriminate between tectonic and mantle geodynamic controls on the geological record of a newly forming magmatic rifted margin. The timing of rift sector development, the three-dimensional focusing of melt, and the ponding of plume material where the rift dramatically narrows, each influence igneous intrusion and volcanism along the MER. However, rifting obliquity plays an important role in localizing intrusion into the crust beneath en-echelon volcanic segments. Along-strike variations in volumes and types of igneous rocks found at rifted margins thus likely carry information about the development of strain during rifting, as well as the physical state of the convecting mantle at the time of breakup.

Dynamics of magma emplacement in centrifuge models of continental extension with implications for flank volcanism

A set of experimental small-scale models of continental extension investigated the emplacement of orthogonal and oblique magma chambers, initially underplated at the base of the crust. The models are driven by a centrifugal body force which simulates the role of gravity in nature. The models represent crustal conditions analogous to relatively mature continental rifts and consider both symmetric and asymmetric extension. The experimental results suggest that magma emplacement is controlled by the interactions between tectonics and rheology of the crustal layers. In particular, extension is mainly accommodated by lateral flow and ductile doming in the viscous layer simulating the lower crust and by listric normal faults, in the overlying brittle layer, associated with the ductile domal uplift. Stretching of the continental crust induces a “reactive” migration of viscous layers into the footwall of major normal faults, where magma accumulation at the core of the domes takes place. This behavior suggests that widespread magmatism is expected to localize in the footwall of major normal faults, a situation that is often observed in core complex structures. Although the models presented here are intended to simulate magma emplacement during extension of a two-layer brittle-ductile system, lateral flow and migration of magma initially underplated at the base of the crust beneath narrow rifts (e.g., the Ethiopian rift) may also provide a similar mechanism and a possible explanation for the occurrence of important volcanoes on the plateaus flanking the rift zones. In the suggested model, such volcanoes are related to large basaltic magma reservoirs, located at the base of the crust, to accommodate the space vacated by lateral flow and thinning of the ductile crust. The similarity in structure of the models with natural examples of continental extension modes (such as rift systems and core complexes) may suggest a close similarity of dynamic processes.

The dem of mt. etna: geomorphological and structural implications

AbstractA Digital Elevation Model (DEM) of Mt. Etna is presented; it has altimetric and planimetric resolution of 1 m and 5 m, respectively, and covers an area of about 120 km . This 3-D view of Mt. Etna allowed both recognition and location of the main morphostructural and volcano-tectonic features of the volcano. A slope map has been generated from the DEM; on the basis of slope distributions and surface textures, five acclivity domains have been recognized. The largest domain, south of the summit craters, reflects the occurrence of old plateau lavas, distinct from central volcanoes which built the present Etnean volcanic system. Interaction between the central volcanoes, with their summit calderas and failed slopes, produced the other recognised domains. Furthermore, newly identified relevant morphostructural lines are discussed.