Giorgio Pasquarè

Continental arc volcanism and tectonic setting in Central Anatolia, Turkey

Abstract The Neogene and Quaternary volcanism of Central Anatolia represents the central sector of the Anatolian Volcanic Arc. related to continental collision between the Afro-Arabian and Eurasian plates. It is closely associated with a complex system of tectonic depressions related to brittle deformations of transtentional type and which commenced in the Late Miocene. The volcanism here considered can be divided into three main periods of activity, separated by important deformative and erosive events. The first period is represented by a mostly andesitic effusive activity. The second period is characterized by the emplacement of a thick ignimbritic sequence and shows an areal distribution up to 11,000 km 2 . Seven ignimbrite units have been recognized. The three main units were found at a distance of more than 100 km from the presumed source area. Geological and sedimentological data lead us to recognize the Melendiz Dag volcanic complex and the Ciftlik caldera as the probable ignimbrites source vent. During the third period great andesitic-basaltic stratovolcanoes and a number of prevalently acid monogenic centres developed. The relationship between the volcanic activity is clearly conditioned by the main transcurrent fault systems present in Central Anatolia. The Neogene-Quaternary volcanic activity prevalently developed along the ENE-WSW Karaman-Sivas lineament. Most of the great central volcanoes developed at the intersection between the ENE-WSW trends and the Ecemis and Tuz Golu transcurrent faults. The structural interpretation of the Quaternary monogenic centres is more difficult. Probably they are related to the very recent N-S fault swarms which cross the Anatolides and the Taurus Range.

Geology of the western Mexican Volcanic Belt and adjacent Sierra Madre Occidental and Jalisco Block

The geology of a large portion of western Mexico, including the entire northern boundary of the Jalisco block, has been compiled at a regional scale. Revision of previous works and new geological surveys allowed the construction of a 1:250,000 scale map with 28 informal geologic units, 25 for the Tertiary and Quaternary volcanic succession of the Sierra Madre Occidental (SMO) and the Mexican Volcanic Belt (MVB). In this region, the MVB covers the boundary between the SMO volcanic province and the Cretaceous to Paleocene batholith and volcano-sedimentary sequences of the Jalisco block (JB). The 1000-m-thick succession of the SMO was emplaced during three discrete volcanic episodes of Eocene, Oligocene, and Early Miocene, separated by volcano-sedimentary deposits or unconformities. This succession, dominated by silicic ash flows, is absent in the deep geothermal wells of La Primavera, San Marcos and Ceboruco areas, indicating that the southern boundary of the SMO is located north of these sites. Within most of the study area, the MVB is clearly separated from the SMO by a tectonic unconformity produced in Middle Miocene. The first episode belonging to the MVB is represented by a widespread and volumetrically significant mafic volcanism, sometimes with an alkaline composition, which occurred at 11‐8 Ma in central-eastern Nayarit and in the Guadalajara region. A period of reduced volcanic activity between 7.2 Ma and 5.5 Ma was followed by the emplacement of large volumes of rhyolites and minor ignimbrites in Early Pliocene. Intermediate to mafic volcanism, also with alkaline composition, becomes again dominant between ~4.5 Ma and the Present, although large rhyolitic and dacitic dome complexes were emplaced between Guadalajara and Tepic. In Late Pliocene and Quaternary times, large andesitic to dacitic strato-volcanoes were built in the northern part of the arc, whereas basaltic shield volcanoes and cinder cones characterize the volcanic front.

Stress pattern evolution in the central sector of the Mexican Volcanic Belt

Abstract Stratigraphic studies and detailed field analyses of brittle and plastic deformations allow the reconstruction of the stress pattern and the tectonic evolution of the central sector of the Mexican Volcanic Belt. Several compressional phases affected the metamorphic basement, while the last one also involved the volcanic cover of Early Miocene age. In Middle Miocene-Early Pliocene times and ENE-WSW extensional phase developed, representing the southern protraction of the Basin and Range Province of the western United States. In Pliocene-Quaternary times there then followed the development of a complicated deformational cycle showing the superficial evidence of a sinistral lateral shear zone. At the beginning the area was affected by a NE-SW compression including reverse, dextral and sinistral strike-slip faulting. This was followed by Early Pleistocene sinistral transtension characterized by a NW-SE direction of least principal stress and sinistral normal-slip faulting. During Late Pleistocene-Holocene times the transtension was concentrated along E-W faults. Tectonic control in the distribution of volcanism has also been recognized. The Basin and Range phase controlled the deposition of the Miocene volcanic and clastic units, particularly the effusion of basaltic lavas connected with the latest extensional movements of the same phase. The Mexican Volcanic Belt sequence is strictly linked with the development of a large E-W sinistral lateral shear zone, active from the Pliocene to the Present, which shows the extensional component progressively prevailing over the shear one.

Geology of Los Azufres Caldera, Mexico, and its relationships with regional tectonics

Ferrari, L., Gardufio, V.H., Pasquar6, G. and Tibaldi, A., 1991. Geology of Los Azufres caldera, Mexico, and its relationships with regional tectonics. In: S.P Verma (Editor), Calderas: Genesis, Structure and Unrest. J. Volcanol. Geotherm. Res., 47: 129-148. The Los Azufres geothermal field is one of several silicic centres of the Mexican Volcanic Belt (MVB) for which a caldera structure was suggested. Geological and structural surveys in a wide area surrounding Los Azufres reveal that this complex is situated in an area of unusual concentration of acid volcanism, consisting of four pyroclastic units and several dome complexes. Although a complete caldera structure is not morphologically evident, several facts suggest a long collapse history at Los A zufres: - the geothermal field lies at the centre of a subcircular depressed area (27 × 26 km in size) filled by a distinctive fluviolaeustrine sequence: - Middle to Late Miocene rocks bound to the south and to the north of this depression and are encountered only at depth inside it; - four large ignimbritic suites of latest Miocene and Pliocenc age outcrop outside the depression; - volume estimations of these pyroclastic products are comparable with the missing volume of the depressed area: - no alternative caldera structures exist in a radius of 200 km from Los Azufres; - dacitic to rhyolitic lavas, principally extruded as dome complexes, were emplaced inside this depression during the Pleistocene. Los Azufres is therefore interpreted as a nested caldera of latest Miocene and Pliocene age, The recent regional tectonic evolution of the central sector of MVB comprises a Late Miocene-Early Pliocene left-lateral transcurrent phase, followed by a Late Pliocene-Quatern ary transtensional one. The silicic volcanism occurring between 6.1 and 2.8 Ma can be linked to the first phase, while in the adjacent areas of MVB a volcanic hiatus can be recognized. The following transtensional phase reached the Los Azufres area only during the Pleistocene and disrupted the caldera structure. Normal faults, developed during this period, controlled the uprising of basic magma which partly interacted with the remaining differentiated one and produced part of the recent intracaldera cycle.

Tectonic evolution of the emergent part of a volcanic ocean island: Lanzarote, Canary Islands

Abstract A reconstruction of the structural evolution of Lanzarote confirms the existence of two principal stages of construction of the island, separated by a main erosional event. The first (pre-erosional) is the shield stage, Miocene-Pliocene in age, which is divided in three stratigraphic groups, where different formations can be distinguished using unconformities mappable over the whole area of outcrop. In the second (post-erosional) stage, developed during the Quaternary, nine stratigraphic levels are tentatively identified and are grouped in two phases of volcanic activity (Shields phase and Pyroclastic cones phase). Data from 580 photogeologic lineaments, as well as field data from 204 fault planes (195 with sense of movement), 273 dykes, and 45 volcanic alignments, are presented. The tectonic style of the deformations is brittle and polyphasic. Strike-slip faults are dominant, but normal and reverse faults are also present. Both the so-called “Atlantic” and “African” directions are well developed in trends of strike-slip faults and volcanic indicators. At least two deformation phases can be identified from the geometrical analysis of faults and from the stress tensor calculated using lineations on fault planes. The reconstructed stress field responsible for the measured deformations changed orientation during the evolution of the island of Lanzarote. It is suggested that a permutation between principal stress axes σ1, and σ3 occurred subparallel to the length of the island between the pre-erosional and post-erosional stages.

Landslides triggered by earthquakes and their relations with faults and mountain slope geometry: an example from Ecuador

Abstract A large number of landslides occurred during two seismic events (respectively, 6.9 and 6.1 Msw) on 5 March 1987 in the Ecuadorian Andes. These landslides have been mapped, digitized, and coregistered with topography at 1:50,000 scale. Geometry of coseismic and Holocene faulting has been assessed integrating field and geophysical data. Landslide distribution and Holocene tectonic features have been compared with earthquake foci, geological deposits, slope area, inclination and orientation, and vegetation cover. The macro-seismic field deduced by landslide distribution is ellipse-shaped with the major axis striking NNE and coinciding with the trend of the coseismic and Holocene faults. Landslide distribution also shows a correlation with respect to the dip-direction of these faults and the orientation of mountain slopes. Slopes parallel to the coseismic fault planes, sloping towards WNW, are almost unaffected by landsliding, while this increases on the slopes at high angle to the fault plane and lying along its strike. If these results could be confirmed in other areas, a method of reconstruction of geometry of seismogenetic faults from topographic effects appears feasible.

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.

Structural Behaviour of a Continental Volcanic Arc: The Mexican Volcanic Belt

Abstract The structural evolution of South-Central Mexico during Neogene and Quaternary times clearly reveals the overlap of several tectonic phases characterized by brittle deformations. Structural lineaments linked with the southern prosecution of the Basin and Range Province were recognized, together with lineaments connected to the opening of the Gulf of California and to the convergence between the Cocos and the North-American Plates. The Mexican Volcanic Belt consists of a volcanic arc structurally controlled by the differential development of the above mentioned lineaments, undergoing strong and repeated reactivations, contrarily to what happens in most adjacent areas. These reactivations structurally represent the unifying motive of the MVB. The morphological and structural analyses have shown the presence of three main sectors. The western sector is characterized by a NW-SE fracture system, representing the southward prosecution of the Gulf of California structures. The central sector is defined by a composite WSW-ENE trending Graben system, probably connected with sinistral lateral shear induced by the aforementioned convergence phenomena. The Eastern sector, mainly developed within Sierra Madre Oriental, shows a reactivation of N-S Basin and Range tensional structures.

Gigantic individual lava flows in the Andean foothills near Malargüe (Mendoza, Argentina)

Payún Volcano is a Quaternary fissural edifice located in the southern part of the Mendoza Province (Argentina) and belonging to an Andean back-arc extensional basin. The volcano eastern portion is covered by a basaltic lava field composed of several very long lava flows, two of which advanced over more than 170 km from their fissural feeding vents. These two gigantic flows covered most of the distance over the nearly flat surface of the Pampean foreland, moving on a 0.3° slope. Since these lava flows are the longest ever documented on our Planet, a full understanding of the processes leading to their emplacement might result in a revision of the current theories on exceptionally long channelized lava flows, both on Earth and on terrestrial planets.RiassuntoIl Vulcano Payún è un edificio fissurale quaternario appartenente ad un bacino estensionale di retroarco della catena andina situato nella parte meridionale della provincia di Mendoza in Argentina. La porzione orientale dell’edificio è mantellata da un campo di lave basaltiche tra cui si riconoscono alcune singole colate canalizzate che si estendono per oltre 170 km di distanza dalla frattura alimentatrice, scorrendo per gran parte del loro percorso sulla superficie piana dell’avampaese pampeano con una pendenza di 0,3°. Esse costituiscono le più lunghe colate laviche individuali finora documentate sulla Terra e la comprensione dei loro processi di messa in posto potrebbe provocare la revisione delle teorie riguardanti le colate laviche di lunghezza eccezionale sia sulla Terra sia sui pianeti terrestri.

Gigantic individual lava flows in the Andean foothills near Malargüe (Mendoza, Argentina)@@@Colate laviche gigantesche alle pendici delle Ande presso Malargüe (Mendoza, Argentina)

Payún Volcano is a Quaternary fissural edifice located in the southern part of the Mendoza Province (Argentina) and belonging to an Andean back-arc extensional basin. The volcano eastern portion is covered by a basaltic lava field composed of several very long lava flows, two of which advanced over more than 170 km from their fissural feeding vents. These two gigantic flows covered most of the distance over the nearly flat surface of the Pampean foreland, moving on a 0.3° slope. Since these lava flows are the longest ever documented on our Planet, a full understanding of the processes leading to their emplacement might result in a revision of the current theories on exceptionally long channelized lava flows, both on Earth and on terrestrial planets.RiassuntoIl Vulcano Payún è un edificio fissurale quaternario appartenente ad un bacino estensionale di retroarco della catena andina situato nella parte meridionale della provincia di Mendoza in Argentina. La porzione orientale dell’edificio è mantellata da un campo di lave basaltiche tra cui si riconoscono alcune singole colate canalizzate che si estendono per oltre 170 km di distanza dalla frattura alimentatrice, scorrendo per gran parte del loro percorso sulla superficie piana dell’avampaese pampeano con una pendenza di 0,3°. Esse costituiscono le più lunghe colate laviche individuali finora documentate sulla Terra e la comprensione dei loro processi di messa in posto potrebbe provocare la revisione delle teorie riguardanti le colate laviche di lunghezza eccezionale sia sulla Terra sia sui pianeti terrestri.