Valerio Acocella

Analogue models of collapse calderas and resurgent domes

Collapse calderas and resurgent domes are a common association related to inflation–deflation processes in volcanic systems. The structure of calderas and domes depends upon the permitted, relative movements of crustal volumes at depth (the so-called “space problem”). In order to study the structures of collapse calderas and resurgent domes and to take the space problem into account, several analogue models were made. Dry-quartz sand was used to simulate the rheology of the brittle crust, while newtonian silicone putty, located at the base of the sand-pack, simulated the ductile behaviour of the magma. A piston moved the silicone putty downward or upward, inducing collapse and doming within the sand. Three separate sets of experiments simulated: (1) caldera collapse; (2) resurgence; and (3) superimposition of resurgence on collapse and vice versa. Collapse experiments are characterized by the development of two concentric depressions; the first-formed depression is bordered by outward dipping reverse ring faults; the subsequent, outer concentric depression is bordered by inward dipping normal ring faults. The deformation pattern during resurgence is a function of the overburden thickness (T) and the dome diameter (D). For higher T/D ratios a dome forms, bordered by inward dipping high angle reverse ring faults; outward dipping normal ring faults develop at late stages. For lower T/D ratios, the dome shows, at late stages, a crestal depression accompanied by radial fractures; subsequently, an apical extrusion of silicone occurs. The superimposition of resurgence over collapse (and vice versa) is characterized by the complete reactivation, with opposite kinematics, of all the pre-existing ring faults during inversion. Both in caldera and resurgence, reverse ring faults form in the early stages due to differential uplift; extensional structures subsequently form to accommodate gravitational collapse during the activity of the reverse faults. The experiments and the overall similarities with nature suggest that the activity of both reverse and normal faults constitutes a possible solution to the space problem during major collapses or resurgences.

Interaction and linkage of extension fractures and normal faults: examples from the rift zone of Iceland

Abstract Field and photogeological studies were made of 90 zones of interacting fracture segments along the rift zone of Iceland. Each zone consists of a pair of extension fractures or a pair of normal faults, with lengths from metres to kilometres. These zones evolve from an underlapping stage, through an overlapping stage (the most common configuration) and, finally, to a linkage stage. Of all the zones, only 7% are underlapping, whereas 93% are overlapping, with hook-shaped fracture pairs. The length/width ratios of the overlapping zones have a mean value of 3.5. The preferred geometry of the overlapping zones depends upon the initial configuration of the interacting fractures (length, overstep) and the development of the interaction. In the overlapping zones, most fracture pairs show moderate shear (strike-slip) components, related to local variations in the extension (opening) directions. Vertical displacements on normal faults decrease as the overstep and length of overlapping zones increase; both, in turn, are proportional to the total lengths of the faults forming the pair. The geometrical and kinematic features of overlapping spreading centres at mid-ocean ridges show close similarities to those reported here. These similarities indicate that the architecture and evolution of overlapping zones are scale independent.

Deformation and eruptions at Mt. Etna (Italy): A lesson from 15 years of observations

This work was partly funded by INGV and the Italian DPC and was supported by ASI, the Preview Project and CRdC-AMRA. DPC-INGV Flank project providing the funds for the publication fees.

The interaction between regional and local tectonics during resurgent doming : the case of the island of Ischia, Italy

Abstract The volcanic island of Ischia is located on the Tyrrhenian margin of Central Italy, characterized by Plio-Quaternary NW–SE- and NE–SW-trending extensional fractures. Ischia displays a resurgent dome uplifted by at least 800 m in the last 33 ka. Remote sensing and field data have been collected to study the structural setting of the island, the deformation pattern associated with resurgence and the superimposition of the regional and the resurgence-induced stress fields. NW–SE and NE–SW extensional fracture systems predominate throughout the island and around the resurgent block, suggesting a relationship with the regional extensional structures. These systems were formed before resurgence and were partly reactivated during resurgence. The reactivation of pre-existing regional systems during resurgence confined the extent of the uplifted area. N–S- and E–W-trending systems have been found exclusively at the borders of the dome and are interpreted as being induced by resurgence. The topmost resurgent block shows an octagonal shape in map view and is tilted at an angle of 15° around a NE–SW-trending horizontal axis; the block is partly bordered by high-angle, inward-dipping regional faults. More than 90% in volume of the volcanic products coeval with resurgence on Ischia have been erupted outside the resurgent block area, suggesting that the resurgence process locally replaced volcanic activity in the last 33 ka.

The role of extensional structures on experimental calderas and resurgence

The structure and shape of collapses and resurgences is often controlled by pre-existing discontinuities, such as normal faults in rift zones. In order to study the role of extensional structures on collapse and resurgence, we used analogue models. Dry sand simulated the brittle crust; silicone, located at the base of the sand-pack, simulated magma. In the experiments, regional extension pre-dated collapse or resurgence, forming normal faults in a graben-like structure; the graben was filled with additional sand, simulating post-rift deposits. A piston then moved the silicone downward or upward, inducing collapse or resurgence within the previously deformed sand. The collapses showed an ellipticity (length of minor axis/length of major axis) between 0.8 and 0.9, with the major axis parallel to the extension direction. The partial reactivation of the pre-existing normal faults was observed during the development of the caldera reverse faults, which, conversely to what was expected (from experiments without pre-existing extension), became partly inward dipping. Resurgence showed an elongation of the uplifted part, with the main axis perpendicular to the extension direction. At depth, pre-existing normal faults were partly reactivated by the reverse faults formed during resurgence; these locally became outward dipping normal faults. A total reactivation of pre-existing faults was also observed during resurgence. The experiments suggest that the observed elongation of calderas and resurgences is the result of the reactivation of pre-existing structures during differential uplift. Such a reactivation is mainly related to the loss in the coefficient of friction of the sand. The results suggest that elliptic calderas and resurgences in nature may develop even from circular magma chambers.

Elliptic calderas in the Ethiopian Rift: control of pre-existing structures

Abstract The Ethiopian Rift is characterized by several Quaternary calderas. Remote sensing and field analyses were used to investigate the regional structural control on three calderas (Fantale, Gariboldi, Gedemsa) in the axial part of the rift. These calderas are located along the Wonji Fault Belt (WFB), a zone of Quaternary NNE–SSW normal faults and extensional fractures. The three calderas show E–W elongation and major E–W vent alignments, oblique with regard to the mean NW–SE extension direction. No significant evidence of E–W tectonic structures has been found near the calderas, the only relevant systems being those of the WFB. Conversely, left-lateral E–W-trending faults are present at the rift borders and on the Nubia and Somalia plateaus, implying a predominant pre-rift activity. The E–W fractures were partly reactivated during rifting, possibly controlling the development of the magma chambers. Thus, the E–W elongation of the calderas would be the surface expression of such a control, rather than the result of regional extension. An evolutionary model on the role of different structures on magmatism at different crustal levels within the rift is proposed.

Oblique back arc rifting of Taupo Volcanic Zone, New Zealand

] Taupo Volcanic Zone (TVZ) is a back arc rift inNorth Island, New Zealand. Its geometry andkinematics are investigated using shaded reliefimages, field examination of faults and offset streamchannels. The results show that TVZ trends NNE, is 250 km long by Hikurangi margin in the North Island of New Zealand [20 km wide and consists of fivesegments. Extension is principally manifest as steeplydipping (60 –90 ) normal faults parallel to TVZ;these, in the last 300 kyr, have experienced acomponent of dextral shear. TVZ is therefore anoblique back arc rift. The dextral shear is 37% of thetotal displacement, which, for previously estimatedspreadingrates 7mm/yr,correspondsto 2.6mm/yr.Thisvalueissimilartopreviousestimatesofthedextralshear from the back arc to the forearc domains in theNorthIsland.DistributeddextralshearacrossTVZthussuggests that strain partitioning across the plateboundary at latitudes of TVZ is less significant thanpreviously thought.

The role of transfer structures on volcanic activity at Campi Flegrei (Southern Italy)

Abstract The Tyrrhenian margin of Central Italy underwent extension during Pliocene and Quaternary. Extension occurred mainly through NW–SE normal faults, bordering a sequence of Plio-Quaternary basins. These basins are offset by coeval NE–SW faults, which show strike–slip and normal motions and have been interpreted as transfer faults. Plio-Quaternary volcanic activity along the margin occurred along a NW–SE belt, systematically in correspondence with NE–SW transverse systems. The Campi Flegrei Volcanic District (CFVD), on the Southern Tyrrhenian margin, consists of an active NE–SW volcanic ridge developed along NE–SW fractures. We performed a structural field analysis with analogue and mechanical models to investigate the role of transverse structures upon volcanism at Campi Flegrei. Field analysis at Campi Flegrei recognized NE–SW and, to a lesser extent, coeval NW–SE active fractures. Analogue experiments have simulated the development of transfer fault systems in brittle extensional domains. The experiments show that subvertical transfer faults connect offset adjacent normal faults dipping 60°. The mechanical model is based on the stress equations in uniaxial lithostatic conditions and absence of regional stresses. It shows how pre-existing subvertical fractures require the smallest magmatic pressures to be penetrated. For a given magmatic pressure, subvertical fractures might be penetrated more deeply, tapping more easily primitive magmas. These results suggest that the CFVD is located along a NE–SW transfer zone connecting NW–SE regional normal faults. Volcanic activity along such NE–SW trend would be induced by the subvertical dip of the transfer faults. The subvertical dip of transfer faults also suggests an explanation for the emission of the more primitive products along NE–SW systems at Campi Flegrei. These considerations find a wider application on the remaining volcanic districts of the margin, located in the same overall structural setting.

The control of overburden thickness on resurgent domes: insights from analogue models

Abstract Resurgent doming consists of the uplift, usually accompanied by volcanic activity, of part of a collapse caldera. Analogue models were used to investigate the architecture of resurgent domes. Dry sand simulates the brittle crust; uprising silicone, located at the base of the sand-pack, simulates magma. The deformation pattern depends mainly upon: (1) the ratio (aspect ratio) between the thickness of the sand overburden and the width of the silicone intrusion; (2) the duration of experiment. For aspect ratios ≈1, two concentric domes develop; the first-formed outer dome is bordered by inward-dipping reverse ring faults, while the inner dome by outward-dipping normal ring faults. The layers inside the dome are uniformly dipping. For aspect ratios ≈0.4, the dome shows a crestal depression, surrounded by radial fractures, followed by an apical extrusion of silicone. The internal structure of the dome is made up of domed layers. Independently from the aspect ratio, the duration of the experiment enhances silicone extrusion. A consistent structure is observed in most resurgent domes in nature. The comparison between experiments and nature suggests that two distinct resurgence modes occur, mainly depending on the aspect ratio (thickness/width) of the crust overlying the magma chamber. Aspect ratios ≈1 develop a resurgent block with uniformly-dipping layers and peripheral volcanic activity (Ischia and Pantelleria type). Aspect ratios ≈0.4 develop a resurgent dome with a crestal depression, domed layers within and peripheral and internal volcanic activity (Valles and Long Valley type).

Dike-fault interaction during the 2004 Dallol intrusion at the northern edge of the Erta Ale Ridge (Afar, Ethiopia)

Received 17 July 2012; revised 29 August 2012; accepted 4 September 2012; published 12 October 2012. [1] During continental rifting the interaction between faulting and magmatic intrusions is not well understood. Using InSAR and seismicity, we show that a 0.06 km 3 dike was intruded along the Dallol segment, Ethiopia and was accompanied by a Mw 5.5 earthquake and associated fault slip along the western flank of the rift. The intrusion was fed by a previously unidentified magma chamber under Dallol. The total seismic moment release was 2.3 10 17 Nm, 10% of the geodetic moment. This is a higher proportion than during the 2005–2009 Dabbahu rifting episode, which ranged between 1–4% of the geodetic moment. A larger component of faulting occurs at Dallol than at Dabbahu segment, a feature we interpret to be related to the proximity (10 km) of the Dallol segment to the rift margin, where welldeveloped faults facilitate slip. Citation: Nobile, A., C. Pagli, D. Keir, T. J. Wright, A. Ayele, J. Ruch, and V. Acocella (2012), Dike-fault interaction during the 2004 Dallol intrusion at the northern edge of the Erta Ale Ridge (Afar, Ethiopia), Geophys. Res. Lett., 39, L19305, doi:10.1029/2012GL053152.