Description: The main issue addressed in this work is the process leading to fluid subsurface entrapment and pressure increase up to hydrofracturing and, possibly, to paroxysm in a hydrothermal setting, in order to envisage such processes and mitigate their effects in the volcanically active study area and elsewhere. A field and laboratory multidisciplinary approach is used in the fossil (late Pleistocene) portion of an active hydrothermal system (Colli Albani volcano, Rome, Italy). In this area, sulfate and sulfide mineralizations and strongly altered ignimbrites are exposed. The alteration acme occurs on top of a buried normal fault, where abundant degassing is still active, and fades away in 2–3 km. Based on pervasive versus discrete alteration styles, mineral assemblages, and further evidence, proximal and distal alteration domains are recognized. Both domains underwent steam-heated advanced argillic alteration with likely temperatures up to ~400 °C in the proximal domain and less than 150 °C in the distal domain. The process of hydrothermal alteration progressively and severely depleted many elements from the most permeable rock units, whereas the lowest-permeable unit (Tufo Lionato) underwent fracture and porosity healing accompanied by both mass and volume gain. In the proximal domain, the advanced argillic hydrothermal alteration eventually formed a substantial barrier to fluids. The hydrothermal fluids accumulated in and below this barrier, which was then suddenly hydrofractured when heat-driven hydraulic pressure overcame the effective stress, thus possibly leading to hydrothermal paroxysm. The decompression associated with hydrofracturing enhanced gas exsolution and mineral precipitation from the entrapped overpressured fluids. Mineral precipitation contributed, in turn, to fracture healing and to reinitiation of a new cycle of hydrothermal fluid entrapment. The key preconditions for the occurrence of the inferred processes are the contrasting compositions of K-alkaline host rocks and acidic alteration fluids, as also previously documented in other similar settings elsewhere.

Description: Paleozoic sequences exposed along the Transantarctic Mountains in Antarctica and in southeastern Australia are segments of a formerly contiguous accretionary orogen that developed along the eastern margin of Gondwana. The margin underwent amalgamation and eastward accretion in the early Cambrian to Early Ordovician Ross-Delamerian orogen and in the Ordovician to Carboniferous Lachlan orogen. Northern Victoria Land plays a key role in many geodynamic reconstructions because it has long been considered the along-strike continuation of Australia in Antarctica; however, the correlation between lithotectonic units in Antarctica (Wilson, Bowers, and Robertson Bay terranes) and those in southeastern Australia (Glenelg, Grampians-Stavely, and Stawell zones), as well as the presence of Lachlan-aged tectono-metamorphic events in northern Victoria Land, are still uncertain. 40 Ar- 39 Ar laser experiments on detrital and syndeformational white micas from low-grade siliciclastic rocks of northern Victoria Land, in conjunction with mineral-textural analysis and whole-rock geochemical and Nd isotope data, are used to constrain provenance and the timing of deformation, and to assess analogies with correlative structural zones in southeastern Australia. Detrital white micas of the western lithotectonic unit (Wilson terrane) yielded an age pattern dominated by late Cryogenian to Ediacaran ages (650–550 Ma), closely matching those of turbidites from the Australian Kanmantoo Group. Detrital white micas from the easternmost lithotectonic units (Bowers terrane and Robertson Bay terrane) yield indistinguishable age patterns, strikingly in agreement with those available for the western subprovince of the Lachlan orogen in Australia, which are dominated instead by younger ages with a dominant Ross orogen fingerprint (550–480 Ma). Deposition of siliciclastic detritus in the three lithotectonic units most likely occurred synchronously in the early–middle Cambrian, and the different signatures suggest that detritus was supplied from different source areas, with sediment supply from the west-southwest (East African orogen or ice-covered regions of East Antarctica) in the Wilson terrane and from the south (Ross orogen) in the Bowers and Robertson Bay terranes. Results also provide evidence for post-Ross orogen (ca. 462 Ma) contractional tectonics at the boundary between the Bowers and the Robertson Bay terranes, suggesting that available Ar data from the literature are variably affected by the presence of detrital micas. This finding establishes a cause-effect relationship between compressional tectonics at the plate margin and Middle–Late Ordovician intraplate reactivation processes in the western Wilson terrane. The accretion of northern Victoria Land was polyphase; it began with the amalgamation of the Bowers and Wilson terranes in the middle–late Cambrian and was followed by the docking of the Robertson Bay and Bowers terranes in the Middle–Late Ordovician. Results further support the link between northern Victoria Land and southeastern Australia. Together with a careful examination of data from the literature, they also suggest that the Bowers terrane and the across-strike contiguous Robertson Bay terrane are correlatives of the whole Stawell zone.

Description: This study describes the structural setting, petrogenesis, and geochronology of a suite of acidic magmatic rocks that are intruded in the metamorphic core of the Tertiary ophiolitic suture zone of the Sabzevar Range, NE central Iran. These intrusive bodies show tabular geometries with solid-state fabrics documenting syntectonic emplacement during crustal shortening. In the total alkalis-silica (TAS) diagram, their compositions define a medium-K calc-alkaline suite, spanning from basaltic andesite to the dacite and rhyolite fields. They show characteristic low MgO (0.15–0.60 wt%) and Ni (〈20 ppm), high Sr contents, a negligible Eu anomaly, and extremely fractionated rare earth element (REEs), with high La/Yb and Sr/Y (up to 900) ratios, but very low Yb and Y contents. They also show zircon Hf isotope compositions compatible with a mid-ocean-ridge basalt (MORB)–type oceanic crust. Inverse and forward thermobarometry constrains conditions of magma crystallization in the upper-pressure field of the amphibolite facies (ca. 1.2–1.5 GPa and 750 °C). Integrated U-Pb zircon and 40 Ar/ 39 Ar white mica and amphibole geochronology constrains the Sabzevar magmatism to the late Paleocene (at ca. 58 Ma). Genesis of the Sabzevar magmatic suite is interpreted in terms of prograde, wet amphibolite melting during oceanic subduction, within a pressure-temperature range between a plagioclase-out and a hornblende-out boundary. Magma differentiation and high-pressure amphibole fractionation of pristine slab melts are proposed as the dominant factors that imparted the adakite signature in the Sabzevar structural zone. Implications in terms of the regional tectonic scenario are discussed and framed within the advancing and retreating evolution of the Neotethyan subduction during the Mesozoic–Tertiary time span.

Description: Central Iran provides an ideal region in which to study the long-term morphotectonic response to the nucleation and propagation of intraplate faulting. In this study, a multidisciplinary approach that integrates structural and stratigraphic field investigations with apatite (U + Th)/He (AHe) thermochronometry is used to reconstruct the spatio-temporal evolution of the Kuh-e-Faghan Fault in northeastern central Iran. The Kuh-e-Faghan Fault is a narrow, ~80-km-long, deformation zone that consists of three main broadly left-stepping, E-W–trending, dextral fault strands that cut through the Mesozoic–Paleozoic substratum and the Neogene–Quaternary sedimentary cover. The AHe thermochronometry results indicate that the intrafault blocks along the Kuh-e-Faghan Fault experienced two major episodes of fault-related exhumation at ca. 18 Ma and ca. 4 Ma. The ca. 18 Ma faulting/exhumation episode is chiefly recorded by the structure and depositional architecture of the Neogene deposits along the Kuh-e-Faghan Fault. A source-to-sink scenario can be reconstructed for this time frame, where topographic growth caused the synchronous erosion/exhumation of the pre-Neogene units and deposition of the eroded material in the surrounding fault-bounded continental depocenters. Successively, the Kuh-e-Faghan Fault gradually entered a period of relative tectonic quiescence and, probably, of regional subsidence, during which a thick pile of fine-grained onlapping sediments was deposited. This may have caused resetting of the He ages of apatite in the pre-Neogene and the basal Neogene successions. The ca. 4 Ma faulting episode caused the final exhumation of the fault system, resulting in the current fault zone and topography. The two fault-related exhumation episodes fit with regional early Miocene collision-enhanced uplift/exhumation, and the late Miocene–early Pliocene widespread tectonic reorganization of the Iranian Plateau. The reconstructed long-term, spatially and temporally punctuated fault system evolution in intraplate central Iran during Neogene–Quaternary times may reflect states of far-field stress changes at the collisional boundaries.

Description: The Neogene–Quaternary Albegna basin (southern Tuscany, central Italy), located to the south of the active geothermal field of Monte Amiata, hosts fossil and active thermogene travertine deposits, which are used in this study to reconstruct the spatio-temporal evolution of the feeding hydrothermal system. Travertine deposition is controlled by regional tectonics that operated through distributed N-S– and approximately E-W–striking transtensional fault arrays. The geochronological data set ( 230 Th/ 234 U, uranium-series disequilibrium) indicates a general rejuvenation (from 〉350 to 〈40 ka) of the travertine deposits moving from north to south and from higher to lower elevations. Negative 13 C and positive 18 O trends with younger deposition ages and lower depositional elevations provide evidence for a change in space and time of the hydrothermal fluid supply, suggesting a progressive dilution of the endogenic fluid sources by increasing meteoric water inputs. Comparison with paleoclimate records suggests increased travertine deposition during humid interglacial periods characterized by highstands of the water table. Travertine deposits of the Albegna basin record the interactions and feedbacks among tectonics, hydrothermalism, and paleoclimate within a region of positive geothermal anomaly during the Quaternary. Our study also sheds light on the neotectonic evolution of the Tyrrhenian margin of central Italy, where hydrothermalism has been distributed along margin-transverse structures during the Pleistocene and Holocene. It is hypothesized that originally upper-crustal, margin-transverse faults have evolved to through-going crustal features during the Quaternary, providing structurally controlled pathways for hydrothermal fluids. We suggest that this was the consequence of a change in the relative magnitude of the principal stress vectors along the Tyrrhenian margin that occurred under a regional stress field dominated by a continuous extensional regime.

Description: The reactivation of faults and shear zones is a widely documented process and represents a fundamental characteristic of deformation in the continental lithosphere. The Ross Sea in Antarctica mainly owes its present-day configuration to an extended period of early Paleozoic subduction-related crustal accretion and the subsequent Mesozoic–Cenozoic tectonics of the West Antarctic Rift System. It thus represents an ideal setting in which to study reactivation processes. This study uses the 40 Ar- 39 Ar laserprobe technique in conjunction with mesostructural, microtextural, and nanotextural analyses to unravel the style and timing of a newly discovered mylonite-pseudotachylyte association from the rift shoulder in northern Victoria Land of Antarctica. Pseudotachylyte-bearing fault rocks overprint a reverse ductile shear zone developed in a Cambrian granite, within an overall transpressional right-lateral kinematic scenario. In situ 40 Ar- 39 Ar analyses of biotite from the mylonite mainly yielded ages of 460–440 Ma, suggesting that ductile shearing occurred during the early Paleozoic orogenic cycle. In contrast, in situ data on the pseudotachylyte matrix yielded scattered and much younger ages of ca. 120–66 Ma. Younger ages of ca. 50 Ma were derived from step-heating experiments on matrix specimens obtained through microtexturally controlled microsampling. Taking into account the possible effects of analytical artifacts due to sample irradiation and of Ar loss due to the ultrafine grain of the matrix, coseismic faulting is assigned to the middle Eocene. Results indicate a minimum 390 m.y. period of tectonic quiescence and prove that brittle reactivation occurred in response to a totally different stress regime. Regionally, the study confirms a post–early Eocene age for the activation and propagation of intraplate dextral strike-slip tectonics in Victoria Land.