Mapping and evaluating kinematics and stress/strain field at active faults and fissures: a comparison between field and drone data at NE Rift, Mt Etna (Italy)

We collected drone data to quantify the kinematics at extensional fractures and normal faults, integrated this information with seismological data to reconstruct the stress field, and critically compared the results with previous fieldwork to assess the best practice. As key site, we analysed a sector of the North-East Rift of Mt Etna, an area affected by continuous ground deformation linked to gravity sliding of the volcano's eastern flank and dyke injection. The studied sector is characterized also by the existence of eruptive craters and fissures and lava flows. This work shows that this rift segment is affected by a series of NE-striking, parallel extensional fractures characterized by an opening mode along an average N105.7° vector. Normal faults strike parallel to the extensional fractures, although they tend to bend slightly when crossing topographic highs corresponding to pyroclastic cones. The extensional strain obtained by cumulating the net offset at extensional fractures with the fault heave gives a stretching ratio of 1.003 in the northeastern part of the study area and 1.005 in the southwestern part. Given a maximum age of 1614 yr AD for the offset lavas, we obtained an extension rate of 1.9 cm/yr for the last 406 yr. The stress field is characterised by a σHmin trending NW-SE. Results indicate that Structure-from-Motion photogrammetry applied to drone surveys allows to collect large amounts of data with a resolution of 2–3 cm, a detail comparable to field surveys. In the same amount of time, drone survey can allow to collect more data than classical fieldwork, especially in logistically difficult rough terrains.

Tectonism and volcanism enhanced by deglaciation events in southern Iceland

Southern Iceland is one of the main outlets of the Icelandic ice sheet and is subject to seismicity of both tectonic and volcanic origins along the South Iceland Seismic Zone (SISZ). A sedimentary complex spanning Marine Isotopic Stage 6 (MIS 6) to the present includes evidence of both activities. It includes a continuous sedimentary record since the Eemian interglacial period, controlled by a rapid deglaciation, followed by two marine glacioisostasy-forced transgressions, separated by a regression phase connected to an intra-MIS 5e glacial advance. This record has been constrained by tephrostratigraphy and dating. Analysis of this record has provided better insights into the interconnectedness of hydrology and volcanic and tectonic activity during deglaciations and glaciations. Low-intensity earthquakes recurrently affected the water-laid sedimentation during the early stages of unloading, accompanying rifting events, dyke injection, and fault reactivations. During full interglacial periods, earthquakes were significantly less frequent but of higher magnitude along the SISZ, due to stress accumulation, favored by low groundwater levels and more limited magma production. Occurrence of volcanism and seismicity in Iceland is commonly related to rifting events. Subglacial volcanic events seem moreover to have been related to stress unlocking related to limited or full unloading/deglaciation events. Major eruptions were mostly located at the melting margin of the ice sheet.

Structural analysis of superposed fault systems of the Bornholm horst block, Tornquist Zone, Denmark.

The Bornholm horst block is composed of Precambrian crystalline basement overlain by Palaeozoic and Mesozoic cover rocks. The cover intervals are separated by an angular unconformity and a hiatus spanning the Devonian through Middle Triassic interval. Late Palaeozoic faulting of the Early Palaeozoic Baltica platform is correlated with early-middle Carboniferous deformation in the Variscan foreland and with faulting associated with dolerite dyke injection in Skåne in the Late Carboniferous – Early Permian. The Palaeozoic fault systems are striking NW-SE and WNW-ESE and the platform series are dipping towards the SE and ESE respectively. The Mesozoic faulting was associated with the development of a horst-graben framework in the Bornholm-Skåne segment of the Sorgenfrei-Tornquist Zone. Mesozoic fault subsidence started in the Rønne Graben in the Triassic. In the Jurassic the Arnager-Sose block became active, cut off from the Bornholm block; in addition the Læså Graben (new) and the Øle Å fault block complex (new) were cut into the central Bornholm block from the south. In the Late Cretaceous the central Bornholm block was perforated by isolated fault blocks, i.e. the Nyker block, the Bøsthøj block and the Lobbæk block (new) along with subsidence of the Holsterhus block and renewed subsidence of the Arnager-Sose block. The Mesozoic series are dipping towards the southwest. The Palaeozoic fault systems were associated with two-dimensional plane strain during ENE-WSW and NNE-SSW extension. By contrast the Jurassic and Cretaceous fault systems were associated with three-dimensional strain with maximum extension striking NE-SW and secondary extension striking NW-SE. The Mesozoic palaeostress fields were associated with the break down of the Pangea supercontinent.

Hybrid magma generation preceding Plinian silicic eruptions at Hekla, Iceland: evidence from mineralogy and chemistry of two zoned deposits

Hekla is a Holocene volcanic ridge in southern Iceland, which is notable for the link between repose periods and the composition of the first-erupted magma. The two largest explosive silicic eruptions, H4 and H3, erupted about 4200 and 3000 years ago. Airfall deposits from these eruptions were sampled in detail and analysed for major and trace elements, along with microprobe analyses of minerals and glasses. Both deposits show compositional variation ranging from 72 % to 56 % SiO2, with mineralogical evidence of equilibrium crystallization in the early erupted rhyolitic component but disequilibrium in the later erupted basaltic andesite component. The eruptions started with production of rhyolitic magma followed by dacitic to basaltic andesite magma. Sparse crystallization of the intermediate magma and predominant reverse zoning of minerals, trending towards a common surface composition, indicate magma mixing between rhyolite and a basaltic andesite end-member. The suggested model involves partial melting of older tholeiitic crust to produce silicic magma, which segregated and accumulated in deep crustal reservoir. Silicic magma eruption is triggered by basaltic andesite dyke injection, with a proportion of the dyke magma contributing to the production and eruption of a mixed hybrid magma. Both the volume of the silicic partial melt, and the proportion of the hybrid magma depend on the pre-eruptive repose time.

Mechanism of emplacement and crystallisation history of the northern margin and centre of the Galway Granite, western Ireland

ABSTRACTThe main phase (∼400 Ma) emplacement of the central and northern part of the reversely zoned Galway Granite was incremental by progressive northward marginal dyke injection and stoping of the 470–467 Ma Connemara metagabbro-gneiss country rock. The space was provided by the synchronous ESE-opening, along the strike of the country rocks, of extensional fractures generated successively northward by a releasing bend in the sinistrally moving Skird Rocks Fault or an equivalent Galway Bay Fault. This fault is a prolongation of the Antrim–Galway (a splay off the Highland Boundary Fault) and Southern Upland Faults. The ESE-strike of the spalled-off rocks controlled the resultant ESE-elongated shape of the batholith. The magma pulses (∼5–30 m in thickness) were progressively more fractionated towards the northern margin so that the coarse Porphyritic (or Megacrystic) Granite (GP; technically granodiorite) in the centre was followed outwards by finer grained, drier and more siliceous granite, until the movements opening the fractures ceased and the magma became too viscous to intrude. ‘Out-of-sequence’ pulses of more basic diorite-granodiorite (including the Mingling–Mixing Zone) and late main phase, more acid, coarse but Aphyric Granite, into the centre of the batholith, complicated the outward fractionation scheme. The outward expansion, caused by the intrusions into the centre, caused a foliation and flattening of cognate xenoliths within the partly crystallised northern marginal granite and in the Mingling–Mixing Zone to the south.Late phase (∼380 Ma) central intrusions of the newly-discovered aphyric Shannapheasteen Finegrained Granite (technically granodiorite), the Knock, the Lurgan and the Costello Murvey Granites, all more siliceous and less dense than the GP, were emplaced by pushing up the already solid and jointed GP along marginal faults. This concentration of lighter granites plus compression shown in thrusting, caused overall fault uplift of the Central Block of the Galway batholith so that the originally deepest part of the GP is exposed where there is the most late phase granite. Chemical analyses show the main and late phase magmas, including late dykes, were very similar, with repetition of the same fractionation except that the late phase magmas were drier and more quickly cooled, giving finer grained rocks.

First direct age determinationfor the Kelseaa Dolerite Dyke, Bornholm, Denmark

The Danish island Bornholm on the southwestern margin of the Baltic Shield was subject to dyke injection during the Proterozoic. The dykes probably result from several magmatic events. We present U-Pb geochronological data for the largest of the dykes, the tholeiitic Kelseaa dyke. The resulting age, 1326 ±10 (2σ) Ma, places the dyke significantly earlier in the Proterozoic than previously assumed. No other dykes of this age have been reported from the western part of the Baltic Shield. The NE–SW strike of the Kelseaa dyke is evidence for extension oblique to the border of the Baltic Shield.The Kelseaa dyke is the first evidence for this event that was subsequent to the emplacement of theBornholm and Karlshamn (SE Sweden) granites and prior to the intrusion of the Central Scandinavian Dolerite Group, and possibly also the majority of mafic dykes on Bornholm.