A. Di Giulio

Combination of Single-Grain Fission-Track Chronology and Morphological Analysis of Detrital Zircon Crystals in Provenance Studies: Sources of the Macigno Formation (Apennines, Italy)

Fission track (FT) analyses on unannealed detrital min- erals provide a powerful tool both for refining provenance models de- rived from traditional methods and for collecting information about erosion rates of the source area. Their power is increased if they are coupled with the study of zircon morphology. This combination of methods is applied to the Chattian-Aquitanian (25-23 Ma) Macigno turbidite complex. Basin-fill patterns and petrographical studies con- sistently identify the uplifting western Central Alps as the main source region for the Macigno Formation. Most zircon grains fall into a young age cluster ( ; 40-30 Ma), de- rived from a rapidly exhuming crystalline source region with a high cooling rate. Within this cluster, two age subgroups can be distin- guished at 30 and 40 Ma. In the younger subgroup, the zircon mor- phology supports the presence of two main populations: (1) from ig- neous rocks (S-type euhedral zircons), which appear to be partly de- rived from airborne tuffs; and (2) from metasedimentary units. In huge volumes of these metamorphic rocks, mica Ar-Ar and zircon fission- track thermochronometers have been reset, because of high geothermal gradients in the vicinity of the Periadriatic intrusives in mid-Oligocene times. At the present surface of the Alps, zircon FT ages around and slightly less than 30 Ma are reported in the Sesia-Lanzo zone, the Gran Paradiso Massif, the Upper Pennine nappes, the Monte Rosa Massif, and the Dent Blanche complex. The older subgroup of the Tertiary zircons (40 Ma) may have been supplied by metamorphic and mig- matitic rocks affected by an Eocene high-temperature phase. A Late Cretaceous age cluster ( ; 70-60 Ma) is related to cooling after the main Austroalpine metamorphic event at 110-100 Ma. Most of the recently exposed Austroalpine nappe complex displays mica cooling ages and zircon FT ages between 95-70 Ma and 99-55 Ma, respectively. Finally, an ill-defined Jurassic age cluster, with a mean in Late Ju- rassic times, is related to rift-shoulder heating of the Austroalpine/ South-Alpine crystalline basement due to rifting of the Pennine oceanic domain. Presently, the Silvretta nappe complex, situated at the western termination of the Austroalpine realm, and the South-Alpine basement west of the Canavese Line, display similar zircon FT ages. Therefore, a westward continuation of the Silvretta complex prior to deep Neo- gene erosion is suggested.

Factors controlling foredeep turbidite deposition: the case of Northern Apennines (Oligocene-Miocene, Italy)

Abstract Three major controlling factors affect turbidite deposition in foredeep basins: tectonics in the source area, tectonics in the belt-basin system, and variations of sea-level (local or global). These factors are expected to have different effects on the volume, grain size, provenance and distribution of clastic sediments during the evolution of the basin. The interplay of these factors is investigated for the latest Oligocene-Middle Miocene Northern Appennines Foredeep turbidite wedges by means of turbidite-system-based lithostratigraphy and field mapping, integrated with nannoplankton biostratigraphy and sedimentary petrography. Almost all recognized turbidite systems, unless tectonically truncated, show an overall stacking pattern formed by a lower sand-rich, thickly bedded stage (depocentre stage) passing upward into mud-rich, thinly bedded stages, eventually grading up to mostly mudstone units (abandonment stage). This rhythmically repeated pattern is interpreted as the result of the abrupt switching on and off of coarse-grained input, coupled with an alternating increase/decrease of depositional rate recorded in all detected systems. Biostratigraphy makes it possible to distinguish the switching-off of turbidite systems due to depocentre migration (a new system is switched on basinward) from that due to a regional decrease of clastic input. Sandstone petrography records the compositional variation related to tectonically induced source reorganization. In the latest Oligocene-Middle Miocene NAF foredeep wedges, this integrated dataset allows us to recognize: two different phases of source tectonics in the latest Oligocene and the middle Burdigalian; two major episodes of basin tectonics and related depocentre shift in the latest Oligocene and the Langhian, plus a minor middle Aquitanian phase; and three intervals of reduced regional turbidite deposition during the Late Aquitanian, Middle Burdigalian and Early Serravallian, possibly linked to sea-level rises.

The sedimentary record of the exhumation of a granitic intrusion into a collisional setting: the lower Gonfolite Group, Southern Alps, Italy

Abstract The clastic wedge of the Gonfolite Lombarda Group (GLW) accumulated during Oligocene–Miocene times in the Southern Alps foreland basin, formed on the southern, inner side of the Alpine belt. It represents the depositional counterpart of the exhumation and erosion of the Central Alps metamorphic–magmatic units. Among the Central Alps units, the Tertiary Bergell Intrusion (TBI) is one of the principal sources of pebbles occurring within the GLW. Geochronologic data, both from intrusive pebbles and present-day outcrops of intrusive rocks, document the rapid uplift history of the GLW source area. The lower Gonfolite clastic wedge (Como Conglomerate and Val Grande Sandstone Formations, Oligocene–Early Miocene) has been investigated through the study of sandstone and conglomerate petrology for detecting the effects in the sedimentary record of this collision-related event. The main results are: (i) sandstone petrology of the Como Conglomerate records an evolution from feldspatholithic to feldspathic sandstones; (ii) the related Q/F–F/L ratios suggest an evolution from a mixed plutonic–metamorphic to a mainly plutonic source; (iii) consistently, conglomerate petrology records a progressive increase of plutonic pebbles (from nearly 0–50% of the total), a corresponding decrease of metamorphic clasts (from nearly 80 to nearly 50%) and the disappearance of cover rock fragments. Considering the high relief/short transport setting of the GLW clastic routing system, these values probably resemble the real proportions of such rocks in the Gonfolite catchment area. During the Aquitanian, the return to a metamorphic-rich source is recorded both by sandstones and conglomerates at the top of the Como Conglomerate and in the Val Grande Sandstone. This last signal is interpreted as the result of the reorganisation of the Gonfolite source area, possibly related to the northward shift of the main Alpine divide.

Mass transfer from the Alps to the Apennines: volumetric constraints in the provenance study of the Macigno–Modino source–basin system, Chattian–Aquitanian, northwestern Italy

A quantitative provenance approach is applied to the Late Oligocene–Early Miocene Macigno–Modino turbidite complex which accumulated in the foreland basin developed in front of the growing Northern Apennines, but was supplied from the rapidly rising Western Alps, at the northwestern end of the basin. As well as the traditional recognition of source rock lithologies and their gross paleogeographic location, the provenance approach adopted here evaluates the relative contributions to the clastic complex from different types of source rock, and estimates the volume of material transferred from the Alps to the Apennines within the Macigno–Modino source–basin system, and the overall volume of rock assumed to have undergone erosion in order to supply it. These elements are used to constrain the possible source of the clastics and to infer the main paleogeological features of the source area. It is concluded that an estimated volume of ca. 17–20×103 km3 of basement (mainly metamorphic) rocks, together with ca. 103 km3 of intermediate composition volcanic rocks was transferred as detritus from the Western Alps area to the Northern Apennines foreland basin, creating the 250–300 km long, 50 km wide and up to 2.7 km thick Macigno–Modino clastic complex. Lithology and paleogeography, combined with volume estimates and current knowledge of the regional geology of the Western Alps, consistently suggest that the bulk of the sediments funnelled into the Macigno–Modino complex were derived from the uplifted Ivrea crustal block, which formed a crystalline massif some 5000 km2 in extent, that rose ca. 3 km during the Oligocene, forming one of the main elements of Western Alpine geology during that time.

Oligocene-Early Miocene tectonic evolution of the northern Apennines (northwestern Italy) traced through provenance of piggy-back basin fill successions

Abstract The provenance history of sediments deposited in the piggy-back basins of the Northern Apennines has been drawn by means of a petrographic study of nearly 200 sandstone samples collected over 250 km of the belt; it allows the evolution of the eroded part of the belt in Oligocene-Early Miocene times to be determined in detail, with special emphasis on the age of the exhumation and the onset of erosion of the high-pressure/low-temperature Pennine metamorphic units of the Ligurian Alps and Corsica that form the innermost part of the chain. Five petrofacies were distinguished, representing three sources that were active separately (three ‘pure’ petrofacies) or together (two ‘mixed’ petrofacies). The resultion sandstone composition reflects the erosion of different source units, changing through time and space along the belt. The stratigraphic distribution of petrofacies records a change in the main clastic source from Ligurian calcareous units to Penninic units. This change occurred over most of the study area, reflecting the complete exhumation of the Penninic metamorphic units within the innermost part of the belt. It occurred at different times along the chain, migrating from northwest to southeast from Late Rupelian to Aquitanian. This time shift is interpreted to be related to the obliquity of the Northern Apennines convergent system.

Contrasting fluid events giving rise to apparently similar diagenetic products; late-stage dolomite cements from the Southern Alps and central Apennines, Italy

Abstract Precipitation of dolomite cements in Jurassic carbonate platform sediments and slope breccias has been studied from well cores and outcrops of the central Southern Alps and central Apennines in Italy. In both areas, an initial, massive dolomite replacement was followed by multiphase precipitation of dolomite cements. The replacement occurred during burial, in a passive margin regime, in response to compaction-driven flow of formational fluids. This interpretation is based on results from fluid inclusion and stable isotope analyses which have been related to the thermal history. The dolomite cements precipitated when both areas were involved in collisional tectonics. In spite of the similar diagenetic evolution, the fluids causing dolomite cementation in each case were compositionally different. In the Alps a decrease in salinity was recorded from sea water to brackish fluids, whereas in the Apennines an increase in salinity from sea water up to >10% NaCl equivalent was recorded. The remarkable salinity differences in diagenetic fluids are considered to be related to the different sub-aerial relief of the two belts during dolomite precipitation. In the Alps, the dilution of fluids is related to the infiltration of meteoric waters from the mountain chain, that was widely emergent. In the Apennines, dolomite cements precipitated whilst the structural units were still widely submerged, preventing meteoric dilution of cementing fluids and promoting an increase in salinity through mixing with fluids rising from older evaporate-bearing layers. In both Alpine and Apennine cases, the same diagenetic trend is observed in thrust-fold belt and foreland basin units; in both structural systems the diagenetic events start precipitating dolomite cements in the inner part of the collision zone and then the diagenetic processes migrate towards the foreland basin along with the structural evolution of the area.

Integrated analyses constraining the provenance of sandstones, a case study: the Section Peak Formation (Beacon Supergroup, Antarctica)

Sandstones of the Section Peak Formation (late Triassic?‐Middle Jurassic), Northern Victoria Land, Antarctica, record a change of palaeotectonic regime, from a stable continental basement setting to the beginning of rifting indicated by widespread basaltic volcanism during the Middle Jurassic. The provenance study of sandstones was performed by integrating conventional microscope study of detrital mode, a varietal study of quartz grains, the detection of mineral inclusions in detrital quartz, and electron microprobe analyses of detrital garnets. This provides the basis for discussing the validity and limits of each of these approaches, highlighting the potential source of pitfalls in using a single provenance approach, especially when clastics related to rapidly changing geotectonic regimes are studied. The resulting palaeogeographic picture is of a braided stream system draining a low relief, crystalline basement area, composed mainly of granitoid and high-grade metamorphic rocks, intermittently interacting with coeval basaltic lava flows that were partially reworked by fluvial currents.

Rock petrophysics v. performance of protective and consolidation treatments: the case of Mt Arzolo Sandstone

Abstract The petrophysical characteristics of a building stone used in the city of Pavia, northern Italy, are analysed in the light of stone conservation through the application of water repellent and consolidant products. The research focuses on the modification of petrophysical properties as a function of the applied products, and on the critical assessment of the performances of fluorinated and siloxane resins as a function of the variable nature of the same lithotype. The studied material is a calcareous sandstone (Mt Arzolo Sandstone), Late Miocene-Early Pliocene in age, extensively employed as a building material during the 11th and 12th centuries. Experimental investigations on samples from historic quarries were performed before and after application of treatments: i.e. petrographical and fabric analyses; ultrasonic measurements; mercury porosimetry; abrasion resistance measurements; contact angle measurements; capillary and low-pressure water absorptions; water vapour permeability; and colour measurements. Two main lithotypes of Mt Arzolo Sandstone were recognized according to their petrophysical features: the open porosity being similar, differences exist concerning their fabric. These differences influence the physical-mechanical properties of the material and consequently the performances of the applied products. In particular, the difficulties in penetration of products when sandstone of smaller pore size is treated can lead to a significant reduction of the overall performances, which is more evident for products containing organic solvent with high molecular weight.

Textural analysis of ancient plasters and mortars: reliability of image analysis approaches

Abstract Different image analysis (IA) methods have been developed to compute textural parameters of ancient plasters and mortars using standard petrographic thin sections. These IA routines were applied to samples of materials with different technological characteristics from three historical buildings in the city of Pavia (Northern Italy), covering a period from the 12th to the 19th century. The IA techniques tested in this study belong both to classical digital image processing and to neural network modelling. In the first case, analyses were performed by commercial IA software whereas in the second case a Multi-Layer Perceptron neural network (MLP) was tested. Digital image analysis was performed on images taken by means of a petrographic microscope; additionally, analysis of back-scattered electron (BSE) images was performed. Textural data obtained through the IA applied to thin sections were compared with the data from traditional point counting and mechanical sieve analysis of disaggregated samples of the same materials. The results show that the IA of thin sections provides robust results in a fast and easy way. However, the reliability of the analyses can be prejudiced by textural and compositional heterogeneity of the samples.