Sets Of Dikes Research Articles

Contrasting constraints on the temporal and spatial extents of normal faults from the Hilltop and Lewis mining districts, northern Shoshone Range, Nevada, USA

Mapping of rock types, structural geology, and hydrothermal alteration, supported by geochronology and thermochronology, sheds light on the original spatial relationships of hydrothermal systems to intrusions in the northern Shoshone Range in north-central Nevada. Rocks in the Hilltop district are tilted ~35–40°E, as indicated by orientations of flattened pumice fiamme and bedding in sedimentary rocks along a single set of presently low-angle normal faults that initiated at 60–70°W dips. New U-Pb zircon geochronology from two sets of dikes in the Lewis district could suggest late Eocene–early Oligocene extension, but definitive crosscutting relations are lacking to demonstrably support this potential earlier period of normal faulting. Reinterpretation of previously reported apatite fission-track cooling ages with a new palinspastic restoration in the Lewis mining district concurs with middle Miocene extension as documented to the south at the Caetano caldera; however, the depth of burial of the Lewis district—and thus the significance of the apatite fission-track cooling ages—is uncertain. The comparable orientations and tilting history, supported by fault scaling relations, suggest that the temporally coincident extension in the Caetano caldera to the south represents the along-strike continuation of the same system of normal faults as in the Hilltop and Lewis districts, with changes in observed offset, percent extension, and fault spacing attributed to the gradual tipping out of the fault system northward.

Genesis of the Shirengou gold deposit, northern North China Craton, based on zircon U[sbnd]Pb, fluid inclusion, sulfide compositional and S isotope data

The newly discovered vein-type Shirengou gold deposit on the northern edge of the North China Craton contains at least 8.69 tons of Au at an average grade of 9.80 g/t. Six auriferous quartz veins have been identified within Upper Jurassic rhyolitic breccia-bearing crystal-lithic tuff, and they follow NW-trending secondary faults. Gold mineralization was closely related to silicification, with pyrite as the predominant ore mineral. Four mineralization stages can be discerned, involving Stage I milky quartz, Stage II quartz-pyrite, Stage III gray quartz-polymetallic sulfide, and Stage IV quartz-calcite ± pyrite. Zircon UPb dating of a pre-ore porphyryritic rhyolite dike sets an upper age limit on the gold mineralization of ~131.1 Ma. Two-phase and single-phase aqueous inclusions were observed in quartz related to all four mineralization stages. From Stage I to Stage IV, the fluid inclusion homogenization temperatures decrease from respectively 257–340 °C to 230–277 °C, 201–250 °C and 161–200 °C, with corresponding salinities of 3.39–9.21, 2.24–8.00, 1.22–7.59, and 0.18–3.55 wt% NaCl equivalent, respectively. Thus, the ore-forming fluid can be described as moderate-high temperature, moderate-low salinity H2O-NaCl fluids. The δ34S values of sulfides vary from 0.0 to 5.3 ‰, indicating that the S was derived primarily from magmatic source(s), with some contribution from the metamorphic rocks of the surrounding Jianping Group. In-situ LA-ICP-MS trace element analysis of sulfides, together with our new age data, suggest that the ore-forming fluid was initially of Early Cretaceous magmatic-hydrothermal origin, and subsequently modified by interaction with the wall rocks of the Jianping Group, extracting the ore-forming metals from these rocks into the fluids.

Geochronology and geochemistry of early Paleozoic magmatism in the Qilian orogen: Constraints on closure of the Proto-Tethys Ocean

The Proto-Tethys Ocean was a Neoproterozoic to early Paleozoic ocean that opened upon the breakup of Rodinia and closed by Paleozoic oceanic and continental subduction. The Qilian orogenic belt is the northernmost orogen of the Proto-Tethys domain and consists of the North Qilian belt, the Central Qilian block and the South Qilian belt. The processes responsible for extensive early Paleozoic magmatism in the Qilian orogen are crucial to understanding the closure of the ocean. This paper presents a systematic investigation of early Paleozoic mafic and felsic rocks in the Qilian orogen. Gabbroic dikes from the Central Qilian block have LA-ICP-MS zircon U-Pb ages of 499–497 Ma with a wide range of εHf(t) values of −9.94 to + 4.28 and TDM1 model ages of 1557–929 Ma, and dioritic dikes have a LA-ICP-MS zircon U-Pb age of 489 ± 3 Ma, positive εHf(t) values of +7.61 to +10.53, TDM1 model ages of 684–790 Ma, and an amphibole 40Ar/39Ar age of 486.3 ± 4.3 Ma. Both sets of dikes were derived from partial melting of a heterogeneous spinel-facies lherzolite lithospheric mantle that was modified by fluids and melts from a subducted slab. The protolith of amphibolites from the Hualong block in the South Qilian belt was a mafic intrusion of 454 ± 2 Ma with negative εHf(t) values of −9.69 to −7.79 and TDM1 model ages of 1390–1478 Ma. Its trace element signature indicates that sediment-derived melts were involved in the magma formation. An amphibole 40Ar/39Ar age of 418.7 ± 3.8 Ma from an amphibolite records the timing of amphibolite facies metamorphism. Felsic volcanic tuffs and granitic intrusions from the Central Qilian block have zircon U-Pb ages of 507–440 Ma with a broad range of zircon εHf(t) values of −9.60 to + 11.87 and TDM2 model ages of 2037–584 Ma. They have geochemical characteristics similar to I-type and S-type granites that were sourced from partial melting of juvenile and ancient continental crust with minor input of mantle materials. The formation of the felsic rocks indicates the transition from an arc-related setting to a post-collisional setting. Combining our results with existing data, we suggest that the Central Qilian block was affected by both southward subduction of the North Qilian ocean and the northward subduction of the South Qilian ocean, resulting in the formation of an Andean-type continental arc along the northern margin of the Central Qilian block and a subduction-accretion system along the southern margin of the Hualong block at ca. 530–440 Ma. Collision of the Central Qilian, Hualong, Quanji and Qaidam blocks at ca. 440–418 Ma marks the final closure of the Proto-Tethys Ocean.

Unraveling the tectonic setting and crystallization history of the Equatorial Atlantic Magmatic Province

In NE Brazil, Early Cretaceous mafic dikes and sills intrude the Precambrian Borborema Province and the Paleozoic Parnaíba basin, where they form the Equatorial Atlantic Magmatic Province. The Rio Ceará-Mirim dike swarm (RCM) is a giant arcuate structure ca. 1100 km in length composed of sets of dikes ranging from E-W to NE-SW. There are two additional giant swarms with WNW and NE-SW orientations spanning 380 km and 355 km, respectively, in the NW and SE regions of the Borborema Province. We investigated the tectonic setting and crystallization history of the mafic magmatism through orientation, dimensions (960 dikes), and 32 representative samples of plagioclase crystal size distributions (CSD) to shed light on the connection between the dikes and sills. Plagioclase compositions in EQUAMP intrusions indicate normal crystallization trends with the magma becoming more evolved towards the final stages of crystallization. CSD data suggests that residence times are similar for dikes and sills (≤ 1.5 yrs), although some dikes display higher residence times (3.6–1.8 yrs) in agreement with a longer crystallization history. These results suggest a deeper crystallization setting for the dikes in agreement with a plumbing system in that dikes penetrate the “stiffer” basement units to feed upward the sills accommodated between sedimentary layers with contrasted rheologies. The regional distribution of the mafic magmatism resulted from a bulk NW-trending extensional deformation, where the brittle (re)activation and control of pre-existing structures allowed the nucleation of the arcuate giant dike system and other satellite swarms. We also examine connections between tholeiitic igneous events in NE South America and tectonomagmatic episodes affecting the Pangea and West Gondwana landscapes in the Mesozoic Era.

Unzipping supercontinent Pangea: Geologic, potential field data, and buried structures, and a case for sequential Atlantic opening

Amalgamation of Pangea culminated with zippered N-to-S closing of the Theic ocean during the Alleghanian orogeny. Transpressional-rotational collision produced widespread dextral faulting throughout the eastern Appalachian hinterland, and thrust faulting in the western hinterland and foreland. The partially buried southern Appalachian Eastern Piedmont fault system is a product of late Paleozoic transpressional dextral faulting. Eastern Piedmont fault system faults are cut by two 200–190 Ma Central Atlantic Magmatic Province (CAMP) diabase dike sets (∼NW and NS), which postdate initial rifting, producing the Late Triassic-Early Jurassic basins. Sinistral reactivation of suitably oriented Paleozoic faults (e.g., Towaliga) that offset Jurassic dikes implies faulting is coeval with CAMP dike emplacement, or occurred very soon after 200–190 Ma dike emplacement. A large NE-striking sinistral fault (Estill fault) offsets magnetic and gravity highs in the Brunswick (Charleston) terrane in South Carolina and Georgia. This fault is at least 185 km long, with ca. 75–80 km maximum of sinistral separation. The Estill fault geometry agrees with the stress field associated with Central Atlantic Magmatic Province dike emplacement. We conclude that movement along the Estill and other sinistral faults represents Early Jurassic displacement that is well documented in eastern North America prior to Atlantic opening. The early Mesozoic post-failed rifting (basin formation) preceded dike emplacement and pre-spreading sinistral faulting. We recognize four stages in the Late Triassic–Jurassic tectonics and kinematics of eastern North America based on field, crosscutting relationships, and available geophysical data: (1) initial W-to-E failed transtensional rifting with formation of the S-to-N-younging Triassic-Jurassic basins; (2) CAMP diabase dikes intruded 200–190 Ma; (3) sinistral movement of large blocks of crust in the North Atlantic region including southeastern North America; and (4) spreading and opening of the Atlantic Ocean 195–170 Ma. These stages mark the reversal of Alleghanian rotational dextral transpressional (zippered) collision forming Pangea supercontinent and ‘unzipping’ the supercontinent prior to Atlantic opening.

Can the geometry of an injection clastic dike swarm indicate a short-term process? Examining the Dead Sea Basin in Israel

Despite the importance of identifying dynamic stress fields, doing so is not straightforward. This is because most geological processes occur slowly and the formation of fractures is generally attributed to long-term processes.Within the seismically active Dead Sea Basin (DSB), radial and concentric injection clastic dikes were emplaced during earthquake events, but a satisfactory explanation for their geometric configuration is lacking. In this study, field observations were used to explore whether the geometry of the injection clastic dike swarm reflects a quasistatic state of stress, such as with a salt diapir, or if it reflects a dynamic short-term stress state associated with the passage of seismic waves. The results show that the radial clastic dikes are not concentrated toward one central point located approximately 3 km east of the Ami'az Plain, that the concentric clastic dikes are arranged by three subsets, and that the spatial density and strain of both fracture sets are focused mainly toward the center of the Ami'az Plain. This evidence does not fit with a quasistatic state-of-stress interpretation of the region that would be associated with a local stress state around a salt diapir. A more plausible explanation is that the geometry of the radial and concentric clastic dike sets was controlled by short-term seismic waves that were probably generated by earthquake events located along an adjacent fault and that were trapped by the sedimentary basin located below the Ami'az Plain itself.The reconstruction of the injection clastic dike pattern may provide an opportunity to more deeply understand the short-term process induced by paleoearthquakes occurring adjacent to sedimentary basins close to the surface.

A new model for the optimal structural context for giant porphyry copper deposit formation

AbstractPorphyry-type deposits are the main global source of copper and molybdenum. An improved understanding of the most favorable structural settings for the emplacement of these deposits is necessary for successful exploration, particularly considering that most future discoveries will be made under cover based on conceptual target generation. A common view is that porphyry deposits are preferentially emplaced in pull-apart basins within strike-slip fault systems that favor local extension within a regional compressive to transpressive tectonic regime. However, the role of such a structural context in magma storage and evolution in the upper crust remains unclear. In this work, we propose a new model based on the integration of structural data and the geometry of magmatic-hydrothermal systems from the main Andean porphyry Cu-Mo metallogenic belts and from the active volcanic arc of southern Chile. We suggest that the magma differentiation and volatile accumulation required for the formation of a porphyry deposit is best achieved when the fault system controlling magma ascent is strongly misoriented for reactivation with respect to the prevailing stress field. When magmas and fluids are channeled by faults favorably oriented for extension (approximately normal to σ3), they form sets of parallel, subvertical dikes and veins, which are common both during the late stages of the evolution of porphyry systems and in the epithermal environment. This new model has direct implications for conceptual mineral exploration.

Geochemistry and Geochronology of the Neoproterozoic Backarc Basin Khzama Ophiolite (Anti-Atlas Mountains, Morocco): Tectonomagmatic Implications

The Khzama ophiolite is a highly dismembered complex located in the Siroua inlier of the Moroccan Anti-Atlas Belt. It consists of ultramafic rocks, cumulate gabbros, sheeted dikes, pillow lavas, and an overlying volcano-sedimentary sequence. Three main tectonic slices of sheeted dike complexes are studied in detail along three rivers, exposing well preserved outcrops where individual dikes are clearly distinguishable from the intruded host rock (Assif n’Tinzla, Assif n’Tasriwine, and Assif n’Iriri). Sheeted dikes of the Khzama ophiolitic complex are basaltic to andesitic in composition, displaying a clear sub-alkaline nature. We identify two sets of dikes that originate from lower High-Ti series (HTS) lavas and overlying upper Low-Ti series (LTS) lava. The immobile trace-element signatures of these rocks point to a genesis on a backarc environment with magmas sourced in a supra-subduction zone (SSZ) at the spinel peridotite zone. The obtained SHRIMP U-Pb data of the gabbro represent the first radiometric age of zircon extracted from the mafic rocks that were intruded by the sheeted dike complex of the Khzama ophiolite. These grains yield a concordia age of 763 ± 5 Ma, which is consistent with the 761.1 + 1.9/−1.6 and 762 + 1/−2 Ma U-Pb zircon ages of plagiogranites of Siroua. Based on their mineralogy, modal proportions, and major element chemistry, the felsic dikes are classified as high silica–low alumina trondhjemites or plagiogranites. These plagiogranites were likely formed by the partial melting of mafic rocks rather than by extreme fractional crystallization. A plagiogranite dated at 777 ± 4.7 Ma (U-Pb on zircon) is significantly older than the ca. 762 Ma plagiogranites previously recorded for the Khzama locality, suggesting a long-lived supra-subduction zone (SSZ) with conditions for the hydrous melting of mafic rocks.

Geologic Map of the Park Reservoir Quadrangle, Sheridan County, Wyoming

This project involved the construction of a detailed geologic map of the Park Reservoir, Wyoming 7.5-Minute Quadrangle (Scale 1:24,000). The Quadrangle occurs entirely in the Bighorn National Forest, which is a popular recreation site for thousands of people each year. This research advances the scientific understanding of the geology of the Bighorn Mountains and the Archean geology of the Wyoming Province. Traditional geologic mapping techniques were used in concert with isotopic age determinations. Our goal was to further subdivide the various phases of the 2.8–3.0 Ga Archean rocks based on their rock types, age, and structural features. This research supports the broader efforts of the Wyoming State Geological Survey to complete 1:24,000 scale geologic maps of the state. The northern part of the Bighorn Mountains is composed of the Bighorn batholith, a composite complex of intrusive bodies that were emplaced between 2.96–2.87 Ga. Our mapping of the Park Reservoir Quadrangle has revealed the presence of five different Archean quartzofeldspathic units, two sets of amphibolite and diabase dikes, a small occurrence of the Cambrian Flathead Sandstone, two Quaternary tills, and Quaternary alluvium. The Archean rock units range in age from ca. 2.96–2.75 Ga, the oldest of which are the most ancient rocks yet reported in the Bighorn batholith. All the Archean rocks have subtle but apparent planar fabric elements, which are variable in orientation and are interpreted to represent magmatic flow during emplacement. The Granite Ridge tear fault, which is the northern boundary of the Piney Creek thrust block, is mapped into the Archean core as a mylonite zone. This relationship indicates that the bounding faults of the Piney Creek thrust block were controlled by weak zones within the Precambrian basement rocks.

Rock and age relationships within the Talkeetna forearc accretionary complex in the Nelchina area, southern Alaska

Subduction zone processes are challenging to study because of the rarity of good exposures and the complexity of rock relationships within accretionary prisms. We report the results of field mapping and petrographic, geochemical, and geochronological analyses of the McHugh Complex accretionary prism mélange in south-central Alaska that was recently exposed due to retreat of the Nelchina Glacier. Our new mapping and analyses of the mélange, as well as adjacent Talkeetna arc intrusives, suggests that the previously mapped trace of the Border Ranges fault should shift northward in this location. Detailed petrographic analysis places this mélange exposure with the Potter Creek assemblage of the McHugh Complex. Blocks of pillow lavas within the mélange have both mid-ocean ridge basalt and intra-plate geochemical affinities, attesting to the complex relations of subduction-zone inputs in an alternating erosive–accretionary margin. A new zircon U–Pb age and geochemical analyses of a set of felsic dikes that cross-cut the accretionary sequence provide constraints on the regional tectonic evolution, including near-trench plutonism associated with the migration of a subducting spreading ridge along the southern Alaska margin during the Paleocene–Eocene. The McHugh section and cross-cutting dikes in this location are pervasively hydrothermally altered, which we attribute to elevated temperatures related to ridge subduction. Late-stage motion along the Border Ranges fault system, which is also recorded in the area, may also have contributed to the widespread alteration. Our data indicate that the Talkeetna volcanic arc and associated accretionary prism sediments were in their current configuration by 55 Ma.

Early syn-rift igneous dike patterns, northern Kenya Rift (Turkana, Kenya): Implications for local and regional stresses, tectonics, and magma-structure interactions

AbstractFour areas (Loriu, Lojamei, Muranachok-Muruangapoi, Kamutile Hills) of well-developed Miocene-age dikes in the northern Kenya Rift (Turkana, Kenya) have been identified from fieldwork and satellite images; in total, >3500 dikes were mapped. Three areas display NNW-SSE– to N-S–oriented dike swarms, with straight, radial, and concentric patterns in zones <15 km long, and indicate NNW-SSE to N-S regional maximum horizontal principal stress (SHmax) directions in the early to middle Miocene. Individual dikes are typically <2 m wide and tens to hundreds of meters long and have accommodated <2% extension. In places (Loriu, Lojamei, Lokhone high), dikes trend at a high angle to the rift trend, suggesting some local influence (e.g., overpressured magma chamber, cracked lid–style dike intrusions over a sill or laccolith, preexisting fabric in basement) on orientation, in addition to the influence from regional stresses. Only a minor influence by basement fabrics is seen on dike orientation. The early- to middle-Miocene dikes and extrusive activity ended a long phase (up to 25 m.y.) of amagmatic half-graben development in central Kenya and southern Turkana, which lay on the southern edge of the early (Eocene–Oligocene) plume activity. The Miocene dike sets and extension on major border faults in Turkana contrast with larger, more extensive arrays of dikes in evolved systems in the Main Ethiopian Rift that are critical for accommodating crustal extension. By the Pliocene–Holocene, magmatism and intrusion along dikes had become more important for accommodating extension, and the tectonic characteristics began to resemble those of rift basins elsewhere in the eastern branch of the East African Rift.

Nature of Cretaceous dolerite dikes with two distinct trends in the Damodar Valley, eastern India: Constraints on their linkage to mantle plumes and large igneous provinces from 40Ar/39Ar geochronology and geochemistry

AbstractTwo distinct sets of Cretaceous dolerite dikes intrude the Chhotanagpur gneissic complex of eastern India, mostly within the Damodar Valley Gondwanan sedimentary basins. One dike set trends NNE to ENE, whereas the other set, which includes the prominent Salma dike, trends NW to NNW. One dike from each set in the Raniganj Basin was dated using the 40Ar/39Ar method in order to resolve a controversy concerning the emplacement age of the Salma dike. The NE-trending dike yielded a plateau age of 70.5 ± 0.9 Ma, whereas the NNW-trending Salma dike is much older, with a plateau age of 116.0 ± 1.4 Ma. These results demonstrate that the Salma dike was emplaced at ca. 116 Ma and not at ca. 65 Ma, as suggested in an earlier study. Geochemical characteristics of the two dikes are also distinct and indicate that they belong to previously identified high-Ti and low-Ti dolerite groups, respectively. The observed geochemical characteristics of both dike sets are comparable with the geochemistry of basalts of the Kerguelen Plateau, Bunbury Island, and Rajmahal Group I and suggest a connection to mantle plumes. The new age data presented herein indicate that these two magmatic episodes in the eastern Indian Shield were related to the ca. 120–100 Ma Kerguelen mantle plume and its associated Greater Kerguelen large igneous province and the ca. 70–65 Ma Réunion plume and its associated Deccan large igneous province, respectively.

Plutonism in three dimensions: Field and geochemical relations on the southeast face of El Capitan, Yosemite National Park, California

Detailed geologic mapping on the ∼1-km-tall, vertical southeast face of El Capitan was completed to determine the chronology and geometry of emplacement. Field relations reveal a complex intrusive history at the boundary between two intrusive suites involving interaction between several granitic units. No resolvable faulting or other postemplacement deformation was observed. New U-Pb zircon geochronologic data (laser ablation and isotope dilution) demonstrate assembly of the El Capitan Granite and diorites of the Rockslides and North America between ca. 106 and 103 Ma. New ages for the Taft (106.6 ± 0.7 Ma), Leaning Tower (104.1 ± 0.10 Ma), and Bridalveil (103.4 ± 0.4 Ma) plutons reveal that they intruded over the same interval as the other plutonic rocks exposed on the face of El Capitan, although field relations and geochronology suggest a distinct order of emplacement. Two sets of aplite dikes are also exposed. Their whole-rock compositions suggest segregation at depths of 5–6 km and derivation from the intrusive suites of Yosemite Valley or Buena Vista Crest. Chemical analysis of samples collected along ∼1-km-tall vertical transects through the El Capitan and Taft Granites reveals no systematic variations in major or trace elements. Analysis of 78 photographs within the El Capitan Granite also shows no systematic variations in texture or mineralogy with elevation. Lack of resolvable vertical variations in both field and petrologic observations is consistent with incremental assembly, and is hard to reconcile with models that envision magma chambers as large fractionating bodies.

Abu Rusheid lamprophyre dikes, South Eastern Desert, Egypt: as physical-chemical traps for REEs, Zn, Y, U, Cu, W, and Ag

Five lamprophyre dikes (L1-L5) intruded Abu Rusheid rocks (cataclastics and monzogranite) in the South Eastern Desert of Egypt along shear zones with NNW-SSE and E-W. The two sets of dikes differ in age, mineralization, and geochemical aspect. The NNW-SSE trending dikes (L1 and L2) are polymineralized and dislocate the E-W (L3, L4, and L5) trending dikes. These dikes underwent multistage of hydrothermal processes (ferrugination, fluoritization, kaolinitization, and calcification). They are characterized by common box works (physical trap) filled by incoming mineralization, and their feldspars and micas are relatively altered to clay minerals (chemical trap). From the mineralogical point of view, the cataclastic rocks are rich in base metals (pyrite, sphalerite, molybdenite, chalcopyrite, and galena), radioactive minerals (thorite, uranophane, kasolite, uranothorite), and accessories phases (zircon, xenotime, cassiterite, molybdenite, copper, columbite, and fluorite). L1 and L2 lamprophyre dikes contain U minerals (uranophane, kasolite, autunite, and torbernite), Mn-franklinite, woodruffite, xenotime, fluorite, silver, copper, and scheelite. L3 contains Zn and U minerals, whereas L4 and L5 contain Zn minerals only. The source magma producing lamprophyre dikes (L1 to L4) are peralkaline to alkaline, whereas L5 lamprophyres are mainly calc-alkaline in character; they were generated from the mantle and had formed in a post-collision tectonic setting with extensive Ti-rich metasomatism. The average ΣREE content in (L1 and L2) lamprophyres is 8006 ppm. REE patterns display clear concave upward pattern of W-type tetrad effect in the REE pattern accompanied by enrichment of the HREE over the LREE and marked negative Eu anomaly due to secondary mobilization of REEs.

A spatiotemporal optimization model for the evacuation of the population exposed to flood hazard

Abstract. Managing the crisis caused by natural disasters, and especially by floods, requires the development of effective evacuation systems. An effective evacuation system must take into account certain constraints, including those related to traffic network, accessibility, human resources and material equipment (vehicles, collecting points, etc.). The main objective of this work is to provide assistance to technical services and rescue forces in terms of accessibility by offering itineraries relating to rescue and evacuation of people and property. We consider in this paper the evacuation of an urban area of medium size exposed to the hazard of flood. In case of inundation, most people will be evacuated using their own vehicles. Two evacuation types are addressed in this paper: (1) a preventive evacuation based on a flood forecasting system and (2) an evacuation during the disaster based on flooding scenarios. The two study sites on which the developed evacuation model is applied are the Tours valley (Fr, 37), which is protected by a set of dikes (preventive evacuation), and the Gien valley (Fr, 45), which benefits from a low rate of flooding (evacuation before and during the disaster). Our goal is to construct, for each of these two sites, a chronological evacuation plan, i.e., computing for each individual the departure date and the path to reach the assembly point (also called shelter) according to a priority list established for this purpose. The evacuation plan must avoid the congestion on the road network. Here we present a spatiotemporal optimization model (STOM) dedicated to the evacuation of the population exposed to natural disasters and more specifically to flood risk.

Deep-seated fragmentation, transport of breccia dikes and emplacement: An example from the Borborema province, northeastern Brazil

Three syenite dike sets, named the Santa Cruz dikes, are coeval with the 627 ± 13 Ma old porphyritic calc-alkalic Princesa Izabel granitoid, northeastern Brazil. Dike set 1 is up to 1.5 m wide, strike 030–040 Az, roughly parallel to the regional foliation. Dike set 2 strikes 120°–130 Az and consists of xenolith-bearing syenites and is roughly parallel to dike set 3, which is up to 1.5 m wide and consists of xenolith-rich syenites forming matrix-supported breccias. Xenoliths in dike sets 2 and 3 are usually up to 3 cm long, angular to sub-rounded, tend to be evenly distributed and occupy ∼50% of the volume of dike set 3. They are amphibolite, mica-pyroxenite and diorite from deep source, and gneiss and feldspar xenocrysts from the conduit. The large amount of dense ultramafic/mafic xenoliths in the breccia dikes indicates rapid ascent of the host magma. The calculated natural viscosity, based on whole-rock chemical data, is 680–4600 Pa.s for 2.3 wt% water and temperatures from 1000 to 900 °C, respectively. A minimum ascent rate of ∼0.3 m/s is estimated from the settling velocity of a 30 cm-long diorite xenolith, the largest one, and an initial fraction of xenoliths of 5%. Progressive addition of xenoliths to the magma during its ascent increased the viscosity of the liquid–solid mixture during emplacement, and this would imply a Bingham rather than Newtonian behavior. These xenoliths were formed by early fracturing of wall rocks during dike propagation associated with thermal spalling of the wall rocks, by intrusion of magma along dike-parallel fractures during the development of a sequential conjugate pair of shear zones. Xenoliths are abundant not only because these magmas have ascended rapidly and could transport them, but also because the initial low viscosity of the magma promoted intense fracturing of the conduit.

Deformation of the substratum of a large shield volcano: Triggering factor for past flank collapses in the old volcanic edifice of La Gomera, Canary Islands

The Basal Complex of La Gomera (Canary Islands) has been analyzed with the aim of deciphering its structural evolution, spanning the middle to late Miocene. Detailed structural maps and cross sections, and the results of a systematic measurement of dike orientation and fault-slip data are presented in this work. The main structural features found in the Basal Complex of La Gomera are four dike sets and two large, normal faults (Guillama and Montana de Alcala faults). Several extensional episodes have been identified. The main deformation phase corresponds to a NNW-SSE– to NW-SE–directed extension that generated first a dense swarm of mafic dikes, and then a large-scale collapse of the Basal Complex with displacement along the Guillama and Montana de Alcala faults and associated rotation of large blocks of La Gomera basement. It is interpreted that the supposed staircase geometry of the Guillama fault is responsible for the observed arrangement of rotated dikes due to development of kilometric-scale fault-bend folds with rounded hinges. Deformation of the Basal Complex shows a complete geometric, kinematic, and chronologic consistency with the large volcanic flank collapses that affected the units belonging to the early growth stages of La Gomera subaerial shield volcano. The results of this work support the models that invoke the importance of the large-scale geodynamic setting on volcano destabilization. More attention should be paid to the structural and geophysical characteristics of volcano basements in order to better evaluate the danger of large, catastrophic volcanic landslide events.

Geophysical characterization of the Azimuth 125° lineament with aeromagnetic data: Contributions to the geology of central Brazil

This study analyzes a portion of the Azimuth 125° lineament (Az 125°), located in the Brazilian states of Goiás and Minas Gerais, based on its magnetic signature and previous studies. In particular, the determination of a chronology of events that shaped the Az 125° lineament is the focus of this contribution. Geological provinces with highly economically valuable mineralizations occur along Az 125, including the most important carbonatite and kimberlite complexes in Brazil. Az 125° is the main structural feature associated with these complexes, consisting of an extensive set of NW-SE oriented faults (approximately 850km long and 70km wide), however it is not mapped on geological maps at a regional scale as a continuous structural feature, because it is not consistently visible on the surface or in satellite images. Magnetic signature characterizes the lineament as a set of linear features with regional continuity in the subsurface. It is characterized by a higher magnetic susceptibility response that contrasts with the response exhibited by surrounding host rocks. Field data support geophysical data: occasional outcrops resulting from active erosion reveal dikes formed by gabbroic rocks and diabase and define the Az 125° lineament as a set of dikes of different ages. Other magnetic anomalies that occur along Az 125° are associated with intrusive rocks from the Goiás and Alto Paranaíba alkaline provinces. Based on magnetic signatures and mapped igneous rocks chronology, the Az 125° is partitioned, within the study area, into three main systems (called L1, L2, and L3). This subdivision, in conjunction with the dates assigned to various basic dikes within the azimuth, reveals that the evolution of the azimuth can be associated with three events. The first event occurred during the Brasiliano (950–520Ma). The second event occurred during the Gondwana fragmentation (starting circa 180Ma). The third event is related to the tectonomagmatic activity of The Trindade plume (90–80Ma). We posit that the L1 system includes the older dikes, and the L3 system includes the youngest dikes; the L2 system which is intersected by L3, is therefore, of intermediate age. The determination of a chronology of events that characterizes the segmented structure of the Az 125° lineament, here defined as L-systems (L1, L2, and L3), based on both geological and geophysical data, and not yet reported in earlier scientific articles, is the main contribution of this study.

Post-convergent structures in lower parts of the 1090–1050 Ma (early-Ottawan) thrust-sheet stack, Grenville Province of Ontario, southern Canadian Shield

Remnants of the early-Ottawan thrust-sheet stack are exposed in the Central Gneiss Belt (CGB, lower portion of stack) and the Composite Arc Belt (upper portion of stack). Post-collisional vertical thinning and associated horizontal extension of the stack produced structures ranging over eight orders of magnitude in horizontal length, and both orogen-parallel and orogen-perpendicular in orientation. At the 100 km scale, the fold-induced constriction in the northern Parry Sound domain appears to have been enhanced, and lineation trend lines in its footwall locally deflected, by a component of NW–SE (i.e., orogen-perpendicular) flattening and a component of NE–SW (i.e., orogen-parallel) ductile extension. At the 10 km scale, four non-cylindrical lenticular bodies of gabbro–anorthosite gneiss within the domain, inferred to be triaxial mega-boudins or heterogeneously strained plutons, are separated by large extensional bending folds, the complementary structures attesting to a component of NW–SE flattening and a component of NE–SW extension. Non-cylindrical lenticular structures in other domains of the CGB, interpreted as triaxial foliation mega-boudins, exceed 30 km in length. Their moderately strained granulite-facies interiors give way to highly strained amphibolite-facies margins, thus documenting subvertical ductile flattening and multi-lateral extension during retrogression. Well-layered, highly strained gneiss is commonly deformed by steep NE–SW-trending extensional faults and associated monoclinal fault-propagation folds (FPFs). The short limbs of the FPFs bend the regional elongation lineation and host a set of fault-parallel, unstrained to slightly deformed, granite–pegmatite dikes. Dilation vectors of most dikes are oblique to the granite–pegmatite contacts, and the sense of their tangential components attests to orogen-perpendicular extension. The fault-parallel dikes and associated FPFs are cut by a set of unstrained dikes. Collectively these observations document a prolonged history of post-collisional extension of the mid crust, from ductile structures indicative of a significant component of orogen-parallel extension shortly after the metamorphic peak at mid-crustal depths, to brittle–ductile structures indicative of a component of orogen-perpendicular extension and associated magmatic dilation following its exhumation and cooling in the upper crust.

Stratigraphy, fossil mammals, and age of the Eocene – Miocene volcano-sedimentary sequences of sierra Huantraico – sierra Negra and cerro Villegas (Neuquén Province, Argentina).

We present here the recent advances in understanding the stratigraphy and age of the Cenozoic volcanic and sedimentary sequences exposed on the area of sierra Huantraico – sierra Negra and cerro Villegas (Pehuenches Department, Neuquén Province). Stratigraphic analyses and recovered fossil remains allow us to recognize and differentiate six litostratigraphic units: a) The “Rodados Lustrosos” (late Eocene – early? Oligocene), composed of coarse-grained texturally mature conglomerates (up to 18 m thick), deposited on a strong paleorelief of eroded pre-Eocene sedimentary rocks (Inca diastrophic phase); b) The Rincón Escondido Formation ( nom. nov .) (early Oligocene – lower late Oligocene), composed of sandy fluvial deposits linked to a strong pyroclastic source (31 m), occupies and fills the same paleorelief on which the “Rodados Lustrosos” lies; c) The Sierra Negra Formation ( nom. nov .) (Aquitanian – Burdigalian, early Miocene) overlies a younger paleorelief that erodes into deposits as old as Early Cretaceous (Pehuenche diastrophic phase); this unit can be subdivided into the Filo Morado Member (up to 250 m of olivine-alkaline basalts, pyroclastic rocks, and volcanogenic deposits) and the Puesto Tillerías Member ( nom. nov. ) (up to 600 m of basalts and andesitic rocks with interbedded tuffs and tuffites); d) The Pichi Tril Formation (late Burdigalian – Langhian) is composed of andesitic intrusive bodies linked to deformation that occurred by the end of the early Miocene (Quechua diastrophic phase ); e) The Desfiladero Negro Formation (early Tortonian) forms a set of basic dikes that cross the rest of the succession – an intrusion related to earliest late Miocene deformation; f) The La Tiza Formation (late? Tortonian – early Messinian, late Miocene), composed of up to 85 m of coarse-grained oligomictic conglomerates with low textural maturity, originated from the erosion of volcanic bodies and basaltic flows of the Sierra Negra Formation. This entire succession was again deformed at the end of the Miocene (late Messinian). The discovery of fossil mammals in the Rincón Escondido and Sierra Negra formations, together with radiometric dates provided by previous authors, have allowed a geochronologic calibration of the volcano-sedimentary units exposed in this region.