Duration Of Volcanic Activity Research Articles

Paleomagnetism of the Permian-Triassic intrusions from the Norilsk region (the Siberian platform, Russia): Implications for the timing and correlation of magmatic events, and magmatic evolution

The Norilsk region is a key area within the Siberian Traps Large Igneous Province, due to the thickest sections of the Permian-Triassic extrusive rocks and high diversity of intrusions and volcanics in composition. The unique Cu-Ni-PGE deposits related to the Siberian Traps intrusions are present here. On the basis of the new representative paleomagnetic and geochemical data, we distinguished three stages of intrusive magmatic activity. At the initial stage, high-Ti trachydolerites of the Yergalakhsky complex were emplaced synchronously with the lower part of the Norilsk volcanic section. The middle stage was manifested by emplacement of ore-bearing intrusions of the Norilsk type, leucogabbro of the Kruglogorsky type, and barren gabbro-dolerites of the Oganer complex, coeval with the Morongovsky and Mokulaevsky volcanic formations (about 251.8–251.6 Ma). At this stage, magmatic activity occurred as a series of brief and powerful events. Finally, the terminal stage is represented by geochemically contrasting dikes of the Avamsky and Daldykan complexes, cutting the entire volcanic section of the Norilsk region. Our new paleomagnetic results together with U-Pb show that the Permian-Triassic boundary in the Norilsk volcanic section should be set at the base of the Morongovsky Formation (the middle part of the Norilsk tuff-lava pile), and the duration of volcanic activity after this boundary can be estimated as ~0.5 Myr.

Temporal‐Geochemical Evolution of the Persani Volcanic Field, Eastern Transylvanian Basin (Romania): Implications for Slab Rollback Beneath the SE Carpathians

AbstractThe Quaternary Persani volcanic field (PVF) consists of alkali basalts formed in an extensional basin at the SE end of the Transylvanian basin, near an important anomaly in the European mantle, the Vrancea slab, a seismically active near‐vertical lithospheric fragment of debated origin. The PVF is the only basaltic field regionally, has been studied geochemically in the past, and is also known for the presence of abundant mantle xenoliths. Here, we describe new geochemical data on rocks recently dated by Ar‐Ar chronometry and show that while we can reproduce virtually all previous results, there is a clear temporal evolution of the magmatic system. There is an increase of over 80°C in temperatures determined by the Si activity thermometer, from 1,300°C to 1,380°C during the ~0.5‐Myr duration of volcanic activity, which is accompanied by several coherent trends in geochemistry, among which the decrease of Zn/Fe and 87Sr/86Sr ratios over time. Earlier, higher Zn/Fe ratios are indicative of a pyroxenite/eclogite‐dominated source, which gradually changed to a peridotite‐dominated source. These characteristics are typical of a dynamic mantle in which vertical mantle lithosphere tectonics, either due to slab rollback or mantle dripping plays a role and are not consistent with simple decompression melting of asthenosphere. Synchronous adakitic rocks found about 25–30 km east of PVF are presumed to be slab melts and are consistent with the Vrancea slab rollback as the trigger for mantle melting responsible for the PVF.

Volcanic Structures Within Niger and Dao Valles, Mars, and Implications for Outflow Channel Evolution and Hellas Basin Rim Development

AbstractOutflow channel formation on the eastern Hellas rim region is traditionally thought to have been triggered by activity phases of the nearby volcanoes Hadriacus and Tyrrhenus Montes: As a result of volcanic heating subsurface volatiles were mobilized. It is, however, under debate, whether eastern Hellas volcanism was in fact more extensive, and if there were volcanic centers separate from the identified central volcanoes. This work describes previously unrecognized structures in the Niger‐Dao Valles outflow channel complex. We interpret them as volcanic edifices: cones, a shield, and a caldera. The structures provide evidence of an additional volcanic center within the valles and indicate volcanic activity both prior to and following the formation of the outflow events. They expand the extent, type, and duration of volcanic activity in the Circum‐Hellas Volcanic Province and provide new information on interaction between volcanism and fluvial activity.

The monogenetic Bayuda Volcanic Field, Sudan – New insights into geology and volcanic morphology

The small monogenetic Bayuda Volcanic Field (BVF; 480km2), comprising at least 53 cinder cones and 15 maar volcanoes in the Bayuda desert of northern Sudan is one of a few barely studied volcanic occurrences of Quaternary age in Sudan. The exact age of the BVF and the duration of volcanic activity has not yet been determined. Furthermore, not much is known about the eruptional mechanisms and the related magmatic and tectonic processes that led to the formation of the volcanic field. In the framework of a larger project focusing on these points it is the purpose of this contribution to provide a first account of the general geology of the BVF volcanoes as well as a first description of a general stratigraphy, including a first description of their morphological characteristics. This was done by means of fieldwork, including detailed rock descriptions, as well as the analysis of satellite images (SRTM dataset at 30m spatial resolution).The BVF cinder cones are dominated by scoracious lapilli tephra units, emplaced mainly by pyroclastic fallout from Strombolian eruptions. Many cones are breached and are associated with lava flows. The subordinate phreatomagmatism represented by maar volcanoes suggests the presence of ground and/or shallow surface water during some of the eruptions. The deposits constituting the rims around the maar volcanoes are interpreted as having mostly formed due to pyroclastic surges. Many of the tephra rings around the maars are underlain by thick older lava flows. These are inferred to be the horizons where rising magma interacted with groundwater. The existence of phreatomagmatic deposits may point to a time of eruptive activity during a phase with wetter conditions and therefore higher groundwater levels than those encountered historically. This is supported by field observations as well as the morphological analysis, providing evidence for relatively high degrees of alteration of the BVF volcanoes and therefore older eruption ages as suggested by some researchers. A Lower Holocene to Upper Pleistocene age is proposed.

40Ar/39Ar geochronology of the Neogene-Quaternary Harrat Al-Madinah intercontinental volcanic field, Saudi Arabia: Implications for duration and migration of volcanic activity

New 40Ar/39Ar ages, based on incremental heating techniques for groundmass separates of 25 samples, are presented for the Harrat Al-Madinah volcanic field, part of Harrat Rahat in the north western part of the Arabian plate. This area is an active volcanic field characterized by the occurrence of two historical eruptions approximately in 641 and 1256AD. Field investigations of the main volcanic landforms indicate dominantly monogenetic strombolian eruptions, in addition to local more explosive eruptions. The lavas consist mainly of olivine basalt and hawaiite flows with minor evolved rocks of mugearite, benmoreite, and trachyte that occur mainly as domes, tuff cones and occasionally as lava flows. Previous K/Ar dating shows that the Harrat Al-Madinah lava flows and associated domes comprise seven units spanning an age range of ca. 1.7Ma–Recent. The new 40Ar/39Ar age determinations confirm, to a great extent, the previously obtained K/Ar ages in the sense that no major systematic biases were found in the general stratigraphy of the different flow units. However, the 40Ar/39Ar plateau ages show that volcanism in this area began in the Neogene (∼10Ma) and continued to Recent, with the most voluminous eruptions occurring in the Quaternary. Neogene volcanism occurred in at least three pulses around 10, 5 and 2Ma, whereas Quaternary volcanism produced at least seven units reflecting lava flow emplacement in the time period of 1.90Ma–Recent. Thus, the whole duration of volcanic activity in the Harrat Al-Madinah (10Ma–Recent) appears much longer than that previously identified. The longevity of volcanism in the same part of the moving Arabian plate and absence of evidence for uni-directional migration of volcanic activity indicate that there is no fixed plume beneath this region. The NNW-trending distribution of the volcanic vents is parallel to the Red Sea, and suggests their origin is related to periodic extensional episodes along the reactivated Red Sea fault system.

Age and duration of volcanic activity at Diamond Craters, southeastern Oregon

Diamond Craters, one of several young basalt lava fields dotted across southeastern Oregon, was active in the time between about 7320 and 7790years ago (calibrated ages). This timing is determined by a two-step analysis. First, new radiocarbon ages from samples that underlie the lava field and existing ages from overlying deposits constrain the volcanic activity to between 6130 and 7790years ago. Second, paleomagnetic directions, specifically a steep inclination of about 71°, further constrain the age of emplacement to the older part of that interval, the time before 7320years ago. Paleomagnetic directions for lava emplaced early and late in the eruptive activity are essentially the same.The duration of Diamond Craters eruptive activity must have been at least 1–2decades in order to account for the extrusion of 1.6km3 of dense rock-equivalent magma at rates typical of long-lived basaltic eruptions. Whether the duration was only that brief or persisted to a century or more is irresolvable from paleomagnetic directions, although the coincidence of directions from beginning to end is evidence that the eruption certainly was not protracted over many centuries.

Shape, rheology and emplacement times of small martian shield volcanoes

This paper focuses on the shape of 31 shield volcanoes of small to intermediate size (a few tens of kilometers in diameter) in 5 regions of Mars: Tempe Terra, Syria Planum, Pavonis Mons, Arsia Mons and central Elysium Planitia. A model for the shape of these shield volcanoes is applied, based upon the concept of porous flow of an unconfined aquifer proposed by Lacey et al. [Lacey, A., Ockendon, J.R., Turcotte, D.L., 1981. On the geometrical form of volcanoes, Earth Planet. Sci. Lett., 54(1), 139–143] and developed by Turcotte and Schubert [Turcotte, D., Schubert, G., 2002. Geodynamics. Cambridge University Press, second edition]. The agreement between the topographic profiles and the theoretical shape suggests that the model is appropriate and can be used to derive the product of flow rate and viscosity ( Qµ). Estimates of Qµ are found to be homogeneous within a given volcanic region, but show large differences from one region to another, values generally decreasing with age (from oldest to youngest). Intrinsically, the method cannot separate the flow rate from the viscosity, but independent observations on individual lava flows in Syria Planum and central Elysium Planitia suggest that this evolution may be explained by decreasing magma viscosity with age. Independent constraints on the viscosity are used to interpret Q variations between volcanic regions. Except for Tempe Terra, and within error bars, relative effusion rates are essentially independent of volcano volumes, suggesting that the size of a shield volcano is principally controlled by the duration of volcanic activity. The specificity of central Elysium Planitia is emphasized, with highest flow rates associated with lowest lava viscosity. However, the lower viscosities at central Elysium Planitia cannot totally explain low Qµ values. Considering the equation of magma flow through a dike from a magma chamber at depth, to the shield, we suggest that lower volatile contents and/or deeper magma chambers may contribute to the decrease of Qµ values with time. Finally, the emplacement times of the 31 shield volcanoes were calculated using a calibration based upon terrestrial volcanoes Mauna Loa, Eldborgir, and Skjalbreiður. Emplacement times of martian volcanoes calculated in this way range from a few hundred thousand years (Tempe Terra) to a few tens of thousands of years (central Elysium Planitia). Despite the simplicity of the model, the quality of the fits and the internal consistency of the results are reasonable, encouraging development of more advanced models linking shield volcano shape to rheological properties of lavas and their characteristic time-scales of emplacement.

Palæomagnetic results and palæointensity of Late Cretaceous Madagascan basalt

For a palæomagnetic study of the Late Cretaceous flood basalts of Madagascar (age ∼ 88 Ma), 144 oriented drill cores from 15 sites located in the northwestern part of the island were sampled. Despite the very common occurrence of secondary isothermal remanent magnetisation related to lightning, the direction of the primary characteristic remanent magnetisation could, in many cases, be isolated using alternating field demagnetisation. Combining the palæodirectional results from this study with previously published results, the authors obtain a well-defined palæomagnetic pole for Madagascar with coordinates 67.8°S, 48.5°E (A95 = 3.7°, K = 34, N = 44). This pole is in good agreement with the synthetic apparent polar wander path (APWP) for Africa. Furthermore, Bingham statistics provided a long axis lying along the APWP indicating that the duration of volcanic activity was sufficiently long to record apparent polar wander. Palæosecular variation, S = 13.9 (+2.4/−1.8), is slightly smaller than expected for this time interval and latitudinal band.The absolute palæointensity, using the Thellier technique, could be obtained only from a single cooling unit, yielding a palæofield of 61.5 ± 5.5 μT. The corresponding virtual dipole moment of 10.7 × 1022 A m2 is somewhat higher than previously published results from the Cretaceous normal superchron; but, obviously, secular variation is not averaged.

Attenuation differences in layer 2A in intermediate- and slow-spreading oceanic crust

In situ seismic attenuationQ−1logs are derived from borehole velocity profiles and reveal sharp boundaries between morphologies of the extrusive volcanic layers in intermediate- and slow-spreading oceanic crust.Q−1logs are calculated from the scattering attenuation associated with vertical velocity heterogeneity in Ocean Drilling Program Holes 504B and 896A and in Hole 395A, located in 5.9–7.3 Ma crust on the Pacific and Atlantic plates, respectively. Our results strongly tie crustal properties to seismic measurables and observed geological structures: we find that the scattering attenuation can be used to identify the extrusive volcanic sequence because it is closely related to changes in the degree of vertical heterogeneity. We interpret a distinct decrease in the Q−1log at the transition below the extrusive volcanic layer to correspond with the seismic layer 2A/2B boundary. The boundary is located at 465 m depth below the sea floor in both Hole 395A and 504B, although this is likely to be a coincidence of the sediment thickness at these sites. Layer 2A is estimated to be approximately 150 m thick in Hole 504B and > 300 m thick in Hole 395A. Cyclic sequences of high-porosity pillows and low-porosity massive units in the uppermost 100 m of volcanics in Hole 395A result in large velocity heterogeneities which cause > 5 times more attenuation in this layer than in Hole 504B. In Hole 896A, by contrast, fewer pillows, more massive flows, and a greater volume of carbonate veins decrease the velocity heterogeneity and attenuation significantly over only 1 km distance from Hole 504B. We conclude that the attenuation in the extrusive volcanics of the ocean crust is largely controlled by variation in local heterogeneity and morphology as well as by subsequent hydrothermal alteration. The observed differences inQ−1profiles and layer 2A thickness at these sites may be attributed to variations in the volume and duration of volcanic activity at mid-ocean spreading centers for these Pacific and Atlantic ridge segments.

Tectonic controls on the geochemical composition of Cenozoic, mafic alkaline volcanic rocks from West Antarctica

Cenozoic, mafic alkaline volcanic rocks throughout West Antarctica (WA) occupy diverse tectonic environments. On the Antarctic Peninsula (AP), late Miocene-Pleistocene (7 to <1 Ma) alkaline basaltic rocks were erupted <1 to 45 million years after subduction ceased along the Pacific margin of the AP. In Marie Byrd Land (MBL), by contrast, alkaline basaltic volcanism has been semi-continuous from 25–30 Ma to the present, and occurs in the West Antarctic rift system. Together, these Antarctic tectono-magmatic associations are analogous to the Basin and Range, Sierran, and Coast Range batholith provinces. Unlike the western US, however, basaltic rocks throughout WA have uniform geochemical characteristics, with especially narrow ranges in initial87Sr/86Sr (0.7026–0.7035),143Nd/144Nd (0.51286–0.51299), and La/Nb (0.6–1.4) ratios, suggesting very limited liput from “old” subcontinental lithosphere or crustal sources during magma genesis. However, there are significant differences in the relative and absolute abundances of the LILE (large-ionlithophile elements), and these divide WA into two provinces. Basalts from the AP region have unusually high K/Ba and K/Rb ratios (50–140 and 500–1500 respectively) and marked Ba depletion (Ba/Nb=2.5–8.0; Ba ppm 66–320) relative to MBL basalts, which have LILE distributions within the range for OIB (ocean-island basalt) (K/Ba <50, Ba/Nb 5–20). This geochemical contrast is accompanied by a three-fold increase in the age range of volcanic activity and a three orders of magnitude increase in the volume of eruptive products, within MBL. The regional differences in geochemistry, and in the volume and duration of volcanic activity, are best explained by a plume-related origin for MBL basalts, whereas alkaline magmatism in the AP is causally related to slab window formation following the cessation of subduction. Plume activity has alreadybeen proposed to explain tectonic doming and associated spatial patterns of volcanism in MBL. Most MBL geochemical traits are shared by the volcanic rocks of the western Ross Sea, suggesting that a large plume head underlies the West Antarctic rift system. The uniformity of basalt compositions throughout WA and the entire rift system suggest uniformly minimal extension throughout this region during late Cenozoic time. Differences in crustal thicknesses can be explained by early Cenozoic or pre-Cenozoic extension, but restraint on extension is suggested by the size of the region and the implied size of the plume. The c. 95% encirclement of the Antarctic plate by mid-ocean ridges and transforms restrains extension on a regional scale, leading to nonadiabatic plume rise and correspondingly little decompression melting.

Magma generation in the Lesser Antilles: geological constraints

Volcanic activity in the Lesser Antilles has been fairly discontinuous in space and time during the past 22 Ma: 1. (1) for the past 100000 years volcano spacing has been highly irregular (between 130 and 10 km); 2. (2) several sections of the arc, about 100 km long, have proved to be volcanically inactive for 1–4 Ma; 3. (3) the present volcanic production rate appears to be ten times higher in the central portion of the arc than in the northern and southern regions; 4. (4) construction of the arc-ridge proceeds from the juxtaposition of volcanic products emitted in areas generally a few tens of km in length (a small island, or a part of a large island e.g. Martinique, the geology of which is presented in some detail in this paper) during several Ma (up to 10 Ma). Such areas are referred to in this paper as arc-compartments. To reconcile the erratic activity of the arc with the coeval continuous process of subduction of Atlantic crust beneath the Caribbean plate, it is tentatively proposed that partial melting of subducted sediments is a sine qua non condition for generation of orogenic magmas in the Lesser Antilles, instead of dehydration of the descending slab. In this speculative scheme, the length along strike and duration of volcanic activity in an arc-compartment are thought to reflect the width and length of a subducted sedimentary basin passing beneath the arc. From the beginning to the end of activity of this arc-compartment, the volcanics typically evolve from tholeiitic to calc-alkaline series, progressively enriched in incompatible elements and radiogenic isotopes. At the same time, the volcanic front migrates away from the trench (westward in the Lesser Antilles), the volume of lavas decreases, and the acidity of most common lavas and occurrences of magma mixing increase. Mg-rich lavas constitute a group of basanitoids, basalts and basaltic andesites long known to crop out in the southern Lesser Antilles. In the Grenadines and in Martinique, this particular and non-voluminous volcanic activity is controlled by faults transverse to the arc-ridge, and unrelated in time and space to common orogenic series (tholeiites and calc-alkaline lavas). It is assumed that Mg-rich magmas are generated in the upper mantle wedge, and that their eruption is related to the tectonic stress generated on the edge of the Caribbean plate during an oceanic-ridge subduction event. The distribution of volcanic activity and the different types of volcanic series in the Lesser Antilles are best explained if related to the morphological features of the subducted ocean floor i.e. particularly the distribution of oceanic basins where sedimentation is more efficient, and seamounts and aseismic ridges where sediments generally do not pond.