Dyke Swarm Research Articles

The Corrugated Hills Continental Flood Basalt: a c. 2.17 Ga flood basalt province related to breakup of the Superior craton

Abstract This study documents a major continental flood basalt province in the central Labrador Trough that produced voluminous mafic magmatism along the palaeo-rifted margin of the Superior craton. This area, representing a preserved magma volume of approximately 25 000 km 3 , is characterized by classic ‘traps’ topography with columnar-jointed flows and sills separated by thin clastic sedimentary layers. A gabbroic sill near the bottom of the sequence yielded a U–Pb zircon age of 2166 ± 4 Ma, and another near the middle of the sequence yielded a 2171 ± 3 Ma U–Pb zircon age. These ages overlap in time with published ages of mafic dyke swarms that radiate outwards into the adjacent Superior craton, namely the Biscotasing (2172–2167 Ma) and Payne River (2170–2160 Ma) dykes, as well as the Otish sills (2172–2162 Ma) and Tasiataq (2170 Ma) dyke. Our data suggest that these central Labrador magmas, named the Corrugated Hills Continental Flood Basalt, were emplaced near or above a mantle plume head, leading to thermal uplift and then rifting of the Superior craton margin. Field relationships suggest that these magmas were erupted through thinned Archean crust as a rift-related continental flood basalt suite. The Corrugated Hills Continental Flood Basalt province represents remnants of one of the oldest continental flood basalt provinces on Earth.

The continuation of the Mull Dyke Swarm into the Southern North Sea

We present new geophysical observations of the Mull Dyke Swarm in the Southern North Sea. Two- and three-dimensional seismic reflection and aeromagnetic data were used to map the dykes. The three main dyke groups recognized onshore (Cleveland, Blyth and Hawick–Acklington) are found to extend to varying distances into the North Sea, crossing a number of major crustal-scale fault zones and domain boundaries, with almost no reorientation. The Blyth Dyke Group extends furthest, for a distance of 672 km from the source on Mull. The seismic data show extensive development of pit chain craters above the upper tips of these dykes, which can be approximately dated to the late Paleocene from the ages of crater fills. Volumetric estimates are made of the intrusive volumes associated with each group, ranging from 90 to 202 km 3 . These three main axes of intrusion probably formed in different intrusive events within a c. 1 myr period, from 59 to 58 Ma, during magnetic chron C26R.

The formation and growth mechanisms of young back-arc spreading ridges from high-resolution bathymetry: The Marsili Seamount (Tyrrhenian Sea, Italy)

The formation and growth mechanisms of Mid-Ocean Ridges (MOR) are relatively well known, whereas those of back-arc spreading ridges are comparatively less known because geophysical, geochemical, and morphological data are scarce and of low density. Here we present a high-resolution bathymetry of the Marsili Seamount (MS; 1 Ma − 3 ka), which represents the inflated spreading ridge of the 2 Ma old Marsili back-arc basin associated to the subduction of the Ionian Sea below the Calabrian Arc and Tyrrhenian Sea. MS is 70 km long, 30 km wide, and its height reaches about 3000 m from surrounding seafloor. Our new digital bathymetric model has a 5 m grid cell size resolution and covers the MS bathymetry from −1670 mbsl to the top at −491 mbsl. We conduct morphometric and morphological analyses of the bathymetry and recognize landforms due to volcanic, tectonic, hydrothermal and gravity processes. MS consists of volcanoes related to fissural and central-type activity, this latter located at the northern and southern tips of the main dike swarms. Dike swarms represent the surface expression of different ridge segments whose strikes are controlled by the larger scale back-arc spreading processes and by the local occurrence of an active hydrothermal field. This latter develops in a flat area between two partly overlapping ridge segments where historical volcanism and extensional processes concentrate. Such ridges represent the embryonic stage of the formation of transform-like faults. Central volcanoes, the northern of which is characterized by a caldera, form at the tips of MS because the decrease in width of the major volcanic fissures promotes vent localization associated with the formation of sill-like reservoirs from which central-type vents may develop. Gravity processes affecting the MS flanks are due to shallow seafloor sliding. Caldera collapses affecting the northernmost central-type polygenic volcano must be included in the evaluation of the hazard related to potential tsunami. Inward dipping faults characterize the MS eastern flank suggesting a moderately asymmetric growth of the spreading ridge possibly associated with the eastward opening of the Marsili back-arc.The Marsili back-arc spreading rate is similar to those of MOR slow spreading ridges. However, the MS morphology resembles that of fast spreading ridges. These two features also characterize more extended back-arc spreading ridges (e.g. the Mariana in Western Pacific). We conclude that, independently from the spatial scale, the increase in the ridge accretion rate is related to the progressive addition of a subduction-related component to a pure spreading mantle source.

Contextual relationship between mechanical heterogeneity and dyking: constraints from magma emplacement dynamics of the ca. 2.21 Ga Anantapur–Kunigal mafic dyke swarm, Dharwar Craton, India

AbstractMafic dykes are typically emplaced through primary hydraulic fracturing of undeformed crust or may make use of pre-existing crustal inhomogeneities, representing the plumbing systems of a large igneous province. The Eastern Dharwar Craton has dense exposures of several generations of Paleoproterozoic mafic dyke swarms ranging from ca. 2.37 Ga to ca. 1.79 Ga. Herein, using anisotropy of magnetic susceptibility fabric data of mafic dykes and associated host granites, the emplacement systematics of the NW- to W-trending ca. 2.21 Ga Anantapur–Kunigal dyke swarm, displaying a radiating geometry, have been studied to understand magma flow dynamics. A low-angle relationship between the silicate and opaque fabrics and good correlation with magnetic lineation, identified via petrographic studies and shape preferred orientation analyses of multiple oriented thin sections, suggest a primary flow-related magnetic anisotropy for the studied dyke samples. The classic subparallel relationship between the trend of the dyke planes and magnetic fabric of the associated host granites suggests that the radiating geometry of the ca. 2.21 Ga dyke swarm was supported by a favourable pre-existing structural grain of the country rock. We interpret the magma for the studied dyke swarm was fed laterally from a distant plume. It was emplaced as laterally propagating primary dyke fractures as well as injected into the pre-existing subparallel crustal inhomogeneities. Corroborating all these inferences, a detailed emplacement model for ca. 2.21 Ga Anantapur–Kunigal dyke swarm is also proposed.

Lithospheric density structure beneath the dyke swarm region of the southern Bastar craton reveals a possible Proterozoic mantle plume: Inference from gravity and geoid study

Lithospheric density structure beneath the dyke swarm region of the southern Bastar craton reveals a possible Proterozoic mantle plume: Inference from gravity and geoid study

Feeding system and mantle sources of the southern and western sector of the Madagascar flood basalt province, and comparisons with Southwest Indian Ridge “anomalous” basalts

The flood basalts of Cretaceous age in southwestern Madagascar range from moderately incompatible-element-enriched, tholeiitic, high-Ti ferrobasalt/basaltic andesite lavas (MgO = 4–5 wt%, Fe2O3t = 14–16 wt%; TiO2 = 3–3.6 wt%, Nb =16–22 ppm, Zr = 236–269 ppm; Lan/Ybn = 7–8) in the southernmost outcrops (Lavanono, Tulear), to incompatible element-poor basaltic dikes (MgO = 4.2–4.9 wt%, Fe2O3t = 12–13 wt%; TiO2 = 1.6 wt%, Nb = 8–9 ppm, Zr = 90–100 ppm; Lan/Ybn = 2.3) around Morondava. The two magma types, at the same degree of magmatic evolution, have markedly different Sr-Nd-Pb isotopic composition (e.g., 87Sr/86Sri = 0.7121–0.7123, 208Pb/204Pb = 40.9–41.3 for the Tulear-Lavanono lavas vs 87Sr/86Sri = 0.7033–0.7040, 208Pb/204Pb = 38.7 for the Morondava dikes), that suggest markedly different sources, magmatic history, and variable interplay between Madagascan Precambrian crust and mantle-derived melts, some with a marked enriched mid-ocean ridge basalt-like composition. The geochemical and isotopic data show no involvement of ancient basement crust in the genesis of the Morondava dikes. The Tulear-Lavanono lavas have high Ba/Nb and La/Nb ratios and Sr-Nd-Pb isotopic characteristic of magmas that have assimilated crustal material. The chemical and isotopic composition of the contaminated tholeiites of southern Madagascar matches some Late Cretaceous group B1 basalts of the Androy volcanic complex and tholeiites of the Ejeda-Bekily dike swarm, and therefore they could derive from the same feeding system or were fed by similar parental magmas. Basalts from the Southwest Indian Ridge between 39 and 41°E section (anomalous magmatic sectors) with low 206Pb/204Pb and 143Nd/144Nd have been interpreted to contain a component of either recycled lower continental crust and/or subcontinental lithospheric mantle. A detailed review of the isotopic and geochemical compositions of the Mesozoic -and Cenozoic- magmatism in Madagascar, and of the lower and upper crustal domains point out that the sources of the anomalous magmatic sectors of the Southwest Indian Ridge do not contain lower crustal (or lithospheric mantle) components of Madagascan origin.Comparison between the samples from the northern and southern parts of the Madagascar flood basalts province indicates that several different parental magmas and mantle sources were involved in the petrogenesis of the Madagascan basalts, and that chemical contributions from a “Marion” hotspot (as a magma source or source component) to the Madagascar primary magmas is not identifiable.

Noble and base metal geochemistry of late- to post-orogenic mafic dykes from central Spain

The post-tectonic and post-orogenic mafic rocks from the Spanish Central System (SCS) (Iberian Massif) include dyke swarms of shoshonitic (microgabbros) and alkaline (lamprophyres and diabases) geochemical affinity, which register the nature of the metasomatic lithospheric mantle under central Spain. Such magmas sometimes show a direct (or indirect) relationship with the formation of orogenic and intrusion-related gold deposits, which are relatively abundant in the Iberian Massif. The noble and base metal composition of these intrusions shows Primitive Mantle-normalized patterns characterized by positive Au and Co anomalies and fractionated platinum group elements (PGE): from lower Ir-group PGE (IPGE; Ir–Ru) to higher Pd-group PGE (PPGE; Rh–Pt–Pd). The low contents of PGE, together with the base metal contents of pyrite (which is the dominant sulphide phase in the alkaline dykes), is in accordance with low degrees of mantle partial melting and the early segregation of sulphides during magma differentiation. The scarcity of PGE mineral deposits in the Iberian Massif could be explained in part by the apparent lack of PGE enrichment in the Iberian lithospheric mantle. On the contrary, the positive Au anomaly of the SCS mafic dykes represents relatively high Au contents, similar to (and higher than) those of mafic rocks derived from metasomatized subcontinental lithospheric mantle underlying Au-endowed cratons. Several geochemical features point to subduction-related metasomatism of either oceanic or continental nature as the main source of Au enrichment. The Au re-fertilization of the lithospheric mantle under central Spain makes it a potential source in the formation of gold mineralizations.

Multiscale Spatial Patterns in Giant Dike Swarms Identified Through Objective Feature Extraction

AbstractDike swarms are ubiquitous on terrestrial planets and represent the frozen remnants of magma transport networks. However, spatial complexity, protracted emplacement history, and uneven surface exposure typically make it difficult to quantify patterns in dike swarms on different scales. In this study, we address this challenge using the Hough transform (HT) to objectively link dissected dike segments and analyze multiscale spatial structure in dike swarms. We apply this method to swarms of three scales: the Spanish Peaks, USA; the Columbia River Flood Basalt Group (CRBG), USA; the Deccan Traps Flood Basalts, India. First, we cluster dike segments in HT space, recognizing prevalent linearly aligned structures that represent single dikes or dike packets, with lengths up to 10 − 30x the mapped mean segment length. Second, we identify colinear and radial dike segment mesoscale structures within each data set, using the HT to segment swarms into constituent spatial patterns. We show that for both the CRBG and Deccan Traps, a single radial or circumferential swarm does not well characterize the data. Instead, multiple and sometimes overlapping mesoscale linear and radial features are prevalent suggesting a complex history of crustal stresses. The HT can provide useful insights in a variety of geologic settings where many quasi‐linear features, at any scale, are superimposed spatially.

On the anatomy and structural control of a dyke swarm that fed caldera-forming ignimbrite eruptions

The evolution of eruptive vents related to calderas is not fully understood. We focus on a structural, rock-magnetic and geochemical investigation of a 314 Ma rhyolite dyke swarm associated with the late-orogenic Altenberg–Teplice Caldera, Bohemian Massif, eastern Variscan belt. The whole-rock major element, trace element and Nd–Pb isotope geochemistry along with the published U–Pb zircon geochronology link the extra-caldera dyke swarm with intra-caldera ignimbrites. The magnetic fabrics determined using the anisotropy of magnetic susceptibility are interpreted to record a continuum from magma ascent to emplacement and eruption during sinistral shearing. The latter evidences an interplay with regional tectonics associated with the activity of crustal-scale shear zones. The sinistral kinematics and strike of the dyke swarm, the elongation of caldera intrusive units, and the kinematics of major caldera faults are consistent with the dextral Riedel shear system, where the dykes correspond to antithetic R’ or X shears. Such a kinematic configuration implies that the maximum and minimum principal stresses were oriented roughly north–south and east–west, respectively. The relation between the stress field and the caldera elongation and orientation is not typical. We suggest that a pre-existing mutually perpendicular set of cross-cutting structural lineaments largely controlled the magma chamber and caldera formation. Supplementary material : The whole-rock major, trace element and isotope geochemical tables, magnetic fabrics source data and details of methods are available at https://doi.org/10.6084/m9.figshare.c.6715893

The Açdif Gold-Bearing Shear Zone (Zenaga Inlier, Central Anti-Atlas, Morocco): New Petro-Structural and Geochemical Data

The Açdif gold deposit is located in the Zenaga Inlier (central Anti-Atlas), approximately 120 km southwest of the city of Ouarzazate. It hosts gold mineralization associated with a shear zone affecting the metamorphic and magmatic formations of the Eburnean basement. It mainly consists of alternating micaschists, augen gneisses, and orthogneiss. These crystalline rock assemblages are intruded by a variety of granitoids. These are the Azguemerzi granitoids, which are locally affected by Eburnean deformation. Subsequently, these facies are intersected by a swarm of mafic dykes, which show a variety of rocks ranging from microgabbro-dolerite to quartz diorite. Detailed mapping, petro-mineralogical investigations, and geochemistry of the major and trace elements of these magmatic intrusions suggests the following: (i) for the granodiorite rocks (deposited before the basic dykes), a calc-alkaline affinity, with a chemical signature similar to a syn-collisional context; (ii) for mafic dykes, a contemporaneous emplacement of these mafic intrusions with an evolutionary process controlled by fractional crystallization of the same magma of continental tholeiites, whose chemical composition is comparable to that of enriched MORBs (EMORBs). These continental tholeiites are related to a distensive tectonic context that would have affected the Zenaga inlier prior to the Pan-African orogeny.

Granitic inselberg erosion controlled by dike swarm array in semiarid Brazil

Numerous inselbergs rise above the regional erosional surface in semiarid northeastern Brazil. The inselbergs are underlain by the Quixadá Pluton and cut by cm-thick granite dike swarms. Five inselbergs with abundant dikes from two different parts of the pluton were selected to investigate the role of dike patterns in the evolution of residual granite topography, including morphological features of hillslopes. The methods included field mapping of landforms, structural measurements of accessible dike arrays, and rock strength determinations using a Schmidt hammer. Unmanned Aerial Vehicle (UAV) photogrammetry was used to build detailed digital surface models of inselbergs and to recognize geometric patterns of dike arrays. In addition, 34 rock samples were collected to determine rock composition and evaluate their resistance to weathering. The results show that >60 % of dikes display a sub-horizontal array composed of low- and medium-angle dikes (up to 45° dip), which tend to increase rock-mass resistance, providing effective barriers against weathering and vertical fracturing, and are associated with the most prominent, steep-sided residual hills. Besides, dike swarm geometry controls the slope outlines of some inselbergs, which commonly conform to dike attitudes. By contrast, steeply-dipping dikes (>60° dip) promote weathering along the dike-host rock interface and are hypothesized to underlie the plains between the inselbergs. At a small scale, dikes control the occurrence and evolution of minor rock landforms such as weathering pans, solution runnels, overhangs, and split boulders. Our findings show that the resistance of inselbergs to erosion is not homogeneous in the granitic pluton but is controlled by local factors such as dike occurrence and dip.

KENGEDE MAFIC DYKE SWARM AND EXPANSION OF THE 1.50 Ga KUONAMKA LARGE IGNEOUS PROVINCE OF NORTHERN SIBERIA

Within the Anabar shield in the northern part of the Siberia, Late Precambrian mafic igneous units are widespread, which form dyke swarms of different ages of different trends. This paper presents new data on the composition, structure and U-Pb dating of the E-W trending Kengede dyke swarm. Three new U-Pb ID-TIMS baddeleyite ages (1496±7, 1494±3 and 1494±5 Ma) were obtained from three dykes, indicating that the Kengede swarm is part of the 1500 Ma Kuonamka large igneous province (LIP). The previously recognized Kuonamka Large Igneous Province (LIP) extends for 700 km from the Anabar shield to the Olenek uplift in the northern part of the Siberia and is potentially linked to coeval dykes and sills of the São Francisco craton and the Congo craton. The newly dated Kengede swarm is parallel to but offset by 50 km from the previously dated 1501±3 Ma Kuonamka swarm, and the identification of these two subparallel dyke subswarms of the Kuonamka LIP supports the earlier interpretation that mantle plume centre was located along the extrapolated trend of the dykes near the eastern or western margin of the Siberia. The paper examines features of sulfide Cu-Ni mineralization in dolerites of the Kengede and East Anabar dyke swarms and discusses potential Cu-Ni-sulfide mineralization linked to the Precambrian mafic dyke swarms of different ages in the north-east of the Siberia.

Aeromagnetic and structural characterization of dyke swarms in southeast Brazil: Evidence for Cenozoic reactivation of the Guapiara lineament, Ponta Grossa Arch

The Guapiara Lineament located on the northern edge of the Ponta Grossa Arch, Southeastern São Paulo state, Brazil, it is an important morphostructural feature defined by dykes oriented in NW-SE direction. The main objective of this work is applying new techniques for enhancing magnetic anomalies of the Guapiara Lineament and other structures, and using aeromagnetic data as support for the analysis of structure reactivation. Aerogeophysical techniques included qualitative and semiquantitative approaches (Euler Deconvolution and Radial Average Spectrum), we used the vertical and horizontal gradients, as well as deep magnetic sources and shallow magnetic sources filters, for an in-depth magnetic-structural characterization that was further correlated with the outcropping geology. We have also applied upward continuation (2000m) in the anomalous magnetic field map which was the basis for applying the Total Horizontal Gradient of the Analytical Signal (TDY), Analytic Signal Amplitude (ASA), Tilt angle (TDR), Total Horizontal Derivative of the Tilt Angle (TDR-THDR) and Tilt Angle of the Horizontal Gradient (TAHG) to improve our interpretations of the regional framework. The magnetic trends of the Guapiara Lineament are superimposed on the NE-SW (e.g.: Cubatão Lineament) and E-W (e.g.: São Sebastião Lineament) anomalies, which are considered structural inheritances of the Proterozoic basement. We report a set of reactivated N40–70W and E-W strike-slip faults showing the same orientation as the mapped magnetic lineaments. Moreover, they seem to be associated to deformational features in the dyke swarm, which was interpreted as evidence of Cenozoic tectonic reactivation.

Fault Striae Analysis and Paleostress Reconstruction of the Northern Tectonic Province (Egyptian Nubian Shield): Insights into the Brittle Deformation History of the Northern East African Orogen

The Egyptian Nubian Shield (ENS), the northwestern continuation of the East African Orogen (EAO), comprises a variety of three lithologically and structurally different tectonic provinces—southern compressional-, central transpressional/wrench-, and northern extensional- provinces. The extensional tectonic province (ETP) extends from Qena‒Safaga shear belt up to the northern tip of the ENS. Several lines of evidence indicate extensional regime in the ETP, such as mantle delamination-induced post-orogenic magmatism and bimodal Dokhan Volcanics, together with the pervasive E‒W dyke swarms and the E‒W (to ENE‒WSW) oriented extensional fractures. The present work is devoted to carry out the fault striae analysis and paleostress reconstruction of the northern tectonic province (ENS) in attempt to add more contribution to the brittle deformation history of the northern EAO. Field measurements collected from six areas (I‒VI) have been processed using Win-Tensor Software. Analysis of fault-slip data revealed four paleostress tensor stages (groups or regimes) that prevailed in the ENS. The stress states and related tectonic regimes are characterized by the stress regime index R' and the horizontal stress axes as follows: 1st stage: transpression (R' = 1.91), E‒W compression (SHmax = N090°‒270° E); 2nd stage: transtension (R' = 1.16), N‒S compression (SHmax = 174°‒354° E); 3rd stage: transpression (R' = 1.81), NE‒SW compression (SHmax = N053°‒233° E); 4th stage: extension (R' = 0.31), E‒W extension (Shmin = N074°‒254° E). The 1st stage can be related to the oblique convergence between E and W Gondwanalands. The 2nd stage was probably concurrent with the N- to NNW-ward migration of the Central Eastern Desert syn-collisional delamination (triggering orogen-parallel extensional collapse) to the North Eastern Desert post-collisional delamination. The 3rd stage was controlled by the generally N‒S shortening which affected the northern and central ENS, post-dating the formation of volcanosedimentary Hammamat Basins. The 4th stage is akin to retreat of the Cadomian arc and the Red Sea rifting.

Magmatic Origins of Extensional Structures in Tempe Terra, Mars

AbstractNumerous graben features transect the Tempe Terra plateau in the northeastern Tharsis Rise, Mars, making it one of the most heavily structured regions of Tharsis. The origin of the complex fault geometries, generated over three distinct stages of tectonic activity, is poorly understood. This work distinguishes between Tempe Terra structures of local and regional origin, to isolate regional deformation patterns related to the general development of the Tharsis Rise from the patterns due to effects of local stress mechanisms. Comparison of structural observations to predicted deformation patterns from different drivers of graben formation in the Martian crust demonstrates the important role of magmatic activity at a variety of scales in driving tectonism in Tempe Terra. Noachian (Stage 1) faulting resulted from local magmatic underplating and associated heating and uplift, which formed part of an incipient stage of widespread Tharsis volcanism that predated development of the main Tharsis Rise. Early Hesperian (Stage 2) faults reflect the interaction of regional stresses from growth of Tharsis with magmatic activity highly localized along the Tharsis Montes Axial Trend—a linear volcanotectonic trendline including the alignment of the Tharsis Montes volcanoes. Early–Late Hesperian (Stage 3) faulting resulted from a series of dyke swarms from a Tharsis‐centered plume, which propagated in a regional stress field generated by growth of the Tharsis Rise. As only Stage 2 NNE faults and Stage 3 ENE faults are linked to regional, Tharsis‐related stresses, other observed Tempe Terra fault trends can be excluded when evaluating models of Tharsis's tectonic evolution.

Paleogeography of India at ∼1.8 Ga: New constraints from baddeleyite geochronology and paleomagnetism of mafic dykes from the Dharwar craton

The paleogeographic position of Dharwar craton/India within Paleoproterozoic supercontinent configurations has been well demonstrated using highly reliable key paleopoles from ∼2.37 Ga to ∼1.86 Ga mafic events in recent studies. Here, we report a paleomagnetic pole at ∼1.8 Ga along with a precise Pb-Pb baddeleyite age. The emplacement age of an NW-SE trending dolerite dyke yields a weighted mean age of 1799 ± 0.8 Ma (MSWD = 0.7), which coincides with earlier reported U-Pb baddeleyite age (1794 ± 7 Ma) and confirms the existence of significant emplacement of Pebbair dyke swarm at ∼1.8 Ga. Paleomagnetic results from eleven sites shows mean D = 306°, I = 31°, α95 = 10° (k = 23), and corresponding paleopole is calculated at 38°N, 357°E (dp = 6.1° and dm = 11°). These remanent directions are carried by the pseudo-single domain (PSD) magnetite. This pole passes the paleosecular variation test of the geomagnetic field and fulfills the reliability criteria (R = 5) of Meert et al. (2020), so, suggested to be a key pole for Dharwar craton/ India (∼1.8 Ga). The paleopositon of Dharwar craton is occupied at shallow paleolatitude and drifted towards the north with a minimum velocity (∼5.7 cm/yr) from ∼1.86 Ga to ∼1.80 Ga. The paleogeographic reconstruction model at ∼1.8 Ga has been attempted with available coeval paleopoles from other global cratons. The Dharwar craton (South India block), the Amazonia craton, and the Baltica craton have assembled at ∼1.86 Ga, and North Australia, Sao Francisco, Rio de la Plata cratons came in close to the South India block, Amazonia, and Baltica assembly at ∼1.80 Ga.

New insights into the tectonics and geodynamics of Botswana from intracrustal structures revealed by receiver function analysis

SUMMARY Recent improvements in seismic coverage allow for an increasingly detailed knowledge of the crustal structure beneath Botswana to be obtained. For the first time, shear wave velocity–depth models throughout Botswana are obtained from genetic algorithm inversion of receiver functions. In this approach, averaging of important structural variations is minimized by separating receiver function waveforms into backazimuth and slowness bins. Division of crustal layers into sediment, upper, middle and lower crust reveals important features that enhance our knowledge of tectonics and geodynamics of Botswana. In the west, beneath the Ghanzi-Chobe belt, as well as at the border region of the Zimbabwe craton with Magondi belt, the crust has been thickened through a thickening of the lower crust, most likely by collisional tectonics during the Damara Orogeny. A significant difference in structure is observed between the western and central-to-eastern Kaapvaal craton. Western Kaapvaal craton has a thin mafic lower crust (<10 km) that has survived the extensive melting of the Ventersdorp tectonomagmatic event at 2.7 Ga, contrary to previous reports of a lack of a mafic lower crust. Meanwhile in central-to-eastern Kaapvaal craton, a lower average ${{{}{V}}}_{{}{s}}$ (∼3.3 km s−1) in the upper crust suggests the presence of a suture zone that separates the two blocks of the Kaapvaal craton of different ages and crustal thicknesses. A relatively low ${{{}{V}}}_{{}{s}}$ in the middle crust from the Okavango rift zone to central and SE Botswana suggests reactivation of Palaeoproterozoic shear zones along a thin and weak crust by intraplate relative motion has enabled fluid infiltration from the mantle which causes seismicity in the region. Furthermore, a NW–SE trending region of high Vs in crustal layers is likely related to Karoo flood magmatism that formed the NW–SE trending Okavango dyke swarm. There is also an indication that a dedicated study is required to further investigate shallow converted phases from mobile belts and sedimentary basins in Botswana, however, the results from this study can provide a good starting point for such a study.

A long time of rest at Llaima volcano following the 2010 MW 8.8 Maule earthquake, Chile

Llaima Volcano (38.692°S, 71.729°W) is one of the most active centers in South America, with ∼56 eruptions since 1852 CE. During the last eruptive cycle (2007–2009), the volcano displayed typical features of open conduit systems, with frequent long-period volcanic seismicity (LP). After the MW 8.8 Maule megathrust earthquake (27 February 2010), Llaima volcano located 305 km to SE of the epicenter, has experienced one of the longest periods of quiescence since 1852, with reduced activity observed since the beginning of its continuous instrumental monitoring in the year 2007. To better understand this behavior, we track the repose time between eruptions in the 20th century to depict the current period of quiescence and provide details of the pattern of volcano seismicity after the earthquake. The number of LP events decreased ∼90% shortly after the earthquake. In turn, crustal faults nearby (∼20 km south of the main crater) showed an increased level of seismicity lasting for approximately six months after the mainshock before returning to background levels. In order to propose driving mechanisms for such behavior, we computed the static changes of the stress tensor on chosen receiver structures. The acting receiver structure differs from those related to the long-term stress regime controlled by regional tectonics and was inferred from the pre-earthquake shallow seismicity (2007–2009). LP and volcanotectonic seismicity (VT) before the earthquake were located in a NW-SE pattern, in contrast to the NE-trending alignment of flank vents (and hence dike swarms) usually interpreted as an indicator of the maximum horizontal stress axis. From the static stress transfer imparted by the earthquake, we find that an opening event would have occurred at a NW-trending receiver structure coupled with closing of other potential structures, which promoted magma storage along this blind structure but precluding further propagation to the surface. Our results thus show a rare example of earthquake-induced suppression of volcanic activity, where the geometry of the fault-fracture network that shape the plumbing system likely plays a major role.

Geochronology, geochemistry, and Sr–Nd isotopes of Permian dike swarms in the Beishan orogenic belt, southernmost Central Asian Orogenic Belt: Petrogenesis and implications for geodynamic setting

ABSTRACT Permian mafic dikes crop out widely within Beishan area in the southernmost Central Asian Orogenic Belt (CAOB). These mantle-derived products provide important clues for elucidating the genesis and geodynamic setting of Permian magmatism. The mafic dikes within the Beishan area display zircons U–Pb ages of 281 ± 4 Ma. Characterized by low SiO2 (44.88–47.73 wt.%) and high Mg# values (62.2–68.6), the type 1 dikes are relatively depleted in light rare earth elements (LREEs; (La/Yb)n = 0.72–1.43) but enriched in large ion lithophile elements (LILEs). They show high positive εNd(t) values (6.6–8.5) with a range of (87Sr/86Sr) i (0.703477–0.705789) ratios. In contrast, the type 2 dikes have higher SiO2 content (47.47–51.13 wt.%) and lower Mg# (40.9–63.0). They are enriched in LREEs ((La/Yb)n = 1.79–5.02) and LILEs, and have lower εNd(t) values (3.8–7.4) with a variable range of (87Sr/86Sr) i (0.702873–0.706949) ratios. Thus, the type 1 dikes resulted from lower degrees of fractional crystallization and contamination than the type 2 dikes. Considering the LILEs and Pb enrichments, high field strength elements depletions, and high Ba/Th (86–460) ratios, the mantle source was metasomatized by subduction-related fluids. These dikes should be derived from high degrees (>20%) of partial melting of a Sr–Nd isotope-depleted, phlogopite-bearing spinel lherzolite lithospheric mantle according to their high Rb/Sr (0.04–0.24), low Ba/Rb (0.9–3.3), low La/Sm, and constant Sm/Yb ratios in addition to Sr–Nd depletion. Combined with the coeval evidence of regional geology and novel tectonic discriminate diagrams, we suggest that the mafic dikes in Beishan area formed in arc-related setting rather than intracontinental setting during the Permian. The metasomatized lithospheric mantle by subduction-related fluids were heated by upwelling asthenosphere mantle triggered by the slab roll-back, resulting in the formation of the parental magma of the mafic dikes.

Upper-plate response to ridge subduction and oceanic plateau accretion, Washington Cascades and surrounding region: Implications for plate tectonic evolution of the Pacific Northwest (USA and southwestern Canada) in the Paleogene

Abstract The interaction between subduction zones and oceanic spreading centers is a common tectonic process, and yet our understanding of how it is manifested in the geologic record is limited to a few well-constrained modern and ancient examples. In the Paleogene, at least one oceanic spreading center interacted with the northwestern margin of North America. Several lines of evidence place this triple junction near Washington (USA) and southern British Columbia (Canada) in the early to middle Eocene, and we summarize a variety of new data sets that permit us to track the plate tectonic setting and geologic evolution of this region from 65 to 40 Ma. The North Cascades segment of the voluminous Coast Mountains continental magmatic arc experienced a magmatic lull between ca. 60 and 50 Ma interpreted to reflect low-angle subduction. During this period of time, the Swauk Basin began to subside inboard of the paleo-trench in Washington, and the Siletzia oceanic plateau began to develop along the Farallon plate–Kula plate or Farallon plate–Resurrection plate spreading center. Farther east, peraluminous magmatism occurred in the Omineca belt and Idaho batholith. Accretion of Siletzia and ridge-trench interaction occurred between ca. 53 and 49 Ma, as indicated by: (1) near-trench magmatism from central Vancouver Island to northwestern Washington, (2) disruption and inversion of the Swauk Basin during a short-lived contractional event, (3) voluminous magmatism in the Kamloops-Challis belt accompanied by major E-W extension east of the North Cascades in metamorphic core complexes and supra-detachment basins and grabens, and (4) southwestward migration of magmatism across northeastern Washington. These events suggest that flat-slab subduction from ca. 60 to 52 Ma was followed by slab rollback and breakoff during accretion of Siletzia. A dramatic magmatic flare-up was associated with rollback and breakoff between ca. 49.4 and 45 Ma and included bimodal volcanism near the eastern edge of Siletzia, intrusion of granodioritic to granitic plutons in the crystalline core of the North Cascades, and extensive dike swarms in the North Cascades. Transtension during and shortly before the flare-up led to >300 km of total offset on dextral strike-slip faults, formation of the Chumstick strike-slip basin, and subhorizontal ductile stretching and rapid exhumation of rocks metamorphosed to 8–10 kbar in the North Cascades crystalline core. By ca. 45 Ma, the Farallon–Kula (or Resurrection)–North American triple junction was likely located in Oregon (USA), subduction of the Kula or Resurrection plate was established outboard of Siletzia, and strike-slip faulting was localized on the north-striking Straight Creek–Fraser River fault. Motion of this structure terminated by 35 Ma. These events culminated in the establishment of the modern Cascadia convergent margin.