Ma Volcanic Rocks Research Articles

Petrogenesis of volcanic rocks in the Sangxiu Formation, central segment of Tethyan Himalaya: A probable example of plume–lithosphere interaction

The ∼133 Ma volcanic rocks of Sangxiu Formation are distributed in the eastern part of the central Tethyan Himalaya and belong paleogeographically to the northeastern margin of Greater India. These volcanic rocks include alkaline basalts and felsic volcanic rocks. Major and trace element abundances and whole-rock isotopic data for selected samples of these volcanic rocks are used to infer their petrogenesis. Geochemically, the Sangxiu basalts are closely similar to the Emeishan high-Ti basalts. Major and trace element data and Sr–Nd isotopic compositions suggest that the Sangxiu basalts may have been derived from an OIB-type mantle source, with discernable contributions from subcontinental lithospheric mantle (SCLM). The basaltic magmas may have formed as a result of the infiltration of plume-derived melts into the base of the lithosphere in a continental rift setting. The Sangxiu felsic volcanic rocks share most of the geochemical features of A-type granite, and have Sr–Nd isotopic compositions which differ considerably from the Sangxiu basalts, suggesting that they originated from the anatexis of ensialic continental crust. The Sangxiu volcanic rocks may be considered as the consequence of an interaction between the Kerguelen hotspot and the lithosphere of the northeastern margin of Greater India at ∼133 Ma, and may represent the initial stage of the separation of Greater India from southwestern Australia.

Cenozoic Tectonic Evolution of the Basin and Range Province in Northwestern Nevada

A regional synthesis of new and existing geologic and thermochronologic data document late Cretaceous - early Cenozoic regional erosion, Oligocene - Miocene volcanism, and subsequent late Miocene extension of the Basin and Range Province in northwestern Nevada and northeastern California. Across an ∼220-km-wide region between the Santa Rosa and Warner Ranges, conformable sequences of 35 to 15 Ma volcanic rocks are cut by only a single generation of high-angle normal faults that accommodated ∼23 km of total east-west extension (∼12%). Fission-track, (U-Th)/He, geologic, and structural data from the Pine Forest Range show that faulting there began at 11 to 12 Ma, progressed at a relatively constant rate until at least 3 Ma, and has continued until near the present time. Extension in the Santa Rosa Range to the east took place during the same interval, although the post-6 Ma part of this history is less well constrained. Less complete constraints from adjacent ranges permit a similar timing for faulting, and we infer that extensional faulting in northwestern Nevada began everywhere at 12 Ma and has continued up to the present. Faulting in the Warner Range in northeastern California can only be constrained to have begun between 14 and 3 Ma, but may represent westward migration of Basin and Range extension during the Pliocene. Compared to the many parts of the Basin and Range in central and southern Nevada, extension in northwestern Nevada began more recently, is of lesser total magnitude, and was accommodated entirely by high-angle normal faults. Fission-track data document Late Cretaceous unroofing of Cretaceous (115 --100 Ma) granitic basement rocks in northwestern Nevada, followed by a long period of relative tectonic quiescence that persisted through Oligocene and Miocene volcanism until the onset of Basin and Range extension at ∼12 Ma. The low magnitude of extension (12%) and early Tertiary stability suggest that the modern ∼31 km thick crust in northwestern Nevada was only slightly thicker (∼35 km) prior to extension at 12 Ma, and was no thicker than ∼38 km in the Late Cretaceous. This stands in contrast to other parts of the Basin and Range, where the crust was thickened to at least 45 to 50 km by Cretaceous thrusting and subsequently thinned to ∼30 km by large magnitude (\>50%) extension.

Terranes of the Western Sierra Nevada Foothills Metamorphic Belt, California: A Critical Review

Previous classifications of the western Sierra Nevada Foothills belt have been modified into five geologic packages: the Northern Sierra terrane, Feather River terrane, Calaveras Complex, Jura-Triassic arc belt, and Middle—Late Jurassic arc sequence. The Northern Sierra terrane comprises the Shoo Fly Complex and an overlying Paleozoic succession of arc volcanics. The Feather River terrane is predominantly serpentinized ultramafics containing tectonic blocks of amphibolite, plagiogranite, and mafic schists. The Permian-Late Triassic Calaveras Complex is a subduction mélange comprising psuedostratigraphic sequences of both chert and argillite with incorporated blocks of limestone, amphibolite, greenschist, phyllite, and basalt. The Red Ant schist is also included in this terrane even though it is of higher metamorphic grade. The Jura-Triassic arc belt consists of a Paleozoic basement of disrupted ophiolite, serpentinite mélange, and ultramafic rocks overlain by uppermost Triassic—Early Jurassic arc volcanics and coeval 200 Ma intrusive rocks. This belt may comprise several small terranes. The Middle—Late Jurassic arc sequence comprises 165-155 Ma volcanic arc rocks, greenstones, and metasedimentary rocks of the Mariposa Formation, and rocks of the Smartville Complex. Coeval intrusive rocks are included within this sequence because they are probably comagmatic with the arc volcanics. The Northern Sierra terrane and Feather River terrane are bounded by pre-Nevadan structures that may represent original sutures or subduction zones and have geologic histories that are relatively decoupled from adjacent rocks; they are therefore designated as terranes. The Calaveras Complex formed either adjacent to the Cordilleran margin as a result of east-dipping subduction, adjacent to arc rocks of the Jura-Triassic arc belt from west-dipping subduction, or a combination thereof. Therefore, the Calaveras Complex has a common geologic history with adjacent rocks and is not designated as a terrane. The Jura-Triassic arc belt comprises several ophiolitic microterranes, which may have been either exotic or fringing relative to North America, and are therefore grouped as a belt instead of individual terranes. The Middle-Late Jurassic arc sequence unconformably overlies rocks of the Jura-Triassic arc belt and is therefore a cover sequence instead of a separate terrane. The accretionary history of the Mesozoic Cordilleran margin is complex. After emplacement of the Northern Sierra terrane and Feather River terrane, the system was dominated by arc volcanism and accretion. The entire Sierra Nevada Foothills metamorphic belt was likely accreted to the margin by the Late Jurassic. The Sierra Nevada Foothills comprise units that appear to be correlative with those in the Klamath Mountains. Possible correlations are drawn between the Northern Sierra terrane and Eastern Klamath terrane, Calaveras Complex + Red Ant Schist and Stuart Fork + Eastern Hayfork terranes, Jura-Triassic arc belt and Rattlesnake Creek terrane, and Middle-Late Jurassic arc sequence and Western Hayfork terrane + coeval intrusives.

Geomagnetic field intensity during the last 5 Myr: LTD-DHT Shaw palaeointensities from volcanic rocks of the Society Islands, French Polynesia

SUMMARY On the basis of the Thellier palaeointensities reported so far it has been thought that the time-averaged virtual dipole moment (VDM) and virtual axial dipole moment (VADM) for the last few million years are almost the same as the present geomagnetic dipole moment (∼8 × 1022 A m2). This estimate has been called into question, however, because recent studies have revealed that the Thellier method occasionally overestimates palaeointensities by as much as twice the true values. In contrast, a recently developed palaeointensity technique, namely the double heating technique of the Shaw method combined with low-temperature demagnetization (the LTD-DHT Shaw method), can yield expected field intensities from samples which give unreliable palaeointensities using the Thellier method. Therefore, we have measured absolute palaeointensities from 0.5–4.6 Ma volcanic rocks from the Society Islands, French Polynesia, mainly using the LTD-DHT Shaw method. As a result, 195 out of 361 samples passed the selection criteria, some of which are compared with additional results obtained with Coe's version of the Thellier method. In the Thellier experiments, 18 out of 40 samples passed the criteria, giving palaeointensities both consistent and inconsistent with the LTD-DHT Shaw results. These samples are characterized mostly by two-segmented Arai diagrams. If we take the LTD-DHT Shaw palaeointensities, 24 reliable site means are available and give a mean VADM of (3.64 ± 2.10) × 1022 A m2. This is nearly half of the mean of the 0–5 Ma Thellier data selected from the latest palaeointensity database [(7.46 ± 3.10) × 1022 A m2, N= 458] as well as the present dipole moment. The LTD-DHT Shaw palaeointensities newly determined in this study suggest that the present-day field is so strong that it may not be typical of the past geomagnetic field.

The Negra Muerta Volcanic Complex, southern Central Andes: geochemical characteristics and magmatic evolution of an episodically active volcanic centre

This petrologic analysis of the Negra Muerta Volcanic Complex (NMVC) contributes to understanding the magmatic evolution of eruptive centres associated with prominent NW-striking fault zones in the southern Central Andes. Specifically, the geochemical characteristics and magmatic evolution of the two eruptive episodes of this Complex are analysed. The first one occurred as an explosive eruption at 9 Ma and is represented by a strongly welded, fiamme-rich, andesitic to dacitic ignimbrite deposit. The second commenced with an eruption of a rhyolitic ignimbrite at 7.6 Ma followed by effusive discharge of hybrid lavas at 7.3 Ma and by emplacement of andesitic to rhyodacitic dykes and domes. Both explosive and effusive eruptions of the second episode occurred within a short time span, but geochemical interpretations permit consideration of the existence of different magmas interacting in the same magma chamber. Our model involves an andesitic recharge into a partially cooled rhyolitic magma chamber, pressurising the magmatic system and triggering explosive eruption of rhyolitic magma. Chemical or mechanical evidence for interaction between the rhyolitic and andesitic magma in the initial stages are not obvious because of their difference in composition, which could have been strong enough to inhibit the interaction between the two magmas. After the initial explosive stages of the eruption at 7.6 Ma, the magma chamber become more depressurised and the most mafic magma settled in compositional layers by fractional crystallisation. Restricted hybridisation occurred and was effective between adjacent and thermally equivalent layers close to the top of the magma chamber. At 7.3 Ma, increments of caldera formation were accompanied by effusive discharge of hybrid lavas through radially disposed dykes whereby andesitic magma gained in importance toward the end of this effusive episode in the central portion of the caldera. Assimilation during turbulent ascent (ATA) is invoked to explain a conspicuous reversed isotopic signature ( 87Sr/ 86Sr and 143Nd/ 144Nd) in the entire volcanic series. Therefore, the 7.6 to 7.3 Ma volcanic rocks of the NMVC resulted from synchronous and mutually interacting petrological processes such as recharge, fractional crystallization, hybridisation, and Assimilation during Turbulent Ascent (ATA). Geochemical characteristics of both volcanic episodes show diverse type and/or depth in the sources and variable influence of upper crustal processes, and indicate a recurrence in the magma-forming conditions. Similarly, other minor volcanic centres in the transversal volcanic belts of the Central Andes repeated their geochemical signatures throughout the Miocene.

Neotectonics in the foothills of the southernmost central Andes (37°–38°S): Evidence of strike‐slip displacement along the Antiñir‐Copahue fault zone

The Antiñir‐Copahue fault zone (ACFZ) is the eastern orogenic front of the Andes between 38° and 37°S. It is formed by an east vergent fan of high‐angle dextral transpressive and transtensive faults, which invert a Paleogene intra‐arc rift system in an out of sequence order with respect to the Cretaceous to Miocene fold and thrust belt. 3.1–1.7 Ma volcanic rocks are folded and fractured through this belt, and recent indicators of fault activity in unconsolidated deposits suggest an ongoing deformation. In spite of the absence of substantial shallow seismicity associated with the orogenic front, neotectonic studies show the existence of active faults in the present mountain front. The low shallow seismicity could be linked to the high volumes of retroarc‐derived volcanic rocks erupted through this fault system during Pliocene and Quaternary times. This thermally weakened basement accommodates the strain of the Antiñir‐Copahue fault zone, absorbing the present convergence between the South America and Nazca plates.

Alligator Ridge District, East-Central Nevada: Carlin-Type Gold Mineralization at Shallow Depths

Carlin-type deposits in the Alligator Ridge mining district are present sporadically for 40 km along the north-striking Mooney Basin fault system but are restricted to a 250-m interval of Devonian to Mississippian strata. Their age is bracketed between silicified ca. 45 Ma sedimentary rocks and unaltered 36.5 to 34 Ma volcanic rocks. The silicification is linked to the deposits by its continuity with ore-grade silicification in Devonian-Mississippian strata and by its similar δ 18 O values (~17‰) and trace element signature (As, Sb, Tl, Hg). Eocene reconstruction indicates that the deposits formed at depths of <300 to 800 m. In comparison to most Carlin-type gold deposits, they have lower Au/Ag, Au grades, and contained Au, more abundant jasperoid, and textural evidence for deposition of an amorphous silica precursor in jasperoid. These differences most likely result from their shallow depth of formation. The peak fluid temperature (~230°C) and large δ 18 O H2O value shift from the meteroric water line (~20‰) suggest that ore fluids were derived from depths of 8 km or more. A magnetotelluric survey indicates that the Mooney Basin fault system penetrates to mid-crustal depths. Deep circulation of meteoric water along the Mooney Basin fault system may have been in response to initial uplift of the East Humboldt-Ruby Mountains metamorphic core complex; convection also may have been promoted by increased heat flow associated with large magnitude extension in the core complex and regional magmatism. Ore fluids ascended along the fault system until they encountered impermeable Devonian and Mississippian shales, at which point they moved laterally through permeable strata in the Devonian Guilmette Formation, Devonian-Mississippian Pilot Shale, Mississippian Joana Limestone, and Mississippian Chainman Shale toward erosional windows where they ascended into Eocene fluvial conglomerates and lake sediments. Most gold precipitated by sulfidation of host-rock Fe and mixing with local ground water in zones of lateral fluid flow in reactive strata, such as the Lower Devonian-Mississippian Pilot Shale.

Mesozoic and Cenozoic tectonic evolution of the Shiquanhe area of western Tibet

In the Shiquanhe area of far‐western Tibet, mid‐Cretaceous strata lie unconformable on ophiolitic melange and Jurassic flysch associated with the Bangong‐Nujiang suture zone. On the basis of our mapping and geochronologic studies, we suggest that these Cretaceous strata were shortened by &gt;57% over a north south distance of 50 km during Late Cretaceous‐early Tertiary time. The Late Cretaceous Narangjiapo thrust placed Permian strata &gt;20 km over ophiolitic melange and Cretaceous strata. North of the Narangjiapo thrust, &gt;40 km of shortening was accommodated by the Late Cretaceous‐early Tertiary south directed Jaggang thrust system that involves Jurassic flysch and Cretaceous strata, and roots into a decollement within ophiolitic melange. The most recent shortening was accommodated to the south of the Narangjiapo thrust, along the north dipping Shiquanhe thrust. The Shiquanhe thrust cuts flat‐lying 22.6 ± 0.3 Ma volcanic rocks and underlying folded, Tertiary nonmarine strata in its footwall and was likely active during slip along the Oligocene Gangdese thrust system of southern Tibet. Ophiolitic melange and structurally overlying Jurassic flysch near Shiquanhe are interpreted to represent remnants of a subduction‐accretion complex and forearc basin, respectively, that were obducted southward onto the margin of the Lhasa terrane during Late Jurassic‐Early Cretaceous closure of the Bangong‐Nujiang Ocean. Subsequent imbrication of the obducted sheet could have produced the two east‐west trending belts of ophiolitic melanges, separated by ∼100 km, in western Tibet. Late Cretaceous‐early Tertiary thin‐skinned shortening may have been accommodated in the deeper crust by northward underthrusting and duplexing of Lhasa terrane rocks beneath the obducted ophiolitic melange and the Qiangtang terrane to the north.

Discussion and reply: West African proximity of the Avalon terrane in the latest Precambrian

The paper of [McNamara et al. (2001)][1] presents high-quality paleomagnetic data from the ca. 580–570 Ma volcanic and interbedded clastic rocks of the Marystown Group in the Avalon terrane of southern Newfoundland (western Avalonia) and is a welcome addition to the literature. We agree that their

Magmatism as a probe to the Neogene shallowing of the Nazca plate beneath the modern Chilean flat-slab

Changes in the Neogene (Miocene to Recent) magmatic style of the modern Chilean flat-slab region and its margins (27–34°S) reflect both variations in Nazca–South American convergence parameters and the effects of subduction of the Juan Fernández Ridge hotspot track. These effects are superimposed on an initially thicker arc crust in a more compressional tectonic regime in the north than in the south. Distinctive magmatic periods across the region reflect relatively steep subduction from ∼27 to 20 Ma, frontal arc migration associated with forearc subduction erosion from ∼20 to 16 Ma, compressional deformation and andesitic volcanism from ∼15 to 9 Ma, a peak of slab shallowing associated with frontal arc migration and arc broadening from ∼8 to 4 Ma, and termination of volcanism over the flat-slab by ∼5 Ma. Changes in magmatic style and distinctive chemical signatures in ∼20 to 16 Ma volcanic rocks coincide with a marginwide transition from a more extensional to a more compressional tectonic regime linked to changing Nazca–South American convergence parameters. North to south differences in ∼15 to 9 Ma magmas partially reflect the southward migration of the northeast-trending arm of the subducting Juan Fernández Ridge, whose effect arrived in the flat-slab region at ∼14 Ma. The termination of andesitic arc volcanism across most of the region at ∼9 Ma and extreme shallowing of the slab beneath the central flat-slab coincides with the arrival and eastward propagation of the east–west-trending segment of the Juan Fernández Ridge. Overall, the subduction of the Juan Fernández Ridge is a perturbation that causes extreme shallowing in part of a regionally shallowing subduction zone. Chemical signatures of the erupted magmas reflect the changing mantle and crustal configuration above the shallowing subduction zone. Those with adakitic-like signatures are derived from subduction zone mafic magmas contaminated by garnet granulitic to eclogitic facies crust in the thickened crust or by crust incorporated into the mantle as a result of forearc subduction erosion. There is no evidence that any of these magmas were generated by melting of the subducted slab.

Evidence for 3650–3600 Ma assembly of the northern end of the Itsaq Gneiss Complex, Greenland: Implication for early Archaean tectonics

The world's most extensive, well preserved, early Archaean (&gt;3500 Ma) structures occur in the northern end of the 3850–3560 Ma Itsaq Gneiss Complex, southern west Greenland, an area which includes the 3800–3700 Ma Isua supracrustal belt. These provide a unique opportunity to study early tectonics. The area is dominated by ∼3800–3650 Ma orthogneisses containing migmatized inclusions of ≥3800 Ma metavolcanic, sedimentary, ultramafic, and gabbroic rocks. The thickest and most extensive of these inclusions forms the southern side of the Isua supracrustal belt. Also present are panels of less migmatized 3710–3690 Ma volcanic and sedimentary rocks, in the northern side of the Isua supracrustal belt, and probably also in the Tussaap supracrustal belt in the south of the area. Combined mapping and geochronology has identified mylonites which were active before regional early Archaean folding and 3580–3560 Ma metamorphism. Sensitive high‐resolution ion microprobe dating of premylonitization, synmylonitization, and postmylonitization granite sheets indicates some mylonites were active between 3650 and 3600 Ma. During this period it is proposed that large horizontal motions and interleaving occurred, results of which included juxtaposition of supracrustal sequences in the Isua supracrustal belt with different depositional ages. The tectonic interleaving was most likely due to horizontal motions in the vicinity of plate margins. The new combined field observations and geochronology support hypotheses that some form of plate tectonics at least contributed to early Archaean sialic crustal development.

Neoproterozoic high‐pressure/low‐temperature metamorphic rocks in the Avalon terrane, southern New Brunswick, Canada

AbstractHigh‐P/low‐T metamorphic rocks of the Hammondvale metamorphic suite (HMS) are exposed in an area of 10 km2 on the NW margin of the Caledonian (Avalon) terrane in southern New Brunswick, Canada. The HMS is in faulted contact on the SE with c. 560–550 Ma volcanic and sedimentary rocks and co‐magmatic plutonic units of the Caledonian terrane. The HMS consists of albite‐ and garnet‐porphyroblastic mica schist, with minor marble, calc‐silicate rocks and quartzite. Pressure and temperature estimates from metamorphic assemblages in the mica schist and calc‐silicate rocks using TWQ indicate that peak pressure conditions were 12.4 kbar at 430 °C. Peak temperature conditions were 580 °C at 9.0 kbar. 40Ar/39Ar muscovite ages from three samples range up to 618–615 Ma, a minimum age for high‐P/low‐T metamorphism in this unit. These ages indicate that the HMS is related to the c. 625–600 Ma subduction‐generated volcanic and plutonic units exposed to the SE in the Caledonian terrane. The ages are also similar to those obtained from detrital muscovite in a Neoproterozoic‐Cambrian sedimentary sequence in the Caledonian terrane, suggesting that the HMS was exposed by latest Neoproterozoic time and supplied detritus to the sedimentary units. The HMS is interpreted to represent a fragment of an accretionary complex, similar to the Sanbagawa Belt in Japan. It confirms the presence of a major cryptic suture between the Avalon terrane sensu stricto and the now‐adjacent Brookville terrane.

The palaeomagnetism of Lesbos, NE Aegean, and the eastern Mediterranean inclination anomaly

Summary Palaeomagnetic results for 44 sites in 16–22 Ma volcanic rocks from Lesbos, NE Aegean, yield a mean pole at 81.8°N, 178.1°E, K = 9.0, A95 = 7.6°. The mean direction for these sites (D = 4.3°, I = 48.5°, k = 10.8, α95 = 6.9°) is 5.9° ± 6.1° shallower than the reference direction for Miocene Lesbos calculated from Besse & Courtillot (1991). Combining these new data with previous work yields a mean inclination that is 5.6° ± 4.7° too shallow. Experimental problems, magnetic anisotropy, the magnetic terrain effect, geomagnetic anomalies, and problems with the reference path all seem unable to account for this observation. Shallow inclinations are a common feature of the Aegean region: 17 of 18 palaeomagnetic studies on Cenozoic igneous rocks have returned an inclination that is shallower than expected. Northward motion of the Aegean block by ∼500 km with respect to northern Europe would account for this observation.

The palaeomagnetism of Lesbos, NE Aegean, and the eastern Mediterranean inclination anomaly

Palaeomagnetic results for 44 sites in 16–22 Ma volcanic rocks from Lesbos, NE Aegean, yield a mean pole at 81.8°N, 178.1°E, K = 9.0, A95 = 7.6°. The mean direction for these sites (D = 4.3°, I = 48.5°, k = 10.8, α95 = 6.9°) is 5.9° ± 6.1° shallower than the reference direction for Miocene Lesbos calculated from Besse & Courtillot (1991). Combining these new data with previous work yields a mean inclination that is 5.6° ± 4.7° too shallow. Experimental problems, magnetic anisotropy, the magnetic terrain effect, geomagnetic anomalies, and problems with the reference path all seem unable to account for this observation. Shallow inclinations are a common feature of the Aegean region: 17 of 18 palaeomagnetic studies on Cenozoic igneous rocks have returned an inclination that is shallower than expected. Northward motion of the Aegean block by ∼500 km with respect to northern Europe would account for this observation.

Reply to: Comment to: "volcanic activity from the Neogene to the present evolution of the Western Mediterranean area. a review" by Michele Lustrino

We recently published a review of geochemical data on volcanic products emplaced during the last 30 Ma in the western Mediterranean area (Lustrino, 2000a). In this paper, the most recent petrogenetic models proposed for various igneous provinces were compared each other and tentatively fitted in a geodynamic scenario of evolution. The aim of that paper was to furnish an updated review of the petrologic models proposed to explain the origin of the main volcanic outcrops of Spain, France, Morocco and Italy, with special emphasis given to the Plio-Pleistocene volcanic rocks of Sardinia. The choice to focus on the recent volcanic activity of Sardinia was thought because of its geographic position (Sardinia represents a lithospheric slice in between two oceanic areas reworked during Hercynian and Alpine orogeneses), its peculiar trace element composition (Plio- Pleistocene volcanic rocks of Sardinia show the highest Ba/Nb and one of the lowest Ce/Pb and Nb/U among the entire Cenozoic European Volcanic Province anorogenic products) and its unique Sr-Nd-Pb isotopic signature (87Sr/86Sr close to bulk Earth values, eNd ~ -5, 206Pb/204Pb down to ~ 17; Lustrino et al., 2000). The comments of Elter seem to not take in consideration the true subject of our paper (geochemistry of 30-0 Ma volcanic rocks) and are entirely focused on the Hercynian geodynamic evolution of Western and Southern Europe. According to Elter, we presented a “continuous” model from Hercynian time to the present, but this is not the case. We don’t present any continuous model: on trace element and Sr-Nd-Pb isotopic grounds (data published elsewhere; Lustrino, 2000c; Lustrino et al., 2000) we simply suppose a relationship between processes responsible for the geochemical signature of mantle sources of Plio-Pleistocene volcanic rocks of Sardinia and those occurred in late Paleozoic, mainly during post-collisional extensional movements. Now we are going to reply to his doubts about the scientific valence of our paper (Lustrino, 2000a); in particular, we will follow its own scheme.

The 1998 edition of the National Geological Map of Botswana

A new National Geological Map of Botswana incorporates data acquired from a variety of sources; the map is produced as a 1:1 million hardcopy as well as in digital format. The new map shows the pre-Kalahari Group geology. The oldest rocks are exposed in eastern Botswana where three Archaean terranes are recognised: the western parts of the Kaapvaal and Zimbabwe Cratons and the western part of the Limpopo Mobile Belt. All three terranes are lithologically similar but differ in their structural styles and in the timing of major thermal events. The oldest (pre-3.0 Gal high-grade metamorphic rocks are found in the Kaapvaal Craton, and the youngest in the Limpopo Mobile Belt, which appears to record Palæoproterozoic ductile shearing. Proterozoic orogenic belts, mostly concealed beneath Karoo rocks, define the western limits of the Archaean terranes and pprogressively young westwards away from the Archaean rocks. The Palwoproterozoic Magondi and Kheis Belts are well-defined by regional magnetic maps, but both are very poorly exposed in Botswana. The Kheis Belt trends due north from South Africa into central Botswana to define the western edge of the Kaapvaal Craton. The western part of the Magondi Belt, as well as all of a Mesoproterozoic (Kibaran) belt and rift are overprinted by the Neoproterozoic Damara Belt; all have pronounced northeasterly trends. During the Palæoproterozoic, there was also significant intraplate magmatism, sedimentation and deformation within the Archæan terranes. Some of the magmatism (in southeastern Botswana) was contemporaneous with, and lithologically similar to, the Bushveld Igneous Complex of South Africa. The main feature of the Mesoproterozoic geology of Botswana is a northeast trending rift that extends right across the northwest of the country and which is partly infilled with ca 1 106 Ma volcanic rocks. Neoproterozoic sedimentary rocks overlie the volcanics within the rift. The various rocks are exposed along the Ghanzi Ridge and to the northeast in the Chobe District. New detailed airborne magnetic surveys in northwest Botswana (Ngamiland) show the detailed geology of the northeast trending inland branch of the Damara Belt and exactly define its northwestern and southeastern boundaries. The southeastern part of the Damara Belt comprises the Mesoproterozoic volcanics of the Kgwebe Formation and the Neoproterozoic Ghanzi Group sedimentary strata. The full extent of the volcanics, and of the three formations recognised in the Ghanzi Group, is shown on the new map. Deformation of these rocks increases to the northwest where they are bounded by the tectono-stratigraphical Roibok Group. To the northwest of the Roibok Group are poorly dated granitoid rocks separated into several units that are locally overlain by carbonate-dominated sequences. A cover sequence of metasedimentary rocks with northnorthwest trending folds lies northwest of the Damara Belt. These sediments may overlie the southernmost part of the Congo Craton in the extreme northwest of Botswana. Neoproterozoic/ Lower Palæozoic sediments of the Nama Group partly infill a foreland basin to the south of the Damara belt in western Botswana. Karoo strata deposited within the Kalahari Basin underlie central Botswana. The distribution of the four major sedimentary groups, as well as of the capping basalts, is shown. The total thickness of the sediments is < 2000 m and the basalts are up to 1000 m in thickness. The sediments comprise a lower sequence (Dwyka and Ecca Groups) related to regional sagging and an upper sequence (Beaufort and Lebung Groups) that succeeded regional uplift that created intra-Karoo unconformities. Karoo sedimentation commenced towards the end of the Carboniferous Period and the basalts were extruded at about 180 Ma before Present. Wherever there have been detailed studies undertaken on the Karoo rocks, they show intense faulting that may or may not mimic structures in the pre-Karoo bedrock. The faulting appears to be post-sedimentation. No evidence was found for growth faults producing abnormal thicknesses of Karoo sediments. It is always possible to correlate the internal stratigraphy, at least at the formational level across the faults. Abnormal thicknesses of the basalts are preserved on the downthrow sides of the major faults. A major dyke swarm coeval with the extrusive basalts trends east-southeast right across north-central Botswana to cut across older structural trends. Over 200 kimberlites are shown on the new map. The kimberlites are distributed throughout Botswana in a number of separate clusters. Most of the kimberlites are of Cretaceous age. Isopachs are shown of the Kalahari Group, which is generally < 180 m in total thickness.

Coeval sedimentation, magmatism, and fold-thrust development in the Trans-Hudson Orogen: propagation of deformation into an active continental arc setting, Wekusko Lake area, Manitoba

Early structures in the southeastern part of the Trans-Hudson Orogen (THO), Wekusko Lake area, Manitoba, comprise northwest-verging folds and a set of steep, southeast-dipping faults that occur at a low angle to bedding. Kinematic indicators are rare, however, the vergence of the associated folds supports reverse movement on the faults. These folds and faults formed prior to the peak of regional metamorphism and therefore must correlate with the widely recognised deformational (D2) event which developed pre- to syn-metamorphism and involved development of a regional, south- to southwest-directed fold-thrust belt. The D2 folds and reverse faults in the study area are interpreted to have originally formed as gently dipping, southwest-verging structures, i.e., as inclined folds and thrust faults, which were then rotated into a steep, northwest-verging orientation during later deformation (D3). In the southeastern THO, regional D2 fold and thrust structures are cut by ca. 1839-1830 Ma plutons, indicating that D2 initiated prior to ca. 1839 Ma. Recognition of these structures in the study area, and their relationships to key stratigraphic units, has provided significant insights into the tectonic setting during the regional D2 event. The D2 fold and thrust structures deform ca. 1834 Ma fluvial sedimentary rocks and ca. 1836-1832 Ma volcanic (subaerial) and plutonic rocks with a "continental" arc geochemical signature. These are the youngest known volcanic and sedimentary rocks within the THO. The subaerial deposition of these young rocks was coeval with initiation of D2 deformation in the region. They were then rapidly incorporated into the thrust fault sequence soon after their formation, which suggests that the fold and thrust structures propagated into a zone of active magmatism which must have been located in the foreland to the belt of thrusting and folding.

Large‐scale right‐slip displacement on the East San Francisco Bay Region fault system, California: Implications for location of late Miocene to Pliocene Pacific plate boundary

A belt of northwardly younging Neogene and Quaternary volcanic rocks and hydrothermal vein systems, together with a distinctive Cretaceous terrane of the Franciscan Complex (the Permanente terrane), exhibits about 160 to 170 km of cumulative dextral offset across faults of the East San Francisco Bay Region (ESFBR) fault system. The offset hydrothermal veins and volcanic rocks range in age from .01 Ma at the northwest end to about 17.6 Ma at the southeast end. In the fault block between the San Andreas and ESFBR fault systems, where volcanic rocks are scarce, hydrothermal vein system ages clearly indicate that the northward younging thermal overprint affected these rocks beginning about 18 Ma. The age progression of these volcanic rocks and hydrothermal vein systems is consistent with previously proposed models that relate northward propagation of the San Andreas transform to the opening of an asthenospheric window beneath the North American plate margin in the wake of subducting lithosphere. The similarity in the amount of offset of the Permanente terrane across the ESFBR fault system to that derived by restoring continuity in the northward younging age progression of volcanic rocks and hydrothermal veins suggests a model in which 80–110 km of offset are taken up 8 to 6 Ma on a fault aligned with the Bloomfield‐Tolay‐Franklin‐Concord‐Sunol‐Calaveras faults. An additional 50–70 km of cumulative slip are taken up ≤ 6 Ma by the Rogers Creek‐Hayward and Concord‐Franklin‐Sunol‐Calaveras faults. An alternative model in which the Permanente terrane is offset about 80 km by pre‐Miocene faults does not adequately restore the distribution of 8–12 Ma volcanic rocks and hydrothermal veins to a single northwardly younging age trend. If 80–110 km of slip was taken up by the ESFBR fault system between 8 and 6 Ma, dextral slip rates were 40–55 mm/yr. Such high rates might occur if the ESFBR fault system rather than the San Andreas fault acted as the transform margin at this time. Major transpression across the boundary between the Pacific and North American plates at about 3 to 5 Ma would have resulted in the transfer of significant slip back to the San Francisco Peninsula segment of the San Andreas fault. Since that time, the ESFBR fault system has continued to slip at rates of 11–14 mm/yr. If this interpretation is valid, the ESFBR fault system was the Pacific‐North American plate boundary between 8 and 6 Ma, and this boundary has migrated both eastward and westward with time, in response to changing plate margin geometry and plate motions.

Early calc-alkaline magmatism in the Mogollon-Data Volcanic Field, New Mexico, USA

The Mogollon-Datil Volcanic Field in southwest New Mexico was active between 40 and 20 Ma. It is situated c. 500 km from the western North American plate margin, and yet the early pre-30 Ma volcanic rocks comprise a high-K calc-alkaline suite ranging from basaltic andesites to dacites. All the samples are characterized by high and variable LILE and LREE contents, and LILE/HFSE ratios, and variable Nd and Sr isotope ratios of 0.51231–0.51250 and 0.7055–0.7065 respectively. In detail, three groups are recognized: Group 1 rocks have the lower SiO 2 , incompatible element abundances and 87 Sr/ 86 Sr ratios; Group 2 preserves the clearest evidence for open system differentiation, and Group 3 exhibits little variation in 87 Sr/ 86 Sr and the most coherent major and trace element arrays. The Group 2 rocks have been modelled in terms of simple binary mixing between selected Group 1 basaltic andesites and a higher silica crustal endmember, which appears to have been derived from the mid-lower crust in the presence of residual garnet. The Group 3 rocks, by contrast, are consistent with near closed system differentiation, modelled in three stages and involving Plag-Aug-O1-Mag and subsequently hornblende and K-feldspar. Published experimental data indicate that the differentiation of the parental basaltic magmas took place at elevated pressures of c . 4-5 kbar, and water contents, c . 2wt% H 2 O. It is argued that the parental magmas to these calc-alkaline suites were generated in a period of lithospheric extension, and that they were derived from within the continental mantle lithosphere, as inferred for mid- to Late Tertiary rocks elsewhere in the western US, with no significant contribution from contemporaneous subduction processes.

Mid‐Cretaceous paleomagnetic results from Marie Byrd Land, West Antarctica: A test of post‐100 Ma relative motion between East and West Antarctica

As part of the tripartite, United States ‐ United Kingdom ‐ New Zealand, 1990–1991 South Pacific Rim International Tectonics Expedition, oriented samples were collected for paleomagnetic analysis from mid‐Cretaceous (circa 100 Ma) intrusive rocks at sampling localities across 350 km of the Ruppert and Hobbs Coast area of Marie Byrd Land, West Antarctica. Paleomagnetic results are presented along with several lines of evidence, including a positive tilt test based on the attitude of circa 117 Ma volcanic rocks that the circa 100 Ma rocks intrude, which argue that these results are a representative estimate of the mid‐Cretaceous magnetic field in Marie Byrd Land (MBL). The new circa 100 Ma mean south pole (224.1°E/75.7°S, A95 = 3.8°, N = 19 site means) is concordant with other West Antarctic results of similar age implying that at least Marie Byrd Land, Thurston Island and the Antarctic Peninsula have not experienced any paleomagnetically resolvable relative motion since the mid‐Cretaceous. However, the poles from these Pacific‐bordering blocks of West Antarctica are significantly offset from a synthetic apparent polar wander path that was produced for the East Antarctic craton, implying relative movement between East Antarctica and Pacific West Antarctica since about 100 Ma. Though the paleomagnetic estimate for east‐west Antarctic relative motion may be reconciled with geologic estimates for extension in the Ross Sea at the extremes of the error envelope, the best paleomagnetic estimate of relative motion suggests a larger amount of total extension between East and West Antarctica (MBL) than previously suspected. Both estimates call for several hundreds of kilometers of post‐100 Ma displacement between East Antarctica and the Pacific‐bordering blocks of West Antarctica.