Tertiary Volcanism Research Articles

Basin structure and Tertiary magmatism on Skye, NW Scotland

The emplacement of igneous material into upper crustal rocks of sedimentary basins is likely to be strongly controlled by the geometry of the pre-existing basin structures. These controls are investigated using examples from the Tertiary igneous complexes of Skye, part of the Sea of Hebrides basin of NW Scotland. The basin consists of an array of half-graben related to SE-dipping normal faults. These pre-volcanic, Mesozoic structures are traced near the igneous complexes using geological relationships preserved unconformably beneath the widespread basaltic lava fields. The unconformity represents a period of Cretaceous uplift and denudation of the basin and its flanks, entirely pre-dating the Tertiary volcanism of NW Scotland. This unconformity seals stratigraphically the major basin faults, preserving field relationships that permit the tracing of these faults in the country rocks to the Tertiary intrusions. The major Camasunary fault is separated from the Raasay fault via a series of minor graben, linked by a series of steep, NW–SE-trending faults that transfered Mesozoic displacements between the principal fault strands. A broad range of igneous material of various compositions was intruded into part of the Mesozoic Sea of Hebrides basins and their flanks during Palaeocene times. Different emplacement styles and different structural controls are found. The major gabbroic centres do not appear to be controlled by upper crustal structures, having been emplaced into the footwalls of major faults. However, minor synmagmatic displacements on the basin faults may have been sufficient to generate dilatational sites in these footwall positions, thereby facilitating emplacement. In contrast, the granitic melts have been emplaced as sheets and domed into the sediments and overlying lava pile, reactivating segments of the basin fault network. Doming occurred from an array of sills, the stratigraphic levels of which can be reconstructed using structural relationships preserved in the roofs and walls of the intrusions. The sill levels and their transgressive forms are strongly related to inferred Mesozoic basin structures. The major fold structures of Tertiary age in southern Skye are interpreted as accommodating granitic emplacement rather than crustal shortening. The NW-SE Mesozoic transfer fault trend appears to have strongly influenced the segmentation of the granite domes. These interpretations are illustrated using field relationships mapped in the vicinity of the Coire Uaigneich granophyre. It is concluded that although the higher parts of the basin faults were reactivated to facilitate the doming of granitic intrusions, the deeper levels of the Mesozoic faults show no evidence of substantial reactivation.

A crustal and uppermost mantle cross-sectional model of the Kenya Rift derived from seismic and gravity data

The most significant implication of a combined seismic and gravity model derived from the data along the KRISP 90 cross-rift line on the Equator concerns the compensation mechanism for the uplifted East African plateau. The seismic model shows that despite an elevation difference of 400 m the thickness of the crust at both ends of the profile is approximately the same at about 34 km b.s.l. A regional gradient needs to be added to the gravity field calculated from the seismic model to agree with the observed. This regional is consistent with Ebinger et al.'s (1989) proposal that the plateau is being dynamically supported by convective processes in the mantle. The principal features revealed by the seismic modelling are: 1. (1) the presence of the thick graben infill, and in particular a very deep (8 km b.s.l.) low-velocity body beneath the Kerio Valley adjoining the rift's western boundary, the Elgeyo fault; 2. (2) the correlation between seismic velocities in the upper crust and different Precambrian (Archaean and Pan-African Mozambique orogenic belt) crustal packages identified along the profile; 3. (3) a thickening of the crust beneath both margins of the rift; 4. (4) the apparent symmetry in crustal thinning mirroring the surface distribution of Tertiary volcanics and sediments, suggesting a pure shear mechanism of crustal extension; 5. (5) the presence of a wide, anomalously low-velocity (7.6–7.8 km s −1) body at the base of the crust identified along the axial line and which it has been argued must include partial melt. This may be continuous with a deep low-velocity zone beneath the rift previously identified from the study of teleseismic residuals; and 6. (6) the presence of an intra-mantle reflector at a depth of approximately 55 km beneath the western margin of the rift.

Lead, sulphur, oxygen and carbon isotopes in the Zletovo ore field (Eastern Macedonia)

Results of isotopic studies of lead, sulphur, oxygen and carbon in the main hydrothermal ore veins and stockwork-disseminated polymetallic ore formations within the Tertiary volcanics in the Zletovo ore field are exposed and discussed in the present paper. Ore metals and sulphur are of endogenous origin, most probably from the deeper parts of the Earth’s crust or/and the upper mantle. On the basis of the isotopic composition of oxygen and carbon it can be supposed that mixed underground meteoric waters influenced to a great extent the ore-bearing fluids. The volcanic activity and the deposition of the polymetallic mineralizations took place within the interval between 27 and 16 Ma BP, according to the K-Ar data obtained.

The structure of the Sea of the Hebrides Basin: an integrated gravity and seismic model

Synopsis By combining seismic and gravity data for the Sea of the Hebrides Basin it has been possible to overcome problems of sea bed multiples and poor energy transmission in seismic data where a sediment starved sea floor and extensive igneous intrusions and volcanic rocks close to the sea bed have previously inhibited detailed interpretation of the basin structure. Models for the basin structure, partly constrained by the seismic data, parallel to the dip and strike of the basin show a good correlation between calculated and observed gravity anomalies and are consistent with the onshore geology. The basin has a half graben geometry with late-Triassic syn-rift sediments thickening toward the eastward dipping Minch Fault, overlain by post-rift Jurassic sediments. Early Tertiary volcanism was followed by Oligocene extensional faulting.

Iceland hotspot track

We use a model of plate motions relative to major hotspots underneath the African, Indian, North American, South American, and Australian plates to compute the track of the Iceland hotspot after 130 Ma. The present-day hotspot is located under eastern Iceland offset about 240 km east of the Reykjanes and Kolbeinsey ridges. At 40 Ma, the Kangerlussuaq region of East Greenland would have been directly above the hotspot. The anomalous postdrift uplift of the East Greenland margin can also be explained by passage of the rifted margin over a hotspot. At 60 Ma, the Umanak Fjord region of the west coast of Greenland was above the hotspot, where picrites and hyaloclastites of nearby Disko Island are dated at ∼64 to 59 Ma. Our reconstruction shows Ellesmere Island above the hotspot between 130 and 100 Ma. Latest Albian to early Cenomanian volcanic rocks on Axel Heiberg Island and northern Ellesmere Island indicate a nearby hotspot at that time. At 130 Ma, our model locates the hotspot near the northern margin of Ellesmere Island, close to the intersection of the Alpha Ridge with the coast. The hotspot would have been located beneath the Arctic Alaska-Chukotka plate when it formed the Mendeleyev Ridge, and as the spreading center migrated over the hotspot, it transferred to the North American plate, where it formed the Alpha Ridge. Our model suggests that the initiation of the Iceland hotspot predates the opening of the North Atlantic by at least 70 m.y. and that the massive early Tertiary volcanism along the North Atlantic plate margins reflects the effect of rifting in the vicinity of existing thinned crust, rather than the arrival of a plume head.

Late Paleozoic Tectonic Evolution of the Eastern Margin of the Diablo Platform, West Texas: ABSTRACT

The Diablo platform is a long-lived positive tectonic feature that forms the western and southwestern margin of the Delaware basin of west Texas and New Mexico. The platform is known to have been episodically active from at least the early Paleozoic to the Holocene. Although the western margin of the platform is well exposed in fault blocks formed during Tertiary Basin-and-Range-style deformation, the eastern margin is largely concealed beneath Cretaceous to Holocene sediments and Tertiary volcanics. Well control along the eastern margin of the Diablo platform is sparse, averaging less than one sub-Permian penetration per 128 km[sup 2] (50 mi[sup 2]). This study is part of a larger effort to utilize newly acquired seismic data, reprocessed older seismic data, and subsurface well control to formulate a structural interpretation of the Delaware basin and its margins. Early 1970s-age Delaware basin group-shoot seismic data were reprocessed to image parts of the eastern margin of the Diablo platform. Data were reprocessed with a high-performance interactive computer workstation and commercially available software. Improved seismic imaging is obtained by detailed velocity analysis and residual statics. Results show that part of the eastern margin of the Diablo platform is bounded by west-dipping reverse faults having upmore » to 1000 m (3300 ft) of vertical displacement. High-angle normal (oblique ) faults having as much as 2000 m (6600 ft) or vertical displacement are, in part, younger than the reverse faults, cutting beds as young as Permian (Ochoan) age (Rustler Formation). These features are interpreted to reflect Pennsylvanian-aged northeast-southwest-oriented compression followed by Middle to Late Permian differential subsidence of the Delaware basin relative to the Diablo platform. Analogy with the basin-bounding margin adjacent to the Central Basin platform suggests that undiscovered compressional structures may exist along the eastern margin of the Diablo platform.« less

Rock magnetism of Tertiary volcanics from North-East Greenland

Rock magnetic results from 8 profiles covering stratigraphically the northern part of the Early Tertiary volcanic province in North-East Greenland (73°30'-75°N) are summarised. Tests in the field revealed only reversed polarities, which are likely to fall in the time span between chron 24r and 26r. Mean magnetic susceptibilities from 60 sites (some 2100 in situ measurements), together with average values of the intensity of the NRM (based upon 160 samples from 37 flows and dykes) and corresponding Q ratios, are tabulated. The overall means and standard deviations are k = 0.010 SI (s.d. = 0.003), J = l.7 Atm (s.d. = l.3) and Q= 2.8 (s.d. = 3), respectively. The susceptibilities are normally distributed, in contrast to the NRM intensity values and the Q ratios, which tend to be log-normal distributed.

Tertiary magmatic belts in Java

Evidence of the earliest magmatic event (Late Eocene-Early Miocene) has recently been discovered in East Java (Pacitan area) as exposures of pillow lava flows. Stratigraphically they represent the lowermost part of Besole Formation (Oligocene-Early Miocene) which is made up mainly of volcanic turbidites. Major element chemistry of the lava flows indicates typical island arc tholieites with a significant Nb anomaly and relatively flat REE patterns. In Central Java the same magmatic event resulted in the emplacement of tholeitic intrusion at Karangsambung, Luk Ulo. The corresponding volcanic products of this event in West Java are represented by the Jampang Formation (Oligocene-Early Miocene) which is limited to the southern part of West Java. The chemistry of Neogene volcanic rocks which have been collected from the Jatiluhur-Sanggabuana area in West Java show that they are calc-alkali basalts and andesites. Some of them are marked by higher K 2O contents leading to the potassic calc-alkali and shoshonitic rock series; similar phenomena are found in the Neogene volcanics of Karangkobar in Central Java. In East Java (Pacitan-Trenggalek area) outcrops of calc-alkali basalt flows underlie the limestones of the Punung Formation (Middle-late Miocene); the latter is also cut by dykes and volcanic necks of more differentiated rocks. From the pattern of distribution of the Tertiary volcanics, two parallel east-west trending magmatic belts may be recognized; the belt along the southern part of Java is related to Paleogene subduction, whereas the other to the north is related to Neogene subduction. The Quaternary volcanic cones in general are distributed within the Neogene magmatic belt.

Age relationships and tectonic implications of late Cenozoic basaltic volcanism in Northland, New Zealand

Abstract An episode of late Miocene ‐ Recent essentially basaltic volcanism is the latest in a sequence of magmatic events recognised in the tectonically complex geological development of the Northland Peninsula. New K‐Ar dates together with an extensive collection of new major and trace element chemical analyses prompt a reassessment of the significance of these late Cenozoic basalts. The main time/ space groupings recognised are Tertiary volcanics in the Kaikohe ‐ Bay of Islands, Puhipuhi, Ti Point, and Stony Batter areas and Quaternary basalts in the Kaikohe ‐ Bay of Islands and Whangarei areas and at Tara. Basalts in the Kaikohe ‐ Bay of Islands area are transitional to alkalic in character, while those in the south are transitional to tholeiitic, with the Ti Point and Stony Batter rocks being geochemically distinct. A consistent model for these observations is that the magmas originate from different levels of a layered mantle source in which the upper part carries a geochemical signature inherited from an earlier subduction event.

Interpretation of three-dimensional seismic refraction data from western Hecate Strait, British Columbia: structure of the crust

As part of a multidisciplinary investigation of the structure and tectonics of the Queen Charlotte Basin and underlying crust, deep multichannel seismic reflection and coincident crustal refraction data were collected in 1988. Energy from the reflection air-gun array source was recorded at land sites at offsets appropriate to record crustal refraction and wide-angle reflection data. Refraction data recorded in a broadside geometry provide good three-dimensional coverage of western Hecate Strait. These data are modelled using tomographic inversion techniques to determine the three-dimensional velocity structure of the crust in this region. The one-dimensional average velocity increases rapidly with depth to 6.5 km/s at 7 km depth. Velocities from 7 to at least 12 km depth remain approximately constant and are associated with rocks of the Wrangellia terrane. Significant lateral velocity variations, including large differences in near-surface velocities attributable to surface features, relatively low velocities representing interbedded Tertiary sediments and volcanics, and a deep high-velocity anomaly that may represent the root of an igneous intrusion, are mapped. Wide-angle reflections from the Moho are used to determine the thickness of the crust. The Moho is at 29 km depth beneath the east coast of the Queen Charlotte Islands. This is deeper than the Moho observed below Queen Charlotte Sound and as deep as, or deeper than, that below Hecate Strait. Crustal thinning during Tertiary extension was thus greatest beneath the surface expression of the Queen Charlotte Basin, leaving the crust under the islands considerably thicker than under the basin. In an alternate or additional explanation, compression at the continental margin during the last 4 Ma may have been taken up by thickening or underplating of the continental crust beneath the islands. If the Pacific plate is subducting beneath the islands, the Moho observations constrain the slab to dip greater than 20–26°.

The Tertiary picrites of West Greenland: contributions from ‘Icelandic’ and other sources

New isotope and trace element data are presented for suites of tholeiitic picrites from Disko, Ubekendt Ejland and Svartenhuk Halvø of the West Greenland Tertiary Volcanic Province. Geographic and stratigraphic subgroups are identified by major element, trace element and Sr and Nd isotope systematics. Early picrites on Disko have ε Nd = +8to+10 and ε Sr = −22to−15 , similar to subaerial Icelandic lavas, and overlap compositionally with North Atlantic MORB. Most other picrites of the province are enriched relative to MORB but still have ε Nd > +6 and ε Sr < −10 . Crustal sources cannot explain the observed geochemical variation. Modelling of the major oxides Al 2O 3, CaO and TiO 2 and FeO′ suggests that the main part of the West Greenland picrites was derived from a source similar to the most enriched Icelandic picrites by rather high degrees of partial melting (high Ca/OAl 2O 3 ) at relatively high pressures (high FeO′). These features and the high MgO (19 wt% in liquids) are in accordance with their derivation from very hot plume mantle below the continental lithosphere. In the same group of picrites, both major elements, incompatible elements and Sr and Nd isotopes are coupled and are thought to identify the Icelandic plume end member at ca. 63 Ma and today: ε Sr = −15to−18 , ε Nd = +7.5 to +8.5, Al/ 2O 3TiO 2= 7–10 and Zr/Nb= 14–26, and a trace element pattern as in the picrites of the Ordlingassoq Member at Disko and Group 1 of the equivalent lower formation at Svartenhuk Halvø. Lithospheric influence in the picrites is only detected in the northern part of the province. The influence from MORB mantle seen in the older Naujanguit Member of Disko decreases with time. The results lead to discussion and revision of recent models for continental flood basalt volcanism and mantle plumes [1,2] with regard to the site of picrite production relative to the plume axis and the timing of continental flood basalt volcanism in West and East Greenland during the early Tertiary.

Differentiation of volcanic ash-fall and water-borne detrital layers in the Eocene Senakin coal bed, Tanjung Formation, Indonesia

The Sangsang deposit of the Eocene Senakin coal bed, Tanjung Formation, southeastern Kalimantan, Indonesia, contains 11 layers, which are thin (<5 cm) and high in ash (> 70%). These layers are characterized by their pelitic macroscopic texture. Examination of eight of the layers by scanning-electron microscopy, energy-dispersive X-ray, and X-ray diffraction analyses show that they are composed primarily of fairly well-crystallized kaolinite, much of which is vermicular. Accessory minerals include abundant Ti oxide, rare-earth element-rich Ca and A1 phosphates, quartz that luminescences in the blue color range, and euhedral to subhedral pyroxene, hornblende, zircon, and sanidine. Although this mineral suite is suggestive of volcanic ash-fall material, only the four pelitic layers in the middle of the bed are thought to be solely derived from volcanic ash-falls on the basis of diagnostic minerals, replaced glass shards, and lithostratigraphic relationships observed in core and outcrop. The three uppermost pelitic layers contain octahedral chromites, some quartz grains that luminesce in teh orange color range, and some quartz grains that contain two-phase fluid inclusions. These layers are interpreted to be derived from a combination of volcanic ash-fall material and hydrologic transport of volcaniclastic sediment. In contrast, the lowermost pelitic layer, which contains large, rounded FeMg-rich chromites, is thought to have been dominantly deposited by water. The source of the volcanic ash-fall material may have been middle Tertiary volcanism related to plate tectonic activity between Kalimantan and Sulawesi. The volcanic ash was deposited in sufficient amounts to be preserved as layers within the coal only in the northern portions of the Senakin region: the southern coal beds in the region do not contain pelitic layers.

Space-time patterns and tectonic controls of Tertiary extension and magmatism in the Great Basin of the western United States

Structural and stratigraphic relations in the Great Basin indicate widespread pre-middle Miocene crustal extension that appears to define two north-trending belts. Most extension in these belts was Oligocene age, but locally it began earlier or lasted into early Miocene time. The eastern belt straddles the Nevada-Utah border and includes the Snake Range, Nevada, area, with its southern end near 37.5°N and its western edge at the Seaman-Butte Mountains breakaway. The southern boundary of the eastern belt is occupied by the 26-15 Ma Caliente caldera complex and approximately coincides with the present east-west-trending margin of the Great Basin north of Saint George, Utah. Crust north of this boundary extended approximately east-west before volcanism began at 30-32 Ma, but to the south, extension began after about 15 Ma. This boundary may have been a rooted zone of left-slip faults that allowed the footwall of the Stampede detachment to move west relative to unextended terrain to the south. The eastern margin of the eastern belt is probably located near the present eastern edge of the Great Basin, but its northern end is poorly defined. The western belt runs from the Funeral and Grapevine Mountains, California, to the Ruby Mountains, Nevada, and north-northeast to the Albion Range, Idaho. Tens of kilometers of crustal extension occurred at least locally in both belts, but magnitude of extension is poorly known for large areas of each. Tertiary volcanism in the Great Basin began in the north in Eocene time with predominantly effusive volcanism and swept southward, ending with voluminous Oligocene-Miocene ignimbrite eruptions from calderas in an irregular, discontinuous belt between Marysvale, Utah, and Reno, Nevada. A result of the south-ward migration of volcanism is that the onset of extension in both belts was syn- or post-volcanic in the north but was pre-volcanic in the south. Late Paleogene extension and crustal magmatism coincided in both time and space only locally, where south-migrating magmatism overlapped active north-south-trending extensional belts. Most calderas in the southern Great Basin formed in previously extended belts or on their margins. Southward migration of ignimbrite sources was apparently blocked by unextended crust to the south. In contrast, volcanism north of the ignimbrite province was dominated by nonexplosive effusion of lava prior to, or during, crustal extension. This is consistent with observations in the southern Basin and Range, where volcanism and crustal extension were generally synchronous, and volcanism was dominantly effusive. Thus, caldera formation may be controlled by the distribution of upper-crustal extension, although the physical mechanism for this control remains speculative. The space-time patterns of late Paleogene extension in the Great Basin are consistent with extension being triggered by thermal weakening of subducted oceanic lithosphere rather than by effects transmitted from the plate margin, but being driven by gravitational collapse of thick crust. Space-time patterns of Tertiary volcanism in the Basin and Range also appear to conform to patterns of thermal weakening or destruction of the subducted slab. Both active and passive rifting mechanisms are inapplicable on the scale of the extensional belts, because both predict close spatial and temporal association of extension and magmatism, which is not generally observed.

Development of the Rockall Trough and the northwest European continental margin

During the Cretaceous and early Tertiary a series of sedimentary basins opened by rifting on the northwest margin of Europe. None of these developed into a fully-fledged ocean until the northern North Atlantic opened at 55 Ma between NW Europe and Greenland. The subsidence, magmatism, and thermal history of these basins and of the continental margin depended critically on the amount and style of stretching and on the temperature of the underlying asthenospheric mantle. Small variations in the mantle temperature can have dramatic effects on both the magmatism produced during rifting and on the subsidence of the basin. An increase of as little as 10% in the mantle temperature can cause uplift above sea-level rather than the more common subsidence when continental crust undergoes large amounts of extension. The initiation of the Iceland mantle plume at about 62 Ma caused abnormally elevated mantle temperatures across a 2000 km diameter region and had a dramatic effect on the subsidence and rifting history of sedimentary basins west of Britain. Their thermal history can be documented from the subsidence and magmatic record in the sedimentary basins and the continental margin. Using normal incidence and wide-angle seismic data and drilling results from the continental margin and from basins of NW Europe, their subsidence and thermal history can be modelled as a function of the amount of stretching and the temperature of the underlying mantle. The Rockall Trough itself is underlain by 5 km of sediments intruded by Tertiary volcanics. The basement imaged on seismic . . .

Geochemical evidence for the tectonic setting of early Proterozoic metavolcanic sequences in southern Lake Superior region.

Samples of metabasaltic dikes and lavas from early Proterozoic rock units in the southcentral Lake Superior region were analyzed for major and selected trace elements in order to compare their chemical composition with those of rocks from modern, known tectonic environments. The rocks exhibit a relatively wide compositional range in which the least evolved samples show a chemical composition similar to that of T-type MORB. Based on geochemical characteristics and tectonic discrimination diagrams, I conclude that the rock suites were emplaced in an extensional regime. The geochemical patterns of these rocks are similar to those of British Tertiary Volcanic Province and continental tholeiitic basalts from Rdendeka. which ernnted in more recent rift environments.

Middle Tertiary Volcanism During Ridge-Trench Interactions in Western California

Bimodal volcanism in the Santa Maria Province of west-central California occurred when segments of the East Pacific Rise interacted with a subduction zone along the California margin during the Early Miocene (about 17 million years ago). Isotopic compositions of neodymium and strontium as well as trace-element data indicate that these volcanic rocks were derived from a depleted-mantle (mid-ocean ridge basalt) source. After ridge-trench interactions, the depleted-mantle reservoir was juxtaposed beneath the continental margin and was erupted to form basalts. It also assimilated and partially melted local Jurassic-Cretaceous sedimentary and metasedimentary basement rocks to form rhyolites and dacites.

Continental rifting and drainage reversal: The clarence river of Eastern Australia

AbstractThe Clarence River on Australia's east coast has an anomalous drainage pattern. Its right‐bank tributaries are markedly barbed, suggesting reversal, whereas Tertiary volcanism has disrupted its left‐bank drainage. The southeast‐flowing Clarence is closely aligned with the northwest‐flowing Condamine River just across the Continental Divide. The Condamine‐Clarence alignment is continued by a large southern tributary, the Orara River, which flows northwest, away from the sea, to meet the southeast‐flowing Clarence. A broad river with a quite different character flows east from near the Orara‐Clarence junction to the sea. This is essentially an overflow channel.This series of aligned streams, the Condamine‐Clarence‐Orara, represents the remains of an earlier northwest‐flowing stream that extended the full length of the Clarence‐Moreton Basin, an eastern extension of the Great Artesian Basin. During the Jurassic, the Clarence‐Moreton Basin was filled with sediments from the surrounding highlands, including those to the east of the present coastline. Continental rifting from Late Cretaceous times onwards led to the opening of the Tasman Sea, causing the reversal and beheading of the original northwest‐flowing streams and the formation of the Great Escarpment.The evolution of the Clarence River does not fit into most conventional geomorphic paradigms such as cycles, climatic geomorphology or steady‐state landforms. It is the result of a succession of unique events on a very long timescale, and as such is a classic example of evolutionary geomorphology.

Timescales and the role of inheritance in long‐term landscape evolution, northern New England, Australia

AbstractLandscape evolution in northern New England is characterized by the persistence of landforms over long time periods. The topography in the study area was initiated by Paleozoic diapiric intrusion of granitic rocks as highs within metasedimentary and volcanic rocks. This framework was probably reinforced by differences in the resistance of the massive igneous and the fractured metasedimentary rocks to erosion. As a result, both the Tertiary and present‐day relief of the area consists of uplands developed on granitic rocks partly overlain by Paleozoic volcanic rocks, and lowlands developed on Paleozoic metasediments. Extensive volcanism in the Tertiary had only a minimal effect on this relationship. A significant impact of Tertiary volcanism, however, was the diversion of major streams across topographic highs. Despite 19‐35 Ma of incision, these streams are still far from establishing equilibrium profiles and dissecting the highs.The persistence of topography for a timescale of the same order of magnitude as that conventionally thought to be required for planation is due to reduced denudation rates in a passive tectonic setting and the presence of erosion‐resistant Paleozoic igneous units. The latter influence is indicated by knickpoint persistence for 19‐35 Ma on streams diverted across these rocks. Since it is generally considered that such disequilibria are short‐lived features formed by Quaternary or ongoing deformation, this knickpoint persistence has important implications for landscape analyses that use disequilibria to assess recent tectonism in a landscape.

Magnetfeldanomalien in der Eifel

The magnetic anomalies over the Laach volcanics and in adjacent areas were interpreted by comparison with the fields of three-dimensional model bodies. In addition to an aeromagnetic survey, magnetic measurements were carried out on the ground and in the laboratory. Most of the model bodies in the Laach volcanic field require a thickness adjustment of only a few tens of meters to achieve a correlation of the magnetic model fields with the anomalies detected at ground level or at flight altitude. The three-dimensional model calculations did not yield any evidence for the existence of a deep-seated body which might be interpreted as a remnant magma chamber from which magma was explosively extruded during the eruption of the Laacher See volcano. The extensive positive magnetic anomaly near Ahrweiler is interpreted as being caused by a magnetized tabular body located at a depth of 8 km. The sharp bend of the isolines in the northeastern part of the Ahrweiler anomaly is a possible indication of the existence of magnetic rock at a maximum depth of 3–4 km. However, the rock need not necessarily occur in the form of a discrete body. The longitudinal axis of the model body lies on the line connecting the Siebengebirge with the Laach volcanic area. Two model body configurations have been developed for the magnetic anomaly near Kelberg. One of them consists of an upper part corresponding to the magma chamber of the Tertiary volcanics of the Hocheifel (high Eifel), and a lower part that can be regarded as a southern extension of the deep-seated body which is probably the source of the Ahrweiler anomaly. The other configuration represents a magnetized zone formed by thermal metamorphism due to an underlying plutonic body or by magma differentiation, possibly during the Paleozoic.

Petrology and petrogenesis of a tertiary bimodal dolerite‐peralkaline/subalkaline trachyte/rhyolite dyke association from Lundy, Bristol Channel, UK

AbstractThe island of Lundy forms the southernmost igneous complex of the British Tertiary Volcanic Province (BTVP) and consists of granite (≈ 90%) emplaced into deformed Devonian sedimentary rocks (Pilton Shale) and associated with a swarm of dykes of dolerite/basalt, minor trachyte and rhyolite composition. The dolerites are of varied olivine basalt composition and are associated with peralkaline trachyte and subalkaline/peralkaline rhyolite with alkali feldspar and quartz ± alkali amphibole ± pyroxene mineralogy. The dyke swarm is therefore an anorogenic bimodal dolerite/basalt–trachyte/rhyolite BTVP association. Although the dyke association is bimodal in major element terms between dolerite/basalt and minor trachyte/rhyolite, the mineralogy and trace element geochemistry indicate that the dykes may be regarded as a cogenetic dolerite—peralkaline trachyte/rhyolite association with minor subalkaline rhyolites. Sr and Nd isotope data indicate derivation of these magmas from a similar BTVP mantle source (with or without minor contamination by Pilton Shale, or possibly Lundy granite).The petrogenesis of the Lundy dyke association is therefore interpreted in terms of extensive fractional crystallization of basaltic magma in a magma chamber of complex geometry below the (exposed) Lundy granite. Fractional crystallization of a representative dolerite magma (olivine ± clinopyroxene ± plagioclase) yields trachyte magma from which the crystallization of alkali feldspar (anorthoclase) ± plagioclase (oligoclase) + Fe–Ti oxide + apatite results in peralkaline rhyolite. Rarer subalkaline rhyolites result from fractionation from a similar dolerite source which did not achieve a peralkaline composition so allowing the crystallization and fractionation of zircon. The basalt–(minor trachyte)/rhyolite bimodality reflects rapid crystallization of basalt magma to trachyte (and rhyolite) over a relatively small temperature interval (mass fraction of melt, F = ≈ 0.15). The rapid high level emplacement of basalt, trachyte and rhyolite dyke magmas is likely to have been associated with the development of a substantial composite bimodal basalt–(minor trachytel)/rhyolite volcano above the BTVP Lundy granite in the Bristol Channel.