Volker Lorenz

Ukinrek Maars, Alaska, I. April 1977 eruption sequence, petrology and tectonic setting

Abstract During ten days of phreatomagmatic activity in early April 1977, two maars formed 13 km behind the Aleutian arc near Peulik volcano on the Alaska Peninsula. They have been named “Ukinrek Maars”, meaning “two holes in the ground” in Yupik Eskimo. The western maar formed at the northwestern end of a low ridge within the first three days and is up to 170 m in diameter and 35 m in depth. The eastern maar formed during the next seven days 600 m east of West Maar at a lower elevation in a shallow saddle on the same ridge and is more circular, up to 300 m in diameter and 70 m in depth. The maars formed in terrain that was heavily glaciated in Pleistocene times. The groundwater contained in the underlying till and silicic volcanics from nearby Peulik volcano controlled the dominantly phreatomagmatic course of the eruption. During the eruptions, steam and ash clouds reached maximum heights of about 6 km and a thin blanket of fine ash was deposited north and east of the vents up to a distance of at least 160 km. Magma started to pool on the floor of East Maar after four days of intense phreatomagmatic activity. The new melt is a weakly undersaturated alkali olivine basalt (Ne = 1.2%) showing some transitional character toward high-alumina basalts. The chemistry, an anomaly in the tholeitic basalt-andesite-dominated Aleutian arc, suggests that the new melt is primitive, generated at a depth of 80 km or greater by a low degree of partial melting of garnet peridotite mantle with little subsequent fractionization during transport. The Pacific plate subduction zone lies at a depth of 150 km beneath the maars. Their position appears to be tectonically controlled by a major regional fault, the Bruin Bay fault, and its intersection with cross-arc structural features. We favor a model for the emplacement of the Ukinrek Maars that does not link the Ukinrek conduit to the plumbing system of nearby Peulik volcano. The Ukinrek eruptions probably represent a genetically distinct magma pulse originating at asthenospheric depths beneath the continental lithosphere.

Plate and intraplate processes of Hercynian Europe during the Late Paleozoic

Abstract It is speculated that until Late Carboniferous time the region of Hercynian Europe was occupied by an elongated island arc system underlain by a segment of continental crust. In the Upper Carboniferous, two subduction zones are assumed to have extended from the north and south beneath Hercynian Europe. An extensive zone of hot, partially molten upper mantle lay above and between these, and diapiric uprise of portions of this material led to separation of mafic magmas, widespread partial melting in the lower and middle crust, high temperature-low pressure metamorphism in crustal rocks, and regional uplift and extension of the crust, as indicated by intermontane troughs and their associated volcanic rocks. In Visean to Westphalian time Hercynian Europe collided with both the large neighbouring plates North America-Europe and Africa. During these diachronous collisions and owing to reduced rigidity of the relatively hot island arc crust, the irregular continental margins of the larger and thicker continental plates induced oroclinal bending of Hercynian Europe. After the collision processes had been terminated, processes of upper mantle activity continued, causing further crustal uplift and even, enhanced crustal extension for several tens of million years into the Lower Permian. Decline of the upper mantle activity beneath Hercynian Europe is indicated by crustal subsidence and formation of a peneplain in Permian time followed by the Upper Permian transgression of both the Zechstein sea and the Tethys sea which mark the end of the Hercynian geodynamic cycle.

Formation of phreatomagmatic maar-diatreme volcanoes and its relevance to kimberlite diatremes

Abstract Studies of maars and diatremes suggest a specific process in their formation. Magma rises along a fissure and contacts ground- or surface derived water. The resulting phreatomagmatic eruptions give rise to base surge and air-fall deposits consisting of juvenile and wall-rock material. Spalling of the wall-rocks enlarges the fissure into an embryonic vent. At a critical diameter of the vent large-scale spalling at depth and slumping near the surface gives rise to a ring-fault of large diameter and subsidence of the enclosed wall-rocks and overlying pyroclastic debris. This subsidence leads to a maar crater at the surface. Fluidization processes are active in the narrow vent and in fractures and faults inside the surrounding structure which subsides en masse . Various features of kimberlite diatremes seem to be consistent with this model. They extend into fissures along which hot kimberlite magma rose. The diatremes, however, indicate emplacement by a cool gas phase, probably steam. Indicators for subsidence along ring-faults may be large diameter of diatremes, slickensides on walls, saucer-shaped structures, subsided “floating reefs”, concentration of xenoliths from specific horizons within certain areas, and zoning of diatreme rocks. It is suggested that formation of kimberlite diatremes may have been influenced by non-juvenile water.

Phreatomagmatism and its relevance

Abstract Phreatomagmatic explosive volcanism results from the interaction of magma with external water, groundwater or surface water, close to or at the Earths surface. Magma of any composition, acid to ultrabasic, may be involved in this type of explosive activity. At present, the relevance of phreatomagmatism is realized increasingly because it takes place in many diverse environments and affects not only small monogenetic volcanoes but also large volcanoes. At some volcanoes, as for example at Lake Taupo, New Zealand, a volume of several tens of km 3 of magma may have been mixed with several km 3 of external water. Phreatomagmatism has thus become an important aspect in volcanic hazard assessments. Other fields involving the interaction of water with a melt are the study of nuclear and other industrial safety hazards, of magma mixing, of diamond carrying volcanism and of hydrothermal ore deposits as well as geothermal and groundwater reservoirs. Experiments on the dynamics of magma-water interactions provide increasing insight into the principles and hazards of phreatomagmatic explosions; this type of research will become a new field in experimental petrology.

Tectonic and volcanic controls on Early Jurassic rift-valley lake deposition during emplacement of Karoo flood basalts, southern Namibia

Abstract The Karoo Igneous Province of southern Africa is one of the classic Mesozoic flood basalt provinces of the world. In the case of the early Jurassic Kalkrand Formation of Namibia the succession comprises three major flood basalt units that are separated by two stratigraphically important fluvio-lacustrine interlayers. These horizons preserve a record of the complex interplay between sedimentation, effusion of Karoo flood basalts and extensional tectonics that predated and accompanied the break-up of Gondwanaland. Both sediment layers start with the dominantly local derivation of weathered and eroded lava debris, followed by the emplacement of subaqueous mass flows and subsequent deposition of chemical sediments. The latter are characterised by interbedded stromatolitic carbonates, grass-like structured gypsum, and plane-bedded sandstones and mudstones containing euhedral displacive gypsum crystals that grew in the subsurface as well as rosettes which nucleated on the sediment surface. The central parts of the lacustrine units are overlain by thin deltaic sandstones showing bottomset, foreset, and topset layering and, finally, braided fluvial, trough cross-bedded sandstones. Evidence of subsidence synchronous with the formation of lake bodies can be explained by two principal mechanisms. The first acted in localised areas only and is reflected by the development of small, centrally subsiding basins. From the repeated occurrence of onlapping geometries within such a pool, a multiphase history of sagging is deduced, being most likely related to periodic magma withdrawal and reduction in magmatic pressures in subsurface lava feeders beneath the basin floor. Abundant syn-subsident fracture features at lava–sediment contacts, such as sediment-, hydrothermal calcite-, and sometimes basic lava-filled fissure systems indicate the pronounced interaction between the underlying volcanics and these small areas of pronounced subsidence. Fluids passing through the volcanic pile exhaled into the lake, giving it the characteristics of alkaline lake systems described from more recent flood basalt areas associated with the modern African Rift System. On the regional scale, however, northerly trending extensional fault systems controlled half-graben basin geometries and both facies and thickness variations across faults indicate that tectonism operated contemporaneously with volcanism and lacustrine sedimentation. The analysis of faults and associated structures, such as regularly aligned sediment-filled fissures, sets of micro-faults, folds and basaltic dykes constrains the extensional opening direction for the Karoo graben structures in this area that heralded the opening of the South Atlantic and thus provides a basis to discuss the extensional history of the Namibian coastal margin within the regional tectonic framework.

FORMATION OF PHREATOMAGMATIC MAAR–DIATREME VOLCANOES AND ITS RELEVANCE TO KIMBERLITE DIATREMES

ABSTRACT Studies of maars and diatremes suggest a specific process in their formation. Magma rises along a fissure and contacts ground– or surface derived water. The resulting phreatomagmatic eruptions give rise to base surge and air–fall deposits consisting of juvenile and wall–rock material. Spalling of the wall–rocks enlarges the fissure into an embryonic vent. At a critical diameter of the vent large-scale spalling at depth and slumping near the surface gives rise to a ring–fault of large diameter and subsidence of the enclosed wall–rocks and overlying pyroclastic debris. This subsidence leads to a maar crater at the surface. Fluidization processes are active in the narrow vent and in fractures and faults inside the surrounding structure which subsides en masse. Various features of kimberlite diatremes seem to be consistent with this model. They extend into fissures along which hot kimberlite magma rose. The diatremes, however, indicate emplacement by a cool gas phase, probably steam. Indicators for subsidence along ring–faults may be large diameter of diatremes, slickensides on walls, saucer–shaped structures, subsided “floating reefs”, concentration of xenoliths from specific horizons within certain areas, and zoning of diatreme rocks. It is suggested that formation of kimberlite diatremes may have been influenced by non-juvenile water.

Geochemistry, tectonomagmatic origin and chemical correlation of altered Carboniferous–Permian fallout ash tuffs in southwestern Germany

Thin, but laterally widespread, fallout ash tuff layers interbedded with complex fluvio-lacustrine successions of the Carboniferous–Permian late Variscan intermontane Saar–Nahe Basin in southwestern Germany provide important tephrostratigraphic markers in the purely continental depositional setting. The tuffs are rhyolitic to rhyodacitic and indicate geochemical affinities to Moldanubian Variscan S-type granitoids. The volcanic ashes are suggested to have been derived from the general region of the central and northern Black Forest (southwestern Germany) and the northern Vosges (eastern France) at 100–150 km distance south of the Saar–Nahe Basin. Six tuff beds from the Jeckenbach and Odernheim subformations (Meisenheim Formation, Glan Group) have been correlated within the basin over a distance of 50 km by mapping and whole-rock geochemical fingerprinting. In each subformation, three tuffs can be well distinguished using geochemical discriminant function analysis. Additional comparisons of trace and rare-earth element contents provide further criteria for the differentiation of individual tuff beds. These discriminations show that the tuffs have unique chemical fingerprints, probably reflecting differences in the original composition of the parent volcanic tephra. Thus, chemical differences between the tephrostratigraphic markers are geologically significant and provide a powerful tool for establishing tuff layer identification and correlation within the complex sedimentary sequence of the Saar–Nahe Basin. They also provide clues to the tectonomagmatic settings of the source volcanoes.

Pleistocene to Recent rejuvenation of the Hebron Fault, SW Namibia

Abstract The Hebron Fault in SW Namibia is associated with a <1 m to 9.6 m high scarp displacing Proterozoic basement and Middle to Late Pliocene crystalline conglomerates. The young age of strata exposed in the fault scarp together with evidence for displacement of aeolian dunes, post-dating the Middle Stone Age, suggests that latest fault displacements occurred during the Late Pleistocene to recent. Recorded historical seismic events show that the fault zone is still active. Latest movements of the fault are recorded by: down-to-the-SW offset of calcrete-cemented conglomerate; fluvially modified, asymmetric hanging wall, graben-like structures; at least two left-stepping jogs in the fault trace and structural data from basement rocks in which late-stage crush zones overprint earlier cataclasite. These features provide consistent evidence that the present scarp formed predominantly by normal dip-slip displacement on a NW-striking, steeply SW-dipping master fault with only a minor dextral strike-slip component. Strongly veined cataclastic fault rocks adjacent to the scarp in basement most probably originated at depths of 4–10 km. The conclusion is therefore that recent fault activity has reactivated a pre-existing, much older fault. Aerial photographic lineaments and similar fault scarps identified NW and SE of the present study area are interpreted as extensions of the same fault structure. Hence the total length of the Hebron Fault is at least 300 km subparallel to the Atlantic margin of southern Africa. Our observations confirm that the Hebron Fault is a neotectonic feature of regional significance that may relate to late Cenozoic and particularly Quaternary neotectonic activity in NE Namibia and NW Botswana.