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Igneous sheet intrusions of various shapes, such as dikes and cone sheets, coexist as parts of complex volcanic plumbing systems likely fed by common sources. How they form is fundamental regarding volcanic hazards, yet no dynamic model simulates and predicts satisfactorily their diversity. Here we present scaled laboratory experiments that reproduced dikes and cone sheets under controlled conditions. Our models show that their formation is governed by a dimensionless ratio (Π 1 ), which describes the geometry of the magma source, and a dynamic dimensionless ratio (Π 2 ), which compares the viscous stresses in the flowing magma to the host rock strength. Plotting our experiments against these two numbers results in a phase diagram evidencing a dike and a cone sheet field, separated by a sharp transition that fits a power law. This result shows that dikes and cone sheets correspond to distinct physical regimes of magma emplacement in the crust. For a given host rock strength, cone sheets preferentially form when the source is shallow, relative to its lateral extent, or when the magma influx velocity (or viscosity) is high. Conversely, dikes form when the source is deep compared to its size, or when magma influx rate (or viscosity) is low. Both dikes and cone sheets may form from the same source, the shift from one regime to the other being then controlled by magma dynamics, i.e., different values of Π 2 . The extrapolated empirical dike-to-cone sheet transition is in good agreement with the occurrence of dikes and cone sheets in various natural volcanic settings.

[1] Magmatic activity tends to concentrate at plate margins. At divergent margins, extensional tectonics provide steep conduits for magma to reach the surface. At rapidly convergent margins, such as the Andes, one might imagine that horizontal compression prevents the rise of magma. Nevertheless, volcanoes are also common. In order to study the mechanisms by which magma rises in a compressional context, we resorted to laboratory experiments, in which a brittle crust was shortened, while magma was intruding. Our model materials were (1) cohesive fine-grained silica powder, representing brittle crust, and (2) molten low-viscosity vegetable oil, representing magma. In general, horizontal shortening and injection were coeval but independent processes. Thrust faults accommodated the shortening, while overpressured oil formed hydraulic fractures. In those experiments where there was no shortening, injection resulted in a saucer-shaped intrusive body. In the other experiments, where there was shortening, oil formed a basal sill, before rising along thrust faults. Once in place, the sill lubricated the base of the model, so that arcuate thrusts formed at the leading edge of a plateau. Uplift of the plateau promoted further intrusion of oil at depth. In general, the pattern of deformation and intrusion depended on the kinematic ratio R between rates of shortening and injection. The lengths of the basal sill and plateau increased with decreasing R. On the basis of these results, we have reexamined two natural examples of magmatic complexes, which were emplaced in compressional tectonic settings, Tromen volcano in Argentina and the Boulder Batholith of Montana.

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A systematic bias towards low palaeomagnetic inclination recorded in clastic sediments, that is, inclination shallowing, has been recognized and studied for decades. Identification, understanding and correction of this inclination shallowing are critical for palaeogeographic reconstructions, particularly those used in climate models and to date collisional events in convergent orogenic systems, such as those surrounding the Neotethys. Here we report palaeomagnetic inclinations from the sedimentary Eocene upper Linzizong Group of Southern Tibet that are ∼20 • lower than conformable underlying volcanic units. At face value, the palaeomagnetic results from these sedimentary rocks suggest the southern margin of Asia was located ∼10 • N, which is inconsistent with recent reviews of the palaeolatitude of Southern Tibet. We apply two different correction methods to estimate the magnitude of inclination shallowing independently from the volcanics. The mean inclination is corrected from 20.5 • to 40.0 • within 95 per cent confidence limits between 33.1 • and 49.5 • by the elongation/inclination (E/I) correction method; an anisotropy-based inclination correction method steepens the mean inclination to 41.3 ± 3.3 • after a curve fitting-determined particle anisotropy of 1.39 is applied. These corrected inclinations are statistically indistinguishable from the well-determined 40.3 ± 4.5 o mean inclination of the underlying volcanic rocks that provides an independent check on the validity of these correction methods. Our results show that inclination shallowing in sedimentary rocks can be corrected. Careful inspection of stratigraphic variations of rock magnetic properties and remanence anisotropy suggests shallowing was caused mainly by a combination of syn-and post-depositional processes such as particle imbrication and sedimentary compaction that vary in importance throughout the section. Palaeolatitudes calculated from palaeomagnetic directions from Eocene sedimentary rocks of the upper Linzizong Group that have corrected for inclination shallowing are consistent with palaeolatitude history of the Lhasa terrane, and suggest that the India-Asia collision began at ∼20 • N by 45-55 Ma.

[1] We document evidence for growth of an active volcano in a compressional Andean setting. Our data are surface structures and

The timing of exhumation of metamorphic rocks and granitoids of the Niğde metamorphic dome, at the southern tip of the Central Anatolian Crystalline Complex, is a matter of debate. According to some authors, the metamorphic rocks are overlain nonconformably by a sedimentary sequence of late Maastrichtian to Late Palaeocene age. In contrast, other authors recently argued that the Niğde dome represents an extensional core complex of Oligocene–Early Miocene age, finally unroofed during late Miocene times. On the one hand, the results of our study contradict the latter interpretation. A sedimentary sequence of earliest Eocene to early Middle Eocene age nonconformably overlies the high-grade rocks of the Niğde dome on its southeastern flank. Pebbles from the metamorphic rocks are ubiquitous in the conglomerates of this sequence. As a result, the Niğde metamorphic rocks must have reached the surface before Eocene times, or at the very beginning of the Eocene at the latest. The Üçkapılı granite, whose crystallization age has been inferred to be Early Miocene, has intruded the metamorphic complex during exhumation. The granite is also found as pebbles within the conglomerates of the Eocene sedimentary sequence and, thus, is actually older than the Eocene. Apatite fission track dates of 12–11 Ma across the Niğde dome do not indicate that the metamorphic rocks were still on their way to the surface at that time; instead, they must reflect a later event, which is most probably heating during late Neogene magmatism. Lastly, there is no ductile-then-brittle extensional detachment in the two areas where it has been invoked, that is, on the western and southern flanks of the dome. An extensional detachment nevertheless exists at the top of the Niğde dome, best documented in its northern part, where the detachment fault superposes a superficial unit made up of massive ophiolitic rocks onto the high-grade metamorphic sequence. Field evidence indicates that this detachment developed before Eocene times. On the other hand, our observations do not confirm the nonconformity of the sedimentary sequence dated as late Maastrichtian–Late Palaeocene onto the Niğde high-grade rocks. Field relations show either a tectonic contact between the two, or the direct nonconformity of the Eocene sediments onto the metamorphic rocks. The lack of coarse clasts originating from the Niğde high-grade rocks within the Maastrichtian–Palaeocene sequence further suggests that the metamorphic dome did not reach the surface before Late Palaeocene times. These results compare well with available data from the north-western part of the Central Anatolian Crystalline Complex, suggesting that exhumation has been broadly synchronous on the scale of the massif, as a result of an episode of high magnitude extension that affected the region in Campanian to Palaeocene times.

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