Laccolithic Intrusion Research Articles

Fracturing and Dome‐Shaped Surface Displacements Above Laccolith Intrusions: Insights From Discrete Element Method Modeling

AbstractInflation of viscous magma intrusions in Earth's shallow crust often induces strain and fracturing within heterogeneous host rocks and dome‐shaped ground deformation. Most geodetic models nevertheless consider homogeneous, isotropic, and linear‐elastic media wherein stress patterns indicate the potential for failure, but without simulating actual fracturing. We present a two‐dimensional Discrete Element Method (DEM) application to simulate magma recharge in a pre‐existing laccolith intrusion. In DEM models, fractures can propagate during simulations. We systematically investigate the effect of the host rock toughness (resistance to fracturing), stiffness (resistance to deformation) and the intrusion depth, on intrusion‐induced stress, strain, displacement and spatial fracture distribution. Our results show that the spatial fracture distribution varies between two end‐members: (a) for high stiffness or low toughness host rock or a shallow intrusion: extensive cracking, multiple vertical surface fractures propagating downward and two inward‐dipping highly cracked shear zones that connect the intrusion tip with the surface; and (b) for low stiffness or high toughness host rock or a deeper intrusion: limited cracking, one central vertical fracture initiated at the surface, and two inward‐dipping fractures at the intrusion tips. Abrupt increases in surface displacement magnitude occur in response to fracturing, even at constant magma injection rates. Our modeling application provides a novel approach to considering host rock mechanical strength and fracturing during viscous magma intrusion and associated dome‐shaped ground deformation, with important implications for interpreting geodetic signals at active volcanoes and the exploitation of geothermal reservoirs and mineral deposits.

A reappraisal of explosive–effusive silicic eruption dynamics: syn-eruptive assembly of lava from the products of cryptic fragmentation

Silicic volcanic eruptions range in style from gently effusive to highly explosive, and may switch style unpredictably during a single eruption. Direct observations of subaerial rhyolitic eruptions (Chaiten 2008, Cordón Caulle 2011–2012, Chile) challenged long-standing paradigms of explosive and effusive eruptive styles and led to the formulation of new models of hybrid activity. However, the processes that govern such hybrid explosive–effusive activity remain poorly understood. Here, we bring together observations of the well-studied 2011–2012 Cordón Caulle eruption with new textural and petrologic data on erupted products, and video and still imagery of the eruption. We infer that all of the activity – explosive, effusive, and hybrid – was fed by explosive fragmentation at depth, and that effusive behaviour arose from sticking and sintering, in the shallow vent region, of the clastic products of deeper, cryptic fragmentation. We use a scaling approach to determine that there is sufficient time available, during emplacement, for diffusive pyroclast degassing and sintering to produce a degassed plug that occludes the shallow conduit, feeding clastogenic, apparently effusive, lava-like deposits. Based on evidence from Cordón Caulle, and from other similar eruptions, we further argue that hybrid explosive–effusive activity is driven by episodic gas-fracking of the occluding lava plug, fed by the underlying pressurized ash- and pyroclast-laden region. The presence of a pressurized pocket of ash-laden gas within the conduit provides a mechanism for generation of harmonic tremor, and for syn-eruptive laccolith intrusion, both of which were features of the Cordón Caulle eruption. We conclude that the cryptic fragmentation models is more consistent with available evidence than the prevailing model for effusion of silicic lava that assume coherent non-fragmental rise of magma from depth to the surface without wholesale explosive fragmentation.

3-dimensional trapdoor structure of laccolith-induced doming and implications for laccolith emplacement, Pampa Amarilla, Mendoza Province, Argentina

Laccolith emplacement and growth are controlled by doming of overburden rocks. Understanding the mechanics and structural evolution of laccolith-induced doming is therefore essential for revealing the emplacement dynamics and growth of laccoliths. In this paper, we present a 3D reconstruction of a subsurface laccolithic intrusion through the structural reconstruction of a well-exposed dome at Pampa Amarilla, Neuquén Basin, southern Mendoza province, Argentina. The 3D reconstruction was made possible by the unique integration of surface geological data and subsurface 3D seismic and borehole data. We estimate that the Pampa Amarilla laccolith is ~400 m thick, ~3 km and 2 km long in the E-W and N-S directions, respectively. Along an E-W cross section, the laccolith exhibits a wedge shape with maximum thickness near its western edge and gradual thinning toward the east. The structure of the dome is typical of a trapdoor, with faulting along the western, northwestern and southwestern edges, and tilting of the overburden to the east. The trapdoor tilting of the laccolith's overburden was the main mechanism controlling the thickening and growth of the Pampa Amarilla laccolith, which exhibits a relatively high thickness-to-length ratio T/D ~ 0.13. Numerous laccolithic intrusions exhibit similar values of T/D, and our study suggests that faulting commonly controls emplacement of laccoliths with T/D > 0.1; conversely, our study suggests that the established mechanical models of laccolith emplacement based on elastic bending of the overburden applies only to thin laccoliths. Finally, our study highlights the necessity and value of integrating field geological measurements with subsurface 3D seismic and borehole data for structural reconstructions of subsurface laccolith intrusions.

Synthesis of the special issue: The formation and evolution of Ceres’ Occator crater

The distinctive bright regions within Occator crater are one of the most remarkable discoveries of the Dawn mission's exploration of Ceres. The central region is named Cerealia Facula and the additional regions in the eastern crater floor are named Vinalia Faculae. Here we summarize and synthesize the results of this special issue, which aimed to identify the driving forces behind the formation of Occator and the faculae, and thus lead us to a new understanding of the processes and conditions that occurred in Ceres’ past, and potentially in its present. The investigations presented here used Dawn data, theoretical modeling and laboratory experiments to deduce the sequence of events that led to the formation of Occator, Cerealia Facula and Vinalia Faculae, which are broken into stages 1–3. Stage 1: Occator's ejecta blanket, terraces and hummocky crater floor material formed during and shortly after crater formation. These features are located in many Cerean complex craters. However, Occator also contains the lobate materials and faculae, which are unique to Occator. We interpret the lobate materials as a slurry of water, soluble salts and boulders of unmelted silicates/salts, which flowed around the crater interior before solidifying. At least portions of the lobate materials were solidified prior to the formation of the central pit, which is suggested to form via the ‘melted uplift model’ and provides insights into central pit formation across the Solar System. We propose the outer edge of Cerealia Facula formed shortly after the Occator-forming impact, via impact-induced hydrothermal brine deposition or via salt-rich water fountaining (perhaps sourced in a pre-existing reservoir). Stage 2: the majority of Cerealia Facula is located within the central pit and is interpreted to have formed later, at least ∼18 Myr after the Occator-forming impact, via multiple depositional events. The Cerealia-Facula-forming brines may have flowed out of fractures in the walls of the central pit and/or been driven to the surface by freezing of a subsurface reservoir and/or deposited via salt-rich water fountains. Further investigations are required to identify whether the formation of the majority of Cerealia Facula is driven by processes triggered by an impact, i.e. an exogenic event, or by a combination of impact-driven and endogenic processes. Cryomagmatic intrusions are suggested to uplift the crater floor, resulting in concentric floor fractures and an asymmetric dome. Injection of a similar material is proposed to inflate part of the lobate materials, giving them a hummocky texture. The resulting stresses formed fractures in the hummocky lobate material, which allowed the Vinalia-Faculae-forming brines to travel to the surface, where they ballistically erupted. The central dome within the central pit was one of the last features to form, by laccolithic intrusion, or by volume expansion from freezing of volatiles, or by extrusion of brines. Stage 3: mixing with Ceres’ average materials and/or space weathering darken the faculae over time. Cerealia Facula and Vinalia Faculae are the brightest and freshest of the bright regions identified on Ceres’ surface. Bright regions darken over time until their eventual erasure. Thus, it is likely that faculae formation has occurred throughout Ceres’ history, but that Occator's faculae are visible today because they are geologically young. Through synthesis of the studies presented in this special issue, we find that entirely exogenic driving forces, triggered by the impact, or a combination of endogenic and impact-derived forces could explain the formation of Occator and its faculae. Whether activity is impact-triggered and/or endogenic in nature is a key question for all investigations of Ceres, and future studies may favor one possibility over the other. The investigations presented in our special issue indicate Ceres is an active world where brines have been mobile in the geologically recent past. As such, Ceres is an intriguing world that we have only begun to explore.

The central pit and dome at Cerealia Facula bright deposit and floor deposits in Occator crater, Ceres: Morphology, comparisons and formation

The prominent bright deposit Cerealia Facula, Ceres, coincides with the central depression (or central pit) of the recently formed 92 km-wide complex crater Occator. The central pit is 9–10 km wide and up to 1 km deep and is partially filled with a 700 m-high 2 km-wide dome. The upper surface of the central dome is densely fractured but the flanks are not, indicating that uplift of the dome surface occurred after the bright deposit was emplaced, and primarily through uplift of the surface from below by laccolithic intrusion or volume expansion. The pit is rimless except for two prominent massifs that flank it to the east and west. This pit-dome morphology bears a strong resemblance in morphology, topography, and dimensions to central pit craters observed on Ganymede and Callisto, but is essentially absent on the similar sized ice-rich moons of Uranus, Saturn, and Pluto. The lack of pit craters on midsize icy moons and the possible lack of central pit craters at the cold poles of Ceres suggest that temperature is a significant controlling factor in central pit formation, beyond the canonical inverse-gravity scaling of complex crater transitions. While renewed modeling will benefit from these new constraints, the mapping of central pits on Ceres, including Occator, suggests that either the rheologically layered target model or more likely the melted central uplift model may be the most probable explanation for central pit formation on Ceres (lack of similar high-resolution imaging on the Galilean icy satellites precludes definitive evaluation of models there). Large volumes of lobate floor-fill material at least 600 m thick in some places cover large areas of the southern and eastern floor of Occator, and small outcrops of this unit are found perched high on closed topographic depressions on the Occator terrace zone and other higher areas of the crater floor. The swirling and knobby surface textures of these lobate deposits are essentially indistinguishable from those on impact melt sheets on large lunar craters such as Tycho. These observations indicate that this unit is most likely an impact melt sheet formed from melted water ice admixed with unmelted particulate or dissolved phyllosilicates, salts and/or carbonates, and large quantities of fragmented debris. Melt of ice phases within Ceres crust is also supported by independent numerical models of impact at Occator and would provide a ready source of fluids to promote hydrothermal deposition of the observed bright Na-carbonates on the surface of the central pit at Occator, if the deposit formed within a million years or so of impact. The central dome finds analogs in salt domes on Earth and pingos (frost-mounds) on Earth and Mars, which form by water and solute migration and freeze expansion. Later freezing of a central reservoir of volatiles beneath the central pit at Occator or mobilization of ice or other phases during the cooling and refreezing process could explain the uplift of the central dome. Finally, no changes were detected in the Cerealia and Vinalia Facula bright deposits during the course of the prime Dawn mapping mission, placing additional constraints on their formation time and limits on ongoing activity.

Small graben in the southeastern ejecta blanket of the lunar Copernicus crater: Implications for recent shallow igneous intrusion on the Moon

Dozens of linear graben that are about 10–400m wide and less than 1km long are recognized in the southeastern continuous ejecta deposits of Copernicus, supporting that Copernican-aged tectonism has occurred on the Moon. Fault geometry analysis suggests that the bounding faults of the graben have formed within the ejecta deposits. The graben are exclusively located on a local high-relief area, but they are not formed by mass wasting, because the topographic slope is substantially less than the repose angle of typical lunar materials, and no other extensional structures are visible on similar high-relief areas at the continuous ejecta deposits of Copernicus. The orientations of the graben all point to the center of Copernicus, but the topography of Copernicus is little compensated after formation, suggesting that the graben were not caused by possible crustal isostatic readjustment. This graben system is one of the three examples on the Moon that were interpreted to be caused by shallow igneous intrusions in the format of laccoliths. The currently available GRAIL gravity data have a lower spatial resolution than the size of the graben, so the gravity data cannot resolve the hypothesized sub-kilometer-scale laccoliths beneath the graben. While laccolith intrusion to a depth of about 80m is required to explain the formation of this graben system, the laccolith intrusion scenario is not consistent with the geological context of the graben. A compressional structure is visible close to the graben system, and their spatial configuration and similar preservation states are consistent with being generically related. A close examination of the other two sets of graben that were also interpreted to have no associated compressional structures actually reveals spatially-related lobate scarps and wrinkle ridges in the vicinity. Therefore, shallow igneous intrusion is not plausible or necessary to explain to formation of Copernican-aged graben on the Moon, and they are most likely formed by the late-stage global contraction of the Moon.

Rapid laccolith intrusion driven by explosive volcanic eruption.

Magmatic intrusions and volcanic eruptions are intimately related phenomena. Shallow magma intrusion builds subsurface reservoirs that are drained by volcanic eruptions. Thus, the long-held view is that intrusions must precede and feed eruptions. Here we show that explosive eruptions can also cause magma intrusion. We provide an account of a rapidly emplaced laccolith during the 2011 rhyolite eruption of Cordón Caulle, Chile. Remote sensing indicates that an intrusion began after eruption onset and caused severe (>200 m) uplift over 1 month. Digital terrain models resolve a laccolith-shaped body ∼0.8 km3. Deformation and conduit flow models indicate laccolith depths of only ∼20–200 m and overpressures (∼1–10 MPa) that likely stemmed from conduit blockage. Our results show that explosive eruptions may rapidly force significant quantities of magma in the crust to build laccoliths. These iconic intrusions can thus be interpreted as eruptive features that pose unique and previously unrecognized volcanic hazards.

Possible sub-glacial eruptions in the Galaxias Quadrangle, Mars

We have identified several landforms in the Galaxias Quadrangle of Mars (MTM 35217), 33.0–35.5°N, 216.0–218.0°W which are consistent with this area having been covered by an ancient ice sheet concurrent with volcanic eruptions. Volcanic activity was initiated by the intrusion of several large dikes measuring ∼50–100m wide and protruding up to ∼35m above the present-day surface. These dikes appear to have originated from Elysium Planitia ∼600km to the SE. In one instance, a dike (at an elevation of −3750m) appears to have produced a subglacial mound (referred to here as “Galaxias Mons 2”) that evolved into an extrusive eruption and produced copious volumes of melt water that carved an outflow channel that extends almost 300km to the north. At a lower elevation (−3980m), a second putative dike may have failed to break the surface of the ice sheet and formed Galaxias Mons as a laccolithic intrusion. We numerically model the formation of Galaxias Mons and find that at least 200m of ice may once have existed at this latitude at the time of the dike intrusions. Such a conclusion supports the idea that enigmatic small domes in the area may be pingoes. Collectively, these observations suggest that the previous interpretations for the origin of near-by Hrad Vallis as a sub-aerial eruption may need to be revised.

Granitoid emplacement by multiple sheeting during Variscan dextral transpression: The Saint-Laurent – La Jonquera pluton (Eastern Pyrenees)

The structural study of the Saint-Laurent – La Jonquera pluton (Eastern Pyrenees), a Variscan composite laccolithic intrusion emplaced in metasedimentary and gneissic rocks of the Roc de Frausa dome, by means of the anisotropy of magnetic susceptibility (AMS) technique has allowed the determination of the nature and orientation of its magmatic fabrics. The magmatic foliation has a predominant NE–SW strike and the mean lineation is also NE–SW trending with a shallow plunge. A strain gradient is measured so that the tonalites to granodiorites that form the basal parts of the pluton, and are intruded into amphibolite-facies metamorphic rocks, recorded the highest anisotropies, whereas the monzogranites and leucogranites, emplaced into upper crustal, low-grade metamorphic rocks, are weakly deformed. These results point to the synkinematic sequential emplacement of multiple granitoid sheets, from less to more differentiated magmatic stages, during the Late Carboniferous D2 event characterized by an E–W-trending dextral transpression. The magmatic foliation appears locally disturbed by the effects of two tectonic events. The first of them (D3) produced mylonitization of granitoids along NW–SE retrograding shear zones and open folds in the host Ediacaran metasediments of the Roc de Frausa massif, likely during late Variscan times. Interference between D2 and D3 structures was responsible for the dome geometry of the whole Roc de Frausa massif. The second and last perturbation consisted of local southward tilting of the granitoids coupled to the Mesozoic–Cenozoic cover during the Alpine.

Mapping real time growth of experimental laccoliths: The effect of solidification on the mechanics of magmatic intrusion

The dynamics of solidification front growth along the margins of magmas have been widely found to be fundamental in controlling magma transport and emplacement. In this vein, the role of solidification fronts in determining the basic growth mechanics of laccoliths are investigated here in a series of scaled experiments using two contrasting magma analogs, water and molten wax that are injected into a visco-elastic gelatin based crustal analog. In non-solidifying, water-style experiments, intrusion is a relatively simple process. In contrast, wax magma emplacement displays a vast palette of compelling behaviors: propagation can slow, stop, and reactivate, and the directionality of lateral growth becomes much more variable. Small flow deviations in water-based intrusions are likely the product of flow instabilities, the result of injecting a viscous fluid along an interface (similar to Hele-Shaw cell experiments). However, the much more complex emplacement style of the wax experiments is attributed to solidification at the leading edge of the crack. The overall effects of solidification during emplacement can be described by a non-dimensional parameter measuring the relative competition between the rates of crack propagation and solidification at the crack leading edge. In this context, laccolith growth mechanics can be separated into three distinctive characteristic stages. Namely, I: A thin pancake style sill initially emanating radially from a central feeder zone, II: As solidification stalls magma propagation at the leading edges, enhanced thickening begins, forming a true, low aspect ratio laccolith, and III: As stresses accumulate, tears and disruptions readily occur in the solidified margin causing fresh breakouts, thus reactivating lateral growth into new lobes. The competitive combination of these latter stages often leads to a characteristic pulsatile growth. The unexpected richness of these results promises to add fundamentally to the basic understanding of laccolith growth mechanisms, and also adds key observations to growth processes to be sought in field studies that have been hitherto suspected but rarely observed. Despite seemingly simple shapes, the growth history of laccolithic intrusions is likely quite complex.

Magma-driven antiform structures in the Afar rift: The Ali Sabieh range, Djibouti

The Ali Sabieh Range, SE Afar, is an antiform involving Mesozoic sedimentary rocks and synrift volcanics. Previous studies have postulated a tectonic origin for this structure, in either a contractional or extensional regime. New stratigraphic, mapping and structural data demonstrate that large-scale doming took place at an early stage of rifting, in response to a mafic laccolithic intrusion dated between 28 and 20 Ma from new K-Ar age determinations. Our ‘laccolith’ model is chiefly supported by: (i) the geometry of the intrusion roof, (ii) the recognition of roof pendants in its axial part, and (iii) the mapping relationships between the intrusion, the associated dyke-sill network, and the upper volcanic/volcaniclastic sequences. The laccolith is assumed to have inflated with time, and to have upwardly bent its sedimentary roof rocks. From the architecture of the ∼1 km-thick Mesozoic overburden sequences, ca. 2 km of roof lifting are assumed to have occurred, probably in association with reactivated transverse discontinuities. Computed paleostress tensors indicate that the minimum principal stress axis is consistently horizontal and oriented E-W, with a dominance of extensional versus strike-slip regimes. The Ali Sabieh laccolith is the first regional-scale magma-driven antiform structure reported so far in the Afro-Arabian rift system.

Lunar intrusive domes: Morphometric analysis and laccolith modelling

This study examines a set of lunar domes with very low flank slopes which differ in several respects from the frequently occurring lunar effusive domes. Some of these domes are exceptionally large, and most of them are associated with faults or linear rilles of presumably tensional origin. Accordingly, they might be interpreted as surface manifestations of laccolithic intrusions formed by flexure-induced vertical uplift of the lunar crust (or, alternatively, as low effusive edifices due to lava mantling of highland terrain, or kipukas, or structural features). All of them are situated near the borders of mare regions or in regions characterised by extensive effusive volcanic activity. Clementine multispectral UVVIS imagery indicates that they do not preferentially occur in specific types of mare basalt. Our determination of their morphometric properties, involving a combined photoclinometry and shape from shading technique applied to telescopic CCD images acquired at oblique illumination, reveals large dome diameters between 10 and more than 30 km, flank slopes below 0.9°, and volumes ranging from 0.5 to 50 km 3. We establish three morphometric classes. The first class, In1, comprises large domes with diameters above 25 km and flank slopes of 0.2°–0.6°, class In2 is made up by smaller and slightly steeper domes with diameters of 10–15 km and flank slopes between 0.4° and 0.9°, and domes of class In3 have diameters of 13–20 km and flank slopes below 0.3°. While the morphometric properties of several candidate intrusive domes overlap with those of some classes of effusive domes, we show that a possible distinction criterion are the characteristic elongated outlines of the candidate intrusive domes. We examine how they differ from typical effusive domes of classes 5 and 6 defined by Head and Gifford [Head, J.W., Gifford, A., 1980. Lunar mare domes: classification and modes of origin. Moon Planets 22, 235–257], and show that they are likely no highland kipukas due to the absence of spectral contrast to their surrounding. These considerations serve as a motivation for an analysis of the candidate intrusive domes in terms of the laccolith model by Kerr and Pollard [Kerr, A.D., Pollard, D.D., 1998. Toward more realistic formulations for the analysis of laccoliths. J. Struct. Geol. 20(12), 1783–1793], to estimate the geophysical parameters, especially the intrusion depth and the magma pressure, which would result from the observed morphometric properties. Accordingly, domes of class In1 are characterised by intrusion depths of 2.3–3.5 km and magma pressures between 18 and 29 MPa. For the smaller and steeper domes of class In2 the magma intruded to shallow depths between 0.4 and 1.0 km while the inferred magma pressures range from 3 to 8 MPa. Class In3 domes are similar to those of class In1 with intrusion depths of 1.8–2.7 km and magma pressures of 15–23 MPa. As an extraordinary feature, we describe in some detail the concentric crater Archytas G associated with the intrusive dome Ar1 and discuss possible modes of origin. In comparison to the candidate intrusive domes, terrestrial laccoliths tend to be smaller, but it remains unclear if this observation is merely a selection effect due to the limited resolution of our telescopic CCD images. An elongated outline is common to many terrestrial laccoliths and the putative lunar laccoliths, while the thickness values measured for terrestrial laccoliths are typically higher than those inferred for lunar laccoliths, but the typical intrusion depths are comparable.

Cambro-Ordovician potassic (alkaline) magmatism in Central Oman: Petrological and geochemical constraints on petrogenesis

Cambro-Ordovician alkaline magmatism is exposed in four small laccolithic intrusions in the Huqf area of Central Oman. 40Ar/ 39Ar dating suggests the presence of two pulses of magmatism at approximately 513–526 Ma and 452–461 Ma. The emplacement of ultramafic foidites characterized the earlier stage of magmatism, while later stage resulted in the intrusion of a sequence of mildly potassic minettes and tephrites. The source of the magmas has many similarities to that of EM1 oceanic island basalts. Trace element evidence however, suggests enrichment of the protolith by supra-subduction zone fluids. The petrogenesis of the suite is therefore explained by mixing of melts derived by small degree partial melting of previously enriched garnet- and spinel–peridotite-facies mantle sources, caused by a perturbed geotherm during minor lithospheric extension.

The Oulad Ouaslam Variscan granitic pluton (Jebilets Massif, Southwestern Moroccan Meseta): A forcibly emplaced laccolithic intrusion characterized by its magnetic and magmatic fabrics

The study of the magmatic fabrics of the Oulad Ouaslam Variscan granitic pluton, based on the Anisotropy of Magnetic Susceptibility technique, allows us to propose that this 30 km long laccolith corresponds to a forcibly emplaced intrusion which proceeded from west to east into its country rock of Carboniferous metapelites. The fact that the magmatic fabrics measured in this pluton are obliquely cut, in the southwestern part, by solid-state structures (cleavage, shear bands) related to the regional main phase of deformation shows that the pluton was emplaced before this phase. Consequently, the tectonic control of this emplacement appears to have been much less important than it was suggested in the previous interpretations which considered, on the basis of the study of the solid-state fabrics, a syn-tectonic emplacement of this granite, possibly linked to a sinistral NNW–SSE shear zone. Our results contribute to the definition of a new framework for the tectonic history of this part of the Variscan chain.

Testing the viability of topographic relaxation as a mechanism for the formation of lunar floor‐fractured craters

Historically, two modes of origin for lunar floor‐fractured craters have been proposed: subcrater laccolith emplacement and topographic relaxation. To test the viability of the topographic relaxation model, we employ an elastoviscoplastic finite element model and ductile rheology measurements of a devolatilized terrestrial diabase, an analog to lunar crustal materials. Trials were run under two situations: (1) under extreme conditions (e.g., high heat flow) to determine the maximum degree of relaxation that the model can accommodate, and (2) under more nominal conditions in order to resolve how much relaxation can be expected to occur under more plausible lunar conditions. We find that in the extreme case, topographic relaxation is a viable means only for explaining the observed shallowing of the largest floor‐fractured craters (>100 km in diameter). Results from the less extreme case demonstrate that lunar materials are simply too rigid to experience degrees of relaxation as large as those observed in the floor‐fractured crater population. These results indicate that topographic relaxation is not the primary mechanism for the formation of floor‐fractured craters. Because the topographic relaxation model does not apply for the vast majority of floor‐fractured craters (i.e., those with diameters less than ∼60 km), we conclude that the laccolith intrusion model is a more viable alternative.

Geochemistry of Basalt from Escanaba Trough: Evidence for Sediment Contamination

Mid-ocean ridge basalts (MORBs) erupted in Escanaba Trough, the southernmost segment of the Gorda Ridge, have greater isotopic and geochemical variability than those from the remainder of the ridge. Samples from the sediment-free northern sites in the trough are depleted normal-MORB similar to those from the northern Gorda Ridge. However, samples from the sediment-covered portion of the trough are significantly enriched in incompatible elements, and Sr, Nd and Pb isotopic ratios are shifted toward the composition of local sediments. Pb isotopic ratios, in particular, lie on a mixing line with local sediments. Assimilation–fractional crystallization (AFC) calculations suggest that 25–40% fractional crystallization coupled with assimilation of 1–2% of local sediments can produce the enrichment observed for some Escanaba basalts. Variations in the amount of enrichment of elements with similar incompatibility for comparably differentiated basalts, and uncorrelated isotopic ratios suggest variable amounts of assimilation of compositionally heterogeneous assimilants by parental magmas. Assimilation of sediment by MORB magmas probably resulted when laccolithic intrusions ponded at the sediment–basement interface beneath the thick sediment cover in the southern part of Escanaba Trough. Geochemical and isotopic evidence for contamination can be detected in these lavas because of the strong compositional contrast between the parental magma and assimilant. The depleted MORB magma and highly enriched sediment assimilant are end-member compositions that provide a sensitive indicator for the smallest amount of contamination of oceanic magma by crustal processes.

Analogue models of laccolith formation

This paper describes dynamically scaled analogue models of laccolithic intrusions. Experiments consisted of the injection of a Newtonian fluid (low-viscosity silicone putty) into a sandpack, with or without an interbedded ductile layer of silicone putty acting as a potential décollement level. Boundary conditions were chosen to analyze the influence of the thickness of the brittle cover and of the décollement layer on the pattern of intrusion. Further experiments were made to examine the effects of an extensional regime during intrusion. Experiments showed that: (1) laccolith formation requires the occurrence of a décollement layer between two competent units, (2) the critical thickness of the décollement layer necessary for laccolith formation decreases with increasing depth, (3) laccoliths change from lenses to bell-shaped with decreasing overburden, (4) for a constant thickness of the décollement layer, the largest diameter of laccoliths is proportional to the thickness of the overburden, and (5) a syn-injection gravitational sliding regime results in an asymmetric laccolith, with amplification of the bell shape.

The Gangotri granite (Garhwal Himalaya): Laccolithic emplacement in an extending collisional belt

The Gangotri Miocene leucogranite is composed of several laccoliths (6–7 km long, 1.5–2 km thick), which can be divided into two sets of lenses: the southern lenses, intruded in the lower part of the High Himalaya sedimentary cover, and the northern lenses, intruded in an older porphyritic biotite‐bearing granite. In both cases, the magmatic fabric is commonly weak with no dominant stretching direction, although a rough E‐W trend is present in the northern lenses. The magnetic fabric is characterized by the fact that drastic changes in the direction, in the magnitude of the magnetic parameters, or in the fabric type (planar or linear) may take place over very short distances. Measurements of preferred orientations in thin sections indicate that the strain regime was largely dominated by a coaxial component for both sets of lenses, in agreement with the large dispersion observed in both the field and magnetic lineations. The comparison of the granite and host rock structures shows that the leucogranite emplacement dates the onset of the extensional tectonism in the High Himalaya range and is not related to a southward directed thrust event associated with the Main Central Thrust. This is exemplified by the vertical attitude of the feeder dikes that intrude the metasedimentary rocks beneath die southern lenses. The presence of these dikes indicates in turn that magma ascent occurred by fracture propagation. The spatial disposition of the southern lenses could have resulted either from the disruption of a single laccolithic intrusion by crustal scale boudinage due to a northern gravity backslide of the top of the Tibetan Slab or from the intrusion of independent laccoliths. The use of the elastic bending theory of Pollard and Johnson (1973) shows that in both hypotheses, the current laccolith sizes are compatible with a laccolithic mode of magma emplacement. However, neither the density contrast between the magma and its enclosing rocks nor the lithological boundary between the Tibetan Slab and the overlying Tibetan metasedimentary series controlled the level of magma emplacement. Rather, flat‐lying collapse structures, which intersected the upward propagating magma dikes, are the most likely causes of magma arrest. Such a mechanism was favored by the schist‐rich lithology of the metasedimentary host rocks. In addition, field relationships indicate that the melt supply through the dike system was a continuous, rather than pulsed, process. Existing numerical treatments on the rates of magma transport through fractures show that in such a case, the laccoliths could have been built in less than 100 years. This short time of emplacement, the small size of the laccoliths, and their peripheral disposition relative to the Badrinath granite suggest that the Gangotri lenses may represent the initial stage of pluton accretion in the High Himalaya which ultimately gave rise to a much larger massif such as the Manaslu granite.

Isotope Study of Nitrogen in Carboniferous, Zechstein And Buntsandstein Reservoirs in Northwest Germany - The Role of Structural Inversion Combined With a Major Heat Event: ABSTRACT

More than 50% of the annual production of natural gas (ca. 20 x 10{sup 9} m{sup 3}) in Germany (1993) derives from Carboniferous, Zechstein and Buntsandstein deposits. Nitrogen contents of these gases vary from 1% to 74%, with {delta}{sup 15}N-values ranging from -2.7 to +15.5{per_thousand}. Based on geological and geochemical coincidences, two main groups of gases can be distinguished: Nitrogen from gas fields with a continuous subsidence history can easily be explained by a generation either from Westphalian coal seams, or from marine precursors in Devonian through Zechstein sediments, or as a gas mixture from both sources. Such gases contain usually less than 10 % of nitrogen with {delta}{sup 15}N-values > +4{per_thousand}. However, many gas deposits are influenced by an Upper Cretaceous structural inversion, accompanied by a large laccolithic intrusion. This event has lead to the loss of early formed and trapped gases. At the same time an extraordinary, short-time maturation of the source rocks has occurred. Hereafter accumulated nitrogen gases are characterized by {delta}{sup 15}N-values < +3{per_thousand}. Although intrusion-related abiogenic nitrogen cannot totally be excluded, we assume that Westphalian coal measures are the precursors of this nitrogen. The variations in nitrogen isotopes in this area may be reflecting the more » release of nitrogen from different functional groups (pyridinic vs. pyrrolic). « less

Structural geometry, lower crustal magmatic underplating and lithospheric stretching in the Ivrea-Verbano zone, northern Italy

The Ivrea-Verbano zone is believed to represent an up-ended cross-section through the lower continental crust as it existed at the end of the Hercynian orogeny in the Southern Alps. Structurally-oriented geological mapping has been carried out in the central and northern parts of the Ivrea-Verbano zone, a region some 30 km along strike. The geology of the southern half of the zone is dominated by a large basic-ultrabasic complex (the Mafic Formation), which is in contact with a strongly banded metasedimentary + metabasic sequence to the north. Particular attention was paid to: (a) the structural relationship between the rocks of the Mafic Formation and their envelope of high-grade metasedimentary and metabasic rocks; (b) the geometrical configuration throughout the Ivrea-Verbano zone of high-temperature shear zones, which accommodated post-Hercynian crustal extension; and (c) the geometry of late, low-temperature faulting, the effects of which have been removed in order to produce a restoration of the structure as it existed during the post-Hercynian extensional phase. The intrusion of large volumes of basic magma (ca 50% of the outcrop area) to form the rocks of the Mafic Formation appears to be coeval with the onset of extension ( ca 280 Ma). The main basic body has a laccolithic form, which was originally more than 10 km thick. Overfolding developed at the northern margin of the laccolith and is interpreted in terms of the gravitational collapse of the hot, immediately subsolidus or partially molten body, incorporating its envelope of hot metasediments into a large, originally recumbent fold. A geometrical association with a high-temperature, low-angle fault zone suggests that faulting was subsequently localized by the several km of uplift associated with the laccolithic intrusion. The Ivrea-Verbano zone may therefore demonstrate at least one particular geometry of lower crustal magmatic underplating, which may aid in the interpretation of present-day deep seismic profiles. It also demonstrates the geometry of a network of conjugate, high-temperature, low-angle shear zones in a well-layered lower crustal section.