Lateral Dike Propagation Research Articles

Seismic Crustal Structure and Morphotectonic Features Associated With the Chain Fracture Zone and Their Role in the Evolution of the Equatorial Atlantic Region.

Oceanic transform faults and fracture zones (FZs) represent major bathymetric features that keep the records of past and present strike‐slip motion along conservative plate boundaries. Although they play an important role in ridge segmentation and evolution of the lithosphere, their structural characteristics, and their variation in space and time, are poorly understood. To address some of the unknowns, we conducted interdisciplinary geophysical studies in the equatorial Atlantic Ocean, the region where some of the most prominent transform discontinuities have been developing. Here we present the results of the data analysis in the vicinity of the Chain FZ, on the South American Plate. The crustal structure across the Chain FZ, at the contact between ∼10 and 24 Ma oceanic lithosphere, is sampled along seismic reflection and refraction profiles. We observe that the crustal thickness within and across the Chain FZ ranges from ∼4.6–5.9 km, which compares with the observations reported for slow‐slipping transform discontinuities globally. We attribute this presence of close to normal oceanic crustal thickness within FZs to the mechanism of lateral dike propagation, previously considered to be valid only in fast‐slipping environments. Furthermore, the combination of our results with other data sets enabled us to extend the observations to morphotectonic characteristics on a regional scale. Our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower‐order Mid‐Atlantic Ridge segmentation around the equator.

Simultaneous Middle Pleistocene eruption of three widespread tholeiitic basalts in northern California (USA): Insights into crustal magma transport in an actively extending back arc

AbstractMapping and chronology are central to understanding spatiotemporal volcanic trends in diverse tectonic settings. The Cascades back arc in northern California (USA) hosts abundant lava flows and normal faults, but tholeiitic basalts older than 200 ka are difficult to discriminate by classic mapping methods. Paleomagnetism and chemistry offer independent means of correlating basalts, including the Tennant, Dry Lake, and Hammond Crossing basalt fields. Paleomagnetic analysis of these chemically similar basalts yield notable overlap, with statistical analysis yielding 7 chances in 1,000,000 that their similar mean remanent directions are random. These basalts also have overlapping 40Ar/39Ar ages of 272.5 ± 30.6 ka (Tennant), 305.8 ± 23.9 ka (Dry Lake), and 300.4 ± 15.2 and 322.6 ± 17.4 ka (Hammond Crossing). Chemical and paleomagnetic analyses indicate that these spatially distributed basalts represent simultaneous (<100 yr uncertainty) eruptions, and thus we use 305.5 ± 9.8 ka (weighted mean) as the eruption age. Their vents align on a N25°W trend over a distance of 39 km. Tennant erupted the largest volume (3.55 ± 0.75 km3) at the highest elevation; both factors decay to the south-southeast at Dry Lake (0.75 ± 0.15 km3) and Hammond Crossing (0.15 ± 0.05 km3). We propose vertical magma ascent beneath the Tennant vent area, where the most evolved, high-SiO2 magma erupted, with lateral dike propagation in the brittle crust. Propagation was near orthogonal to east-west extension (0.3–0.6 mm/yr) along north-northwest–trending normal faults.

What Drives the Lateral Versus Vertical Propagation of Dikes? Insights From Analogue Models

AbstractVolcanic eruptions are usually fed by dikes. Understanding how crustal inhomogeneities and topographic loads control the direction (lateral/vertical) and extent (propagation/arrest) of dikes is crucial to forecast the opening of a vent. Many factors, including buoyancy, crustal layering, and topography, may control the vertical or lateral propagation of a dike. To define a hierarchy between these factors, we have conducted analogue models, injecting water (magma analogue) within gelatin (crust analogue). We investigate the effect of crustal layering (both rigidity and density layering), topography, magma inflow rate, and the density ratio between host rock and magma. Based on the experimental observations and scaling considerations, we suggest that rigidity layering (a stiffer layer overlying a weaker one) and topographic gradient favor predominantly lateral dike propagation; inflow rate, density layering, and density ratio play a subordinate role. Conversely, a softer layer overlying a stiffer one favors vertical propagation. Our results highlight the higher efficiency of a stiff layer in driving lateral dike propagation and/or inhibiting vertical propagation with respect to the Level of Neutral Buoyancy proposed by previous studies.

Propagation and arrest of dikes under topography: Models applied to the 2014 Bardarbunga (Iceland) rifting event

AbstractDikes along rift zones propagate laterally downslope for tens of kilometers, often becoming arrested before topographic reliefs. We use analogue and numerical models to test the conditions controlling the lateral propagation and arrest of dikes, exploring the presence of a slope in connection with buoyancy and rigidity layering. A gentle downslope assists lateral propagation when combined with an effective barrier to magma ascent, e.g., gelatin stiffness contrasts, while antibuoyancy alone may be insufficient to prevent upward propagation. We also observe that experimental dikes become arrested when reaching a plain before opposite reliefs. Our numerical models show that below the plain the stress field induced by topography hinders further dike propagation. We suggest that lateral dike propagation requires an efficient barrier (rigidity) to upward propagation, assisting antibuoyancy, and a lateral pressure gradient perpendicular to the least compressive stress axis, while dike arrest may be induced by external reliefs.

Evidence of an axial magma chamber beneath the ultraslow-spreading Southwest Indian Ridge

Abstract Ultraslow-spreading ridges are a novel class of spreading centers symbolized by amagmatic crustal accretion, exposing vast amounts of mantle-derived peridotites on the seafloor. However, distinct magmatic centers with high topographies and thick crusts are also observed within the deep axial valleys. This suggests that despite the low overall melt supply, the magmatic process interacting with the tectonic process should play an important role in crustal accretion; however, this has been obscured due to the lack of seismic images of magma chambers. Using a combination of seismic tomography and full waveform inversion of ocean bottom seismometer data from the Southwest Indian Ridge at 50°28′E, we report the presence of a large low-velocity anomaly (LVA) ∼4–9 km below the seafloor, representing an axial magma chamber (AMC) in the lower crust. This suggests that the 9.5-km-thick crust here is mainly formed by a magmatic process. The LVA is overlain by a high-velocity layer, possibly forming the roof of the AMC and defining the base of hydrothermal circulation. The steep velocity gradient just below the high-velocity layer is explained by the ponding of magma at the top of the AMC; this could provide the overpressure for lateral dike propagation along the ridge axis, leading to a complex interaction between magma emplacement, tectonic, and hydrothermal processes, and creating a diversity of seafloor morphology and extremely heterogeneous crust.

Deep fluid transfer evidenced by surface deformation during the 2014–2015 unrest at Piton de la Fournaise volcano

Identifying the onset of volcano unrest and providing an unequivocal identification of volcano reawakening remain challenging problems in volcanology. At Piton de la Fournaise, renewal of eruptive activity in 2014–2015, after 41months of quiescence and deflation, was associated with long-term continuous edifice inflation measured by GNSS. Inflation started on June 9, 2014, and its rate progressively increased through 2015. Inflation onset was rapidly followed by an eruption on June 20–21, 2014, showing that volcano reactivation can be extremely fast, even after long non-eruptive phases. This short-lived eruption involved a shallow source (1.3–1.9km depth below the summit). The inflation that followed, and eruptions in 2015, involved a larger depth range of fluid accumulation, constrained by inverse modeling at ca. 3.9 to 1.2–1.7km depth. This time evolution reveals that volcano reawakening was associated with continuous pressurization of the shallowest parts of its plumbing system, triggered by progressive upwards transfer of magma from greater depth. A deep magma pulse occurred in mid-April 2015 and was associated with deep seismicity (3 to 9.5km depth) and CO2 enrichment in fluids emitted by summit fumaroles. From this date, ground deformation accelerated and the output rates of eruptions increased, culminating in the long-lasting, large-volume, August–October eruption (~36Mm3). This evolution suggests that deep magma/fluid transfer through an open conduit system first provoked the expulsion of the top of the plumbing system in June 2014, and then induced the progressive vertical transfer of the entire plumbing system down to 9km (four eruptions in 2015). The new sustained feeding of the volcano was also at the origin of the hydrothermal system perturbation and the acceleration of the eastern flank motion, which favor lateral dike propagation and the occurrence of frequent and increasingly large eruptions. Our results highlight the fast and progressive way in which basaltic magmatic systems can wake up.

On the mechanisms governing dike arrest: Insight from the 2000 Miyakejima dike injection

Magma stored beneath volcanoes is sometimes transported out of the magma chambers by means of laterally propagating dikes, which can lead to fissure eruptions if they intersect the Earth's surface. The driving force for lateral dike propagation can be a lateral tectonic stress gradient, the stress gradient due to the topographic loads, the overpressure of the magma chamber, or a combination of those forces.The 2000 dike intrusion at Miyakejima volcano, Izu arc, Japan, propagated laterally for about 30 km and stopped in correspondence of a strike-slip system, sub-perpendicular to the dike plane. Then the dike continued to inflate, without further propagation. Abundant seismicity was produced, including five M>6 earthquakes, one of which occurred on the pre-existing fault system close to the tip of the dike, at approximately the time of arrest. It has been proposed that the main cause for the dike arrest was the fault-induced stress.Here we use a boundary element numerical approach to study the interplay between a propagating dike and a pre-stressed strike-slip fault and check the relative role played by dike–fault interaction and topographic loading in arresting the Miyakejima dike. We calibrate the model parameters according to previous estimates of dike opening and fault displacement based on crustal deformation observations. By computing the energy released during the propagation, our model indicates whether the dike will stop at a given location. We find that the stress gradient induced by the topography is needed for an opening distribution along the dike consistent with the observed seismicity, but it cannot explain its arrest at the prescribed location. On the other hand, the interaction of dike with the fault explains the arrest but not the opening distribution. The joint effect of the topographic load and the stress interaction with strike-slip fault is consistent with the observations, provided the pre-existing fault system is pre-loaded with a significant stress, released gradually during the dike–fault interplay.Our results reveal how the mechanical interaction between dikes and faults may affect the propagation of magmatic intrusions in general. This has implications for our understanding of the geometrical arrangement of rift segments and transform faults in Mid Ocean Ridges, and for the interplay between dikes and dike-induced graben systems.

Forecasting the path of a laterally propagating dike

AbstractAn important aspect of eruption forecasting is predicting the path of propagating dikes. We show how lateral dike propagation can be forecast using the minimum potential energy principle. We compare theory to observed propagation paths of dikes originating at the Bárðarbunga volcano, Iceland, in 2014 and 1996, by developing a probability distribution for the most likely propagation path. The observed propagation paths agree well with the model prediction. We find that topography is very important for the model, and our preferred forecasting model considers its influence on the potential energy change of the crust and magma. We tested the influence of topography by running the model assuming no topography and found that the path of the 2014 dike could not be hindcasted. The results suggest that lateral dike propagation is governed not only by deviatoric stresses but also by pressure gradients and gravitational potential energy. Furthermore, the model predicts the formation of curved dikes around cone‐shaped structures without the assumption of a local deviatoric stress field. We suggest that a likely eruption site for a laterally propagating dike is in topographic lows. The method presented here is simple and computationally feasible. Our results indicate that this kind of a model can be applied to mitigate volcanic hazards in regions where the tectonic setting promotes formation of laterally propagating vertical intrusive sheets.

Seismicity during lateral dike propagation: Insights from new data in the recent Manda Hararo–Dabbahu rifting episode (Afar, Ethiopia)

Seismicity released during lateral dike intrusions in the Manda Hararo–Dabbahu Rift (Afar, Ethiopia) provides indirect insight into the distribution and evolution of tensile stress along this magma‐assisted divergent plate boundary. In this paper, 5 dike intrusions among the 14 that form the 2005–present rifting episode are analyzed with local and regional seismic data. During dike intrusions, seismicity migrates over distances of 10–15 km at velocities of 0.5–3.0 km/h away from a single reservoir in the center of the rift segment, confirming the analogy with a slow spreading mid‐ocean ridge segment. Comparison with geodetic data shows that the reservoir is located 7 km down rift from the topographic summit of the axial depression. Dikes emplaced toward the north are observed to migrate faster and to be more voluminous than those migrating southward, suggesting an asymmetry of tension in the brittle‐elastic lithosphere. Seismicity during dike injections is concentrated near the propagating crack front. In contrast, faults and fissures in the subsurface appear to slip or open aseismically coeval with the intrusions. The seismic energy released during dike intrusions in the Manda Hararo Rift appears to be primarily modulated by the local magnitude of differential tensile stress and marginally by the rate of stress change induced by the intrusion. The low level of seismic energy accompanying dike intrusions, despite their significant volumes, is likely an indicator of an overall low level of tension in the lithosphere of this nascent plate boundary.

Evidence that coupling to magma chambers controls the volume history and velocity of laterally propagating intrusions

In this paper, I present a simple analytical solution for the unsolved problem of a propagating dike coupled to a magma chamber. As recognized by Segall et al. (2001), the flow of the magma from a chamber to a dike is described by an ordinary differential equation for the unknown pressures of chamber and dike. The most intuitive assumption on magma exchange is that the volume gained by the intrusion equals the volume lost by the chamber. This constraint, however, implies a chamber so large that its pressure does not drop during the feeding process and an incompressible magma that does not expand or vesiculate as it intrudes. Models assuming constant driving pressure fail to explain observations. Here, mass conservation constrains pressure as magma flows from the chamber to the dike and the compressibilities of magma and sources control volume changes. These assumptions allow me to decouple the equations and solve the system analytically. The model predicts that chamber and dike volume change exponentially with time as V(t) = V∞[1 − exp(−t/τ)]. The asymptotic volume V∞ and the timescale τ are found to be functions of rock, magma, chamber and dike parameters and of the initial pressure conditions. Assuming that the intrusion is shaped as an elongating cuboid, its velocity is found to change as v = v0exp(−t/τ), where v0 is the initial dike velocity. The model is successfully tested against the best observations available for lateral dike propagation events: the volume history of chamber and dike during the 2000 Miyakejima intrusion (Japan) and dike velocity during the 1978 Krafla event (Iceland) and during some intrusions following the 2005 event in Afar (Ethiopia). This paper confirms and extends the results of a previous study by Rivalta and Segall (2008), who found the final ratio between dike volume and the volume withdrawn from the chamber to be rV = 1 + 4μβm/3 > 1, where μ is the host rock rigidity and βm is the magma compressibility. Here, I demonstrate that the formula for rV holds at any time during the intrusion, not just at the end. This model confirms that some magma chambers behave as stiff magma‐tanks, able to inflate large dikes as balloons, and demonstrates that high volume ratios are unlikely if the chamber has a pure sill shape. Fitting observations with the model allows us to estimate several dike, chamber, magma and rock parameters.

A constant influx model for dike propagation: Implications for magma reservoir dynamics

Most observations of seismicity rate during dike propagation on basaltic volcanoes show (1) rate stationarity despite possible variations of the dike tip velocity, (2) frequent lack of clear and monotonic hypocenter migration following dike propagation, and (3) event occurrences located backward with respect to the dike tip position. On these bases, the origin of the seismicity contemporary to dike intrusion within basaltic volcanoes cannot be solely related to the crack tip propagation. Seismicity rather appears to be the response of the edifice itself to the volumetric deformation induced by the magma intruding the solid matrix. The volume change induced into the volcano edifice over time by the intruding magma is equal to the magma flux injected into the dike from the reservoir. The consequence of this is that the stationary seismicity rate observed during the intrusion is a proxy for the magma flux withdrawn from the reservoir. We consider a two‐phase dike propagation model, including a first vertical propagation followed by a lateral migration along a lithological discontinuity. We explore (1) under which geophysical conditions the vertical dike is fed at constant flow rate of magma and (2) dike propagation patterns. Implications entailed by constant volumetric flux on the Piton de la Fournaise volcano case study suggest a minimum size for the magma reservoir of about 1 km3 and a maximum value for the initial magma reservoir overpressure of about 2.2 MPa. Considering similar magma inflow rates during vertical and lateral dike propagation phases, we reproduce independent estimates of propagation velocities, rise times, and injected volumes when applying the model to the August 2003 Piton de la Fournaise eruption.

Compositions of dikes and lavas from the Pito Deep Rift: Implications for crustal accretion at superfast spreading centers

The northwest trending walls of the Pito Deep Rift (PDR), a tectonic window in the southeast Pacific, expose in situ oceanic crust generated ∼3 Ma at the superfast spreading southern East Pacific Rise (SEPR). Whole rock analyses were performed on over 200 samples of dikes and lavas recovered from two ∼8 km2 study areas. Most of the PDR samples are incompatible‐element‐depleted normal mid‐ocean ridge basalts (NMORB; (La/Sm)N < 1.0) that show typical tholeiitic fractionation trends. Correlated variations in Pb isotope ratios, rare earth element patterns, and ratios of incompatible elements (e.g., (Ce/Yb)N) are best explained by mixing curves between two enriched and one depleted mantle sources. Pb isotope compositions of most PDR NMORB are offset from SEPR data toward higher values of 207Pb/204Pb, suggesting that an enriched component of the mantle was present in this region in the past ∼3 Ma but is not evident today. Overall, the PDR crust is highly variable in composition over long and short spatial scales, demonstrating that chemically distinct lavas and dikes can be emplaced within the same segment over short timescales. However, the limited spatial distribution of high 206Pb/204Pb samples and the occurrence of relatively homogeneous MgO compositions (ranging <2.5 wt %) within a few of the individual dive transects (over distances of ∼1 km) suggests that the mantle source composition evolved and magmatic temperatures persisted over timescales of tens of thousands of years. The high degree of chemical variability between pairs of adjacent dikes is interpreted as evidence for along‐axis transport of magma from chemically distinct portions of the melt lens. Our findings suggest that lateral dike propagation occurs to a significant degree at superfast spreading centers.

Gravitational spreading causes en-echelon diking along a rift zone of Madeira Archipelago: an experimental approach and implications for magma transport

Many volcanic rift zones show dikes that are oriented oblique rather than parallel to the morphological ridge axis. We have evidence that gravitational spreading of volcanoes may adjust the orientation of ascending dikes within the crust and segment them into en-echelon arrays. This is exemplified by the Desertas Islands which are the surface expression of a 60 km long submarine ridge in southeastern Madeira Archipelago. The azimuth of the main dike swarm (average = 145°) deviates significantly from that of the morphological ridge (163°) defining an en-echelon type arrangement. We propose that this deviation results from the gravitational stress field of the overlapping volcanic edifices, reinforced by volcano spreading on weak substratum. We tested our thesis experimentally by mounting analogue sand piles onto a sand and viscous PDMS substratum. Gravitational spreading of this setup produced en-echelon fractures that clearly mimic the dike orientations observed, with a deviation of 10°–32° between the model’s ridge axis and that of the main fracture swarm. Using simple numerical models of segmented dike intrusion we found systematic changes of displacement vectors with depth and also with distance to the rift zone resulting in a complex displacement field. We propose that at depth beneath the Desertas Islands, magmas ascended along the ridge to produce the overall present-day morphology. Above the oceanic basement, gravitational stress and volcano spreading adjusted the principal stress axes’ orientations causing counterclockwise dike rotation of up to 40°. This effect limits the possible extent of lateral dike propagation at shallow levels and may have strong control on rift evolution and flank stability. The results highlight the importance of gravitational stress as a major, if not dominant factor in the evolution of volcanic rift zones.

Temporal and spatial history of the 1999–2000 Endeavour Segment seismic series, Juan de Fuca Ridge

Two large clusters of earthquake activity in June of 1999 and January of 2000 have dominated recent seismicity along the Endeavour Segment of the Juan de Fuca Ridge. The impacts of the June 1999 sequence on the hydrologic system, which include changes in vent temperature and chemistry within the Main Endeavour Vent Field, have been well documented previously. Analysis of seismic and hydroacoustic data indicates that both sequences exhibit a swarm‐like behavior, characterized by the absence of a dominant main shock event. The epicentral locations of events within the two swarms overlap spatially, with centroid positions near 47°49′ and 47°46′N latitude. During the June 1999 swarm, the initial activity spans the along‐axis region where a shallow axial magma chamber reflector was later imaged. The epicenters then migrate ∼12 km to the south at a rate of 0.3 m/s, consistent with lateral dike propagation. A distinct subcluster of events also occurred in the vicinity of Surveyor Volcano on the overlapping portion of the Cobb Segment. Given its distance from the main swarm, this activity may represent a triggered response to dynamic shaking. The January 2000 swarm has a more limited along‐axis extent, relative to the June 1999 swarm, with no indication of lateral migration. Much of this activity is concentrated in a region predicted to have undergone extension due to dike propagation in 1999. Although it contains fewer total events and is of shorter duration, relative to the June 1999 swarm, the January 2000 activity exhibits a higher peak rate of seismicity and greater mean event magnitude. As in situ temperature monitoring was not in place during January 2000 and vent fluids were not sampled until June 2000, the impacts of this swarm on the hydrothermal system are unknown. The southernmost tip of the Endeavour Segment also is found to be a region of repeating swarm activity. Although morphologic evidence indicates the Cobb Segment has been propagating northward recently, this seismic activity suggests that the western limb of the Endeavour‐Cobb overlap zone remains active.

Effects of crustal heterogeneity on the morphology of chasmata, Venus

The Ix‐Chel, Kuanja, and Vir‐ava Chasmata (IKVC) are a 7000‐km‐long collection of discontinuous topographic troughs and deformation suites. Collectively, they account for approximately 12.5% of the ∼55,000 linear km of chasmata on Venus; however, morphologically the IKVC differs significantly from the majority (75%) of Venusian chasmata because it lacks corona features. Coronae represent notable volumes of material both erupted onto the surface and intruded into the crust. Together with chasmata, coronae may also be important indicators of Venus's interior convective processes as well as significant contributors to Venus's heat loss. Although IKVC lacks coronae, detailed geologic and structural mapping along its length highlights previously unrecognized structural and volcanic relationships indicative of local uplift, collapse, and volcanism we call pseudocoronae. In light of this mapping, previous corona formation models (theoretical and numerical) are evaluated, and an amended hypothesis (diapir stagnation) is presented for pseudocorona formation. The diapir stagnation model proposes that variations of the crustal thickness perturb corona formation and thus affect their resultant surface morphology. Chasmata are accompanied by corona development when forming in regions of relatively thin, or normal (plains‐type), crust (<20 km) but lack corona features when influenced by crustal heterogeneities (mechanical and chemical) at depth associated with thick crustal plateau roots (35–80 km (e.g., Ovda, Thetis, and Phoebe Regios)). Beneath crustal plateaus, diapiric rise is inhibited by (1) the presence of a low‐density crustal keel that creates a deep‐seated density trap and (2) a composition‐dependent mechanically weak crust‐mantle boundary layer. Diapir stagnation can be characterized or accompanied by minimal broad doming of the surface, formation of radial and concentric extension structures, lateral dike propagation, and collapse, all of which may be accompanied by limited volume surface eruptions of the lowest density melt phases. Through time, the crust‐mantle boundary layer may repeatedly lift and subside through conductive heating and viscous flow, which may allow more voluminous melt migration and eruption; however, true coronae fail to develop.

What controls the along‐strike slopes of volcanic rift zones?

We investigate the dynamics of viscous pressure losses associated with lateral magma transport in volcanic rift zones by performing (1) coupled elastic‐hydrodynamic simulations of downrift magma flow in dikes and (2) analog experiments mimicking lateral dike propagation in the presence of an along‐rift topographic slope. It is found that near‐source eruptions are likely to be favored by shallow slopes while distant downrift eruptions may be encouraged by steeper slopes, provided that along‐rift variations in the tectonic stress are negligible or uncorrelated on the timescale of multiple dike intrusions. This implies the existence of a critical slope to which a volcanic rift zone would naturally evolve. Such behavior is produced by three‐dimensional (3‐D) elastic effects and is controlled by the ratio of the driving pressure gradient due to the along‐strike topographic slope to the vertical gradient in the excess magma pressure in the dike. This model may be viewed as complementary to commonly cited mechanisms that appeal to magma viscosity and the dynamics of freezing of lava flows at the surface to explain the low profiles of basaltic shield volcanoes. Our estimated values of the critical slopes are in general agreement with observations in Hawaiian rift zones, but further development of fully 3‐D models is required for more accurate predictions.

Thermodynamics of lateral dike propagation: Implications for crustal accretion at slow spreading mid‐ocean ridges

We consider solidification of hot fluid flowing through a rigid‐wall channel of infinite extent. The calculated “thermal arrest” lengths are used to investigate the role of magma freezing in limiting the propagation distance of lateral dike intrusions. Our results demonstrate that for reasonable parameters the propagation distances of meter‐wide dikes do not exceed the wavelength of crustal thickness variations or transform fault spacing along slow spreading ridges. This suggests that thermal controls on the crustal melt delivery system could be an important factor in modulating these variations. Unlike published results for a finite channel, which predict unlimited meltback of the channel walls if the prefreezing fluid velocity exceeds some critical value, any flow into an infinite channel will eventually freeze, provided that shear heating in the magma is negligible. The thermal arrest distances depend strongly on the average dike thickness h (∝h4 for dikes driven by an along‐strike topographic slope and ∝h2 for dikes driven by an excess source pressure). Thermal erosion of the country rocks associated with lateral dike intrusions is likely to be confined to a very small region near the magma source. Substantial correlations between the along‐strike bathymetry and geochemistry of the erupted lavas along individual ridge segments may be consistent with high‐level basalt fractionation in the laterally propagating dikes.

Topography of Closed Depressions, Scarps, and Grabens in the North Tharsis Region of Mars: Implications for Shallow Crustal Discontinuities and Graben Formation

Using Viking Orbiter images, detailed photoclinometric profiles were obtained across 10 irregular depressions, 32 fretted fractures, 49 troughs and pits, 124 solitary scarps, and 370 simple grabens in the north Tharsis region of Mars. These data allow inferences to be made on the shallow crustal structure of this region. The frequency modes of measured scarp heights correspond with previous general thickness estimates of the heavily cratered and ridged plains units. The depths of the flat-floored irregular depressions (55-175 m), fretted fractures (85-890 m), and troughs and pits (60-1620 m) are also similar to scarp heights (thicknesses) of the geologic units in which these depressions occur, which suggests that the depths of these flat-floored features were controlled by erosional base levels created by lithologic contacts. Although the features have a similar age, both their depths and their observed local structural control increase in the order listed above, which suggests that the more advanced stages of associated fracturing facilitated the development of these depressions by increasing permeability. If a ground-ice zone is a factor in development of these features, as has been suggested, our observation that the depths of these features decrease with increasing latitude suggests that either the thickness of the ground-ice zone does not increase poleward or the depths of the depressions were controlled by the top of the ground-ice zone whose depth may decrease with latitude. Deeper discontinuities are inferred from fault-intersection depths of 370 simple grabens (assuming 60° dipping faults that initiate at a mechanical discontinuity) in Tempe Terra and Alba Patera and from the depths of the large, flat-floored troughs in Tempe Terra. The frequency distributions of these fault-intersection and large trough depths show a concentration at 1.0-1.6 km depth, similar to data obtained for Syria, Sinai, and Lunae Plana. The consistency of these depth data over such a large region of western Mars suggests that a discontinuity or a process that transcends local and regional geology is responsible for the formation of these features. If this discontinuity is represented by the base of the cryosphere, its uniform depth over 55° of latitude suggests that the cryosphere did not thicken poleward. Alternatively, the concentration of depths at 1.0-1.6 km may represent the upper level of noneruptive dike ascent (lateral dike propagation) of Mars, which is controlled by gravity and atmospheric pressure and magma and country-rock characteristics, and was probably controlled, in part, by ground ice. Fault-intersection depths in the north Tharsis region locally extend down to a depth of 5-7 km. The depth data between 2 and 3 km are attributed to the discontinuity at the interface of megaregolith and basement or to the upper limit of noneruptive dike ascent of magma with a high volatile content. Intersection depths greater than 3 km, which were found at Alba Patera, may be due to the megaregolith-basement discontinuity, which was buried and depressed by volcanic loading, or to the upper level of noneruptive dike ascent of magma with a low volatile content. The near absence of narrow simple grabens with fault-initiation depths less than 0.6-1.0 km in this study area, as well as in most of western Mars, suggests that this depth represents the minimum depth that normal faults can initiate; at shallower depths tension cracks or joints would form instead. This hypothesis is supported by the application of the Griffith failure criterion to this minimum depth of normal fault initiation, which suggests that shallow crustal materials have a tensile strength of 2-4 MPa throughout most of western Mars, in close agreement with previous estimates of tensile strength of martian basaltic rock.

Dike‐induced faulting and graben subsidence in volcanic rift zones

Field observations and geodetic data indicate that dike intrusion in volcanic rift zones typically generates normal faulting and graben subsidence at the Earth's surface. Elastic models indicate that two‐dimensional (infinite strike length) dikes do not lower the ground surface above the dike and that normal faults do not lower the surface significantly, more than one down‐dip fault length from the fault trace. Dikes of finite length produce subsidence above the dike, but not by an appreciable amount, for appropriate dike lengths. Therefore the observed graben subsidence can be achieved only if fault slip extends virtually to the dike plane at depth. Dike intrusion increases the horizontal compression adjacent to the dike and decreases the compression beyond the dike perimeter. Therefore fault slip extending to the dike plane is most likely to occur above or in front of the laterally propagating dike. Two data sets documenting the change in surface elevation accompanying dike intrusion in the Krafla rift zone, Iceland, were inverted to determine the dike and fault geometry at depth. Ten kilometers south of the Krafla caldera, subsidence of a graben 1.5 km wide was produced by fault slip to 1.5–2 km depth, essentially to the dike top. Forty kilometers north of the caldera, subsidence of a graben 6 km wide was produced by fault slip to 4–5 km depth, well within the zone of compression adjacent to the dike. In order to determine if fault slip in front of the dike could have produced the observed subsidence north of the caldera, a three‐dimensional boundary element model that computes fault slip during lateral dike propagation was developed. Results indicate that fault slip in front of the dike is capable of producing most of the subsidence observed. Additional subsidence could result from reasonable mechanical anisotropy of the rift zone. The lack of deep fault slip south of the caldera is attributed to a less favorable initial stress state. This is consistent with differences in the tectonic history of the two regions over the past several centuries.

The role of rift zone storage in controlling the site and timing of eruptions and intrusions of Kilauea Volcano, Hawaii

Repeated eruptive and intrusive activity on the East Rift Zone of Kilauea volcano allows dikes to exist within the rift zone for extended periods of time. Leakage of magma from the summit storage region into the rift zone can cause inflation of a rift zone dike. Inflation raises the excess pressure at the dike centre and increases the stress intensity at the dike edges. If the inflation is sufficiently large, the stress intensity may exceed the fracture toughness of the country rocks and allow vertical or lateral dike emplacement to occur. Theoretical modelling can be used to predict the conditions under which vertical or lateral dike propagation and hence eruptive or intrusive activity may occur. Over typical ranges of dike half height and regional tension gradient, vertical propagation and hence eruption are favored by high fracture toughness, relatively low dike half heights, and low regional tension gradients. Lateral propagation can occur when the fracture toughness is low, when dike half height is higher, and when the regional tension gradient is high. The results of from the model constrain the fracture toughness within the East Rift Zone to be 100–110 MPa m½ and the regional tension gradient to be ≪ 1000 Pa/m.