Subhorizontal Sill Research Articles

Rhyolitic volcano dynamics in the Southern Andes: Contributions from 17 years of InSAR observations at Cordón Caulle volcano from 2003 to 2020

In this article I present a review of InSAR observations of ground deformation at Cordón Caulle volcano, whose 2011–2012 VEI 4-5 eruption is the best scientifically observed and instrumentally recorded rhyolitic eruption to date. I document a complete cycle of pre-eruptive uplift, co-eruptive subsidence and post-eruptive uplift with InSAR data between March 2003 and May 2020, and produced by a complex interplay of magmatic processes. Pre-eruptive data show ~0.5 m of ground uplift in three distinct episodes between 2003 and 2011, with uplift rates between ~3 and ~30 cm/yr. The uplift was likely caused by magma injection resulting in pressurization of the magmatic system at depths of 4–9 km. Data spanning the first 3 days of the eruption show ~1.5 m of deflation produced by two distinct sources at 4–6 km depth located 18 km from each other and up to 10 km from the eruptive vent -- suggesting hydraulic connectivity of a large magma mush zone. A third source of deformation was recorded during the rest of the eruption at a depth of ~5 km, resulting in a total subsidence of ~4.2 m during the whole eruption. On a much smaller spatial scale (~25 km2), InSAR-derived digital elevation models recorded ~250 m of uplift in the area of the eruptive vent interpreted as the intrusion of a shallow laccolith during the first 2.5 months of the eruption and time averaged lava discharge rates up to ~150 m3/s. The co-eruptive time series of reservoir pressure drop and extruded volume follow exponential trends that can be explained by a model of magma reservoir depressurization and conduit flow. Since the end of the eruption, the surface of the volcano was uplifted ~1 m in a sequence of three transient episodes of unrest during 2012 and 2019, with uplift rates between 6 and 45 cm/yr and lasting between 0.5 and 3.2 years. These pulses can be modeled by the same source, a sub-horizontal sill at a depth of ~6 km. Viscoelastic relaxation is not significant on these time scales, hence I interpret these uplift signals as being produced by episodic pulses of magma injection in the crystal mush that likely underlies the volcano. The episodic and abrupt changes of the ground deformation suggest a restless trans-lateral magmatic system at depths of 4–9 km, and active across multiple spatial and temporal scales. Finally, I also discuss challenges of the InSAR technology that should be addressed to detect ground deformation on short time scales, particularly under the low coherence conditions of Cordón Caulle.

Crustal seismicity associated to rpid surface uplift at Laguna del Maule Volcanic Complex, Southern Volcanic Zone of the Andes

Laguna del Maule Volcanic Complex (LMVC, Southern Andes of Chile) has been experiencing large rates (ca. 30cm/yr) of surface uplift as detected since 2008 by satellite geodetic measurements. Previous works have modeled the source of this deformation as an inflating rectangular sub-horizontal sill underlying LMVC at 5km depth, which is supposedly related to an active process of magmatic replenishment of a shallow silicic reservoir. However little is known about the tectonic context on which this activity is taking place, particularly its relation with crustal seismicity that could help understanding and monitoring the current deformation process. Here we present the first detailed characterization of the seismic activity taking place at LMVC and integrate it with structural data acquired in the field in order to illuminate the possible connection between the ongoing process of surface uplift and the activation of crustal faults. Our main finding is the recognition of repetitive volcano-tectonic (VT) seismic swarms that occur periodically between 2011 and 2014 near the SW corner of the sill modeled by InSAR studies. A cross-correlation analysis of the waveforms recorded for these VT events allows identifying three different seismic families. Families F1 and F3 share some common features in the stacked waveform and its locations, which markedly differ from those of family F2. Swarms belonging to this later family are more energetic and its energy was increasing since 2011 to a peak in January 2013, which coincide with maximum vertical velocities detected by local GPS stations. This points to a common process relating both phenomena. The location of VT seismic swarms roughly coincides with the intersection of a NE-SW lineament with a WNW-ESE lineament. The former shows clear field evidences of dextral strike-slip that are fully consistent with one nodal plane of focal mechanism for well-recorded F2 events. The conjugate nodal plane of these focal mechanisms could coincide with the WNW-ESE lineament, for which our field reconnaissance suggests a dominant normal motion. Events belonging to families F1 and F3 are also dominantly strike-slip but with some mixture with thrust and normal components. Our results, in conjunction with results of previous authors, suggest a complex mechanical interaction between the arrangement of crustal faults forming the structural framework on which the magmatic plumbing system of LMVC is emplaced and the inflating source at depth.

Continuous estimation of 3-D reflector orientations along 2-D deep seismic reflection profiles

Interpretation of 2-D deep seismic reflection data can be limited by the recording of out-of-plane reflections that cannot be readily distinguished from those originating beneath a seismic line. Here I present a method analogous to semblance velocity analysis that utilizes varying source-receiver azimuths to derive continuous estimates of 3-D reflector orientations along onshore 2-D reflection profiles. For each zero-offset time within a common depth point supergather, the semblance is calculated along 3-D travel time curves, and the dip and strike of the most coherent reflection is determined. Relative errors in these angles are derived from the range of travel time curves that have semblance values greater than a specified fraction, for example 90%, of the maximum. The method is illustrated using a section from line 10GA-YU1 from the Youanmi terrane of the Yilgarn craton in Australia in which the original field data have been replaced with synthetic in-line and cross-line reflections. Reflector orientations are generally well recovered where the range of available source-receiver azimuths is greater than 20o, but the method fails at lower ranges where the seismic line is almost linear, and this behavior is also observed in analysis of the field data. Nevertheless when using a realistic 1-D velocity function the orientation of upper crustal shear zones can be readily determined, and on unmigrated sections subhorizontal sills can be distinguished on the basis of their geometry from the mid-crustal reflectivity. In future surveys, reflector orientations can be determined along the near-linear sections of a seismic profile by deploying additional receivers, perhaps as cross-line recording spreads, to supplement the limited range of azimuths available from the in-line acquisition. The method can in principle be extended to marine reflection surveys, and more complex sub-surface velocity models.

Crustal accretion at a sedimented spreading center in the Andaman Sea

The magmatic, tectonic, and hydrothermal processes forming the oceanic crust along mid-ocean spreading centers are strongly modified at sedimented spreading centers but poorly understood owing to the lack of good crustal images. Here we present high-resolution deep seismic reflection images across the sedimented slow-spreading Andaman Sea spreading center. Several sub-horizontal sills are injected within the sedimentary strata, and no surface eruption is observed. On-axis reversed-phase reflections within the igneous crust correspond to axial magma lenses at different depths. The faults within the axial valley are steeply dipping (65°–75°) in a staircase pattern forming the axial graben. Their base coincides with a shallow-dipping (30°) reflection, defining the zone of extension and magmatism. As the sill-sediment sequences are rafted away from the axis, they are rotated and buried due to subsidence and faulting, forming the upper oceanic crust. The gabbroic lower oceanic crust is separated from the mantle by a complex Moho transition zone probably containing dunite lenses.

Long period seismicity and very long period infrasound driven by shallow magmatic degassing at Mount Pagan, Mariana Islands

AbstractLong period (LP) seismicity and very long period infrasound (iVLP) were recorded during continuous degassing from Mount Pagan, Mariana Islands, in July 2013 to January 2014. The frequency content of the LP and iVLP events and delay times between the two arrivals were remarkably stable and indicate nearly co‐located sources. Using phase‐weighted stacking over similar events to dampen noise, we find that the LP source centroid is located 60 m below and 180 m west of the summit vent. The moment tensor reveals a volumetric source modeled as resonance of a subhorizontal sill intersecting a dike. We model the seismoacoustic wavefields with a coupled earth‐air 3‐D finite difference code. The ratios of pressure to velocity measured at the infrasound arrays are an order of magnitude larger than the synthetic ratios, so the iVLP is not the result of LP energy transmitting into the atmosphere at its epicenter. Based on crater shape and dimensions determined by structure from motion, we model the iVLP as acoustic resonance of an exponential horn. The source of the continuous plume from gas analysis is shallow magmatic degassing, which repeatedly pressurized the dike‐sill portion of the conduit over the 7 months of observation. Periodic gas release caused the geologically controlled sill to partially collapse and resonate, while venting of gas at the surface triggered resonance in the crater. LP degassing only accounts for ~12% of total degassing, indicating that most degassing is relatively aseismic and that multiple active pathways exist beneath the vent.

The effects of thermomechanical heterogeneities in island arc crust on time‐dependent preeruptive stresses and the failure of an andesitic reservoir

AbstractUsing ground deformation data from Soufrière Hills volcano (SHV), we present results from numerical modeling of the temperature‐ and time‐dependent stress evolution in a mechanically heterogeneous crust prior to reservoir failure and renewed eruptive activity. The best fit models do not allow us to discriminate between a magmatic plumbing system consisting of either a single vertically elongated reservoir or a series of stacked reservoirs. A prolate reservoir geometry with volumes between 50 and 100 km3, reservoir pressure changes between 4 and 7 MPa, and reservoir volume changes between 0.03 and 0.04 km3 with magma compressibility between 4 × 10−11 and 1 × 10−9 Pa−1 provide plausible thermomechanical model parameters to explain the deformation time series; around an order of magnitude less overpressure than is generally inferred from homogeneous, elastic crustal models. Reservoir failure is predicted to occur at the crest of the reservoir except for reservoirs with highly compressible magma ( Pa) for which subhorizontal sill formation is predicted upon reservoir failure. Introducing a deep‐crustal hot zone modulates the partitioning of strains into the hotter underlying crust and results in a further reduction in overpressure estimates to values of around 1–2 MPa upon reservoir failure. Deduced volume fluxes are consistent with constraints from thermal modeling of active subvolcanic systems and imply dynamic failure of a compressible magma mush column feeding eruptions at SHV. Our interpretation of the results is that the combined thermomechanical effects of a deep‐crustal hot zone and hot encasing rocks around a midcrustal andesitic reservoir fundamentally alter the time‐dependent subsurface stress and strain partitioning upon reservoir priming. These effects substantially influence surface strains recorded by volcano geodetic monitoring.

A new model for the growth of basaltic shields based on deformation of Fernandina volcano, Galápagos Islands

Space-geodetic measurements of surface deformation produced by the most recent eruptions at Fernandina – the most frequently erupting volcano in the Galápagos Archipelago – reveal that all have initiated with the intrusion of subhorizontal sills from a shallow magma reservoir. This includes eruptions from fissures that are oriented both radially and circumferentially with respect to the summit caldera. A Synthetic Aperture Radar (SAR) image acquired 1–2 h before the start of a radial fissure eruption in 2009 captures one of these sills in the midst of its propagation toward the surface. Galápagos eruptive fissures of all orientations have previously been presumed to be fed by vertical dikes, and this assumption has guided models of the origin of the eruptive fissure geometry and overall development of the volcanoes. Our findings allow us to reinterpret the internal structure and evolution of Galápagos volcanoes and of similar basaltic shields. Furthermore, we note that stress changes generated by the emplacement of subhorizontal sills feeding one type of eruption may control the geometry of subsequent eruptive fissures. Specifically, circumferential fissures tend to open within areas uplifted by sill intrusions that initiated previous radial fissure eruptions. This mechanism provides a possible explanation for the pattern of eruptive fissures that characterizes all the western Galápagos volcanoes, as well as the alternation between radial and circumferential fissure eruptions at Fernandina. The same model suggests that the next eruption of Fernandina will be from a circumferential fissure in the area uplifted by the 2009 sill intrusion, just southwest of the caldera rim.

A Detailed Geochemical Study of a Shallow Arc-related Laccolith; the Torres del Paine Mafic Complex (Patagonia)

The shallow crustal Torres del Paine Intrusive Complex in southern Patagonia offers an opportunity to understand the chemical evolution and timing of crystallization processes in shallow plutonic rocks. It is characterized by hornblende-gabbros, gabbronorites, monzodiorites and granitic plutonic rocks.The exceptional exposure of the intrusion permits the identification of two structurally and petrographically different zones. Layered gabbronorite, olivine-bearing pyroxene-hornblende gabbronorite and monzodiorite forming vertical sheets and stocks in the west are referred to here as the feeder zone. These mafic rocks are in vertical contact with younger granitic rocks on their eastern border. The eastern part is a laccolith complex. It is characterized by three major units (I, II, III) of granitic rocks of over 1000 m vertical thickness; these are underlain in places by a sequence of hornblende-gabbro sills intermingled with evolved monzodiorite granite. Chilled, crenulated margins as well as flame structures between gabbroic rocks and monzodiorites suggest that the mafic sill complex remained partially molten during most of its construction. Bulk-rock major and trace element data indicate that the Paine mafic rocks follow a high-K calc-alkaline to shoshonitic differentiation trend. The parental magmas were basaltic trachyandesite liquids, with variable H2O and alkali contents.The majority of the feeder zone gabbronorites have high Al2O3 contents and positive Eu and Sr anomalies, consistent with accumulation of plagioclase and efficient extraction of intercumulus melt.The mafic sill complex largely lacks these cumulate signatures. Comparisons of the intercumulus groundmass in the hornblende-gabbros with intra-sill dioritic stocks and pods reveal similar rare earth element patterns and trace element ratios indicating incomplete extraction of evolved interstitial liquids.The Sr, Nd and Pb isotopic compositions of the mafic and granitic rocks exhibit ranges of 87Sr/86Sr of 0·704-0·708, eNd +3·8 to -1.2, 206Pb/ 204Pb 18·61-18·77, 207Pb/ 204Pb 15·67-15·67 and 208Pb/ 204Pb 38·56-38·77. Crystal fractionation and assimilation-fractional crystallization modelling, combined with high-precision U-Pb dating of zircons, indicates that the western feeder zone gabbronorites are linked to the uppermost Paine granite (granite I), whereas the mafic sill complex is younger and not directly related to the voluminous granite units II and III. These results are interpreted to indicate that crystal-liquid separation is facilitated in subvertical, dynamic feeder systems whereas subhorizontal sill complexes are inefficient in separating large volumes of mafic cumulates and complementary felsic rocks.

Deformation of Stromboli Volcano (Italy) during the 2007 eruption revealed by radar interferometry, numerical modelling and structural geological field data

The activity of Stromboli volcano is characterized by very low energy explosions, occurring every 10–15 min, throwing out lava above the crater rim. During 2007, the volcano showed an anomalous activity in which new effusive vents developed (27 February and 9 March) and a major explosion took place on 15 March. This paper presents an integrated study based on a structural geological field survey, interferometric radar monitoring and numerical modelling on the deformations of the upper NW flank of the volcano (Sciara del Fuoco — SdF). The field survey carried out during the 2007 events shows the development of mainly NE-striking fissures and fractures on the northern part of the upper SdF. In January 2007, the ground-based synthetic aperture radar (GB-InSAR), installed on the flank of the SdF, showed a progressive acceleration of movement on the NE crater preceding the eruption of 27 February 2007. By the first half of February, the acceleration also involved the upper portion of the SdF and on 27 February the eruption started with the opening of new vents and deformation rates higher than the measurement capability of the radar device. On 8–9 March 2007, the GB-InSAR highlighted the formation of a bulge on the NW sector of the SdF, preceding the opening of an additional vent. To understand the deformation pattern recorded during the eruption, a numerical simulation was carried out by using the FLAC 3D code. Based on the integration of all the data, the deformations observed in the pre-effusive phase (before 27 February) seem to be related to the intrusion of a SW–NE striking dyke. The bulging recorded before the 9 March vent opening is instead to be associated with the intrusion of a sub-horizontal sill.

Age and significance of the Marble Hall breccia, Bushveld Complex, South Africa

Diorite and associated breccia form scattered outcrops on the Marble Hall Fragment, in the vicinity of the town of Marble Hall, Mpumalanga Province, South Africa. The diorite (2055.6 ± 3.1 Ma) intruded as sub-horizontal sills in highly deformed and metamorphosed supracrustal rocks of the Transvaal Supergroup. The breccia, which ranges from polymictic to monomictic, consists of angular to sub-rounded fragments of meta-dolomite, chert and adinole, as well as intensely scapolitized Marble Hall diorite and Bushveld B1-type sill rock. Predominantly sodic pargasite constitutes the breccia-fill of the monomictic breccia and represents the fractionated liquid of transitional basalt that crystallized approximately 20% amphibole and 40% plagioclase (An40). The breccia-fill of the polymictic breccia comprises ferroactinolite-albite rock, with minor diopside, calcite, sphene, scapolite, grossular garnet, chalcopyrite and pyrrhotite. Cryptocrystalline silica or calcite is less common as the matrix of the polymicitc breccia. 40Ar/39Ar step-heating analyses on the sodic pargasite yields a plateau age of 2062 ± 8 Ma (55.4% 39Ar; MSWD = 0.28). The bulk of the evidence suggests that the emplacement of the Marble Hall diorite and breccia formation overlap with the B1 sill emplacement. The former rocks may be marginally younger than the Lower Zone of the Bushveld Complex.

Petrogenesis of the Squirrel Hills granite and associated magnetite-rich sill and vein complex: Lightning creek prospect, Cloncurry district, Northwest Queensland

The Lightning creek Fe-oxide Cu–Au prospect is hosted within a Mesoproterozoic granitoid batholith, composed dominantly of high-K, calc-alkaline, porphyritic quartz monzodiorite. The quartz monzodiorite contains enclaves of quartz diorite and has been intruded by more felsic granitoids (monzogranite and alkali-feldspar granite) and a series of subhorizontal sills. The quartz monzodiorite crystallised at crustal depths in excess of 10 km ( P∼4 kb, T=800–850°C, melt X H 2O =>4 wt.%, f O 2 ∼NNO buffer), and was probably fluid-saturated ( X H 2O : X CO 2 of >0.5). As might be expected, the more fractionated granitoids crystallised at progressively lower temperatures (quartz monzonite ∼800°C, alkali-feldspar granite <800°C). Subsolidus re-equilibration took place at temperatures between 700 and 600°C under increasingly oxidising conditions (amphiboles become increasingly TiO 2-poor and MgO-rich). Although there is abundant petrographic evidence for magma mingling in the petrogenesis of this granitoid suite, the curvilinear nature of many of the chemical trends indicates an additional contribution from crystal fractionation. Sm–Nd isotopic characteristics ( εNd=−2.7 to −3.3) indicate an older crustal component in the source region. Sill emplacement was contemporaneous with the development of a large magnetite-rich vein system. The sills are remarkable for their mineralogical and textural complexity. In addition to aplitic rocks, there are zones of albite–magnetite–quartz rock, which display a variety of unusual spherulitic textures. These Fe-rich portions of the sills are marginally younger than the aplites and are mineralogically similar to the magnetite-rich veins. The aplites are interpreted as late-stage differentiates of the plutonic suite, with which they share many mineralogical and compositional characteristics. It is argued that the textural and compositional variation within the sills reflect crystallisation under different degrees of melt saturation, and that the Fe-rich spherulitic rocks reflect the transition from magmatic to hydrothermal conditions within the crystallising pluton. Fluid inclusion studies indicate that this transition occurred at (hydrostatic?) pressures in excess of 1.5 kb (and possibly >2.5 kb) and temperatures greater than 500°C. Phase separation occurred in the fluid phase, producing in a CO 2-rich vapour and a metal-rich (Fe, Cu) brine.