Marginal Shear Zone Research Articles

Results of the Study of Epigenetic Changes of Famennian–Tournaisian Carbonate Rocks of the Northern Marginal Shear Zone of the Caspian Syneclise (Kazakhstan)

Various genetic and morphological types of voids in carbonate reservoirs make it difficult to diagnose them, which can be seen in the determination of reservoir properties in the northern marginal shear zone of the Caspian Syneclise. A macro- and microscopic study of rocks was carried out by staining carbonates in thin sections with alizarin (determination of the mineral composition, structure, texture, void and fracture spaces, rock genesis). Instrumental methods (X-ray, DTA—differential thermal analysis, TGA—thermo-gravimetric analysis, and probe microanalysis) established the composition of rocks, the nature of their diagenetic transformations, and the formation of void space. The elemental and oxide composition of a number of samples was carried out using the X-ray probe microanalysis method, and mineral formations with intermediate thermochemical properties were found. The results of X-ray, DTA, and TGA measurements and the data of probe microanalysis made it possible to reveal thermally inert formations of oxides of calcium, magnesium, silicon, iron, and other compounds in the composition of carbonates. A relatively low-cost express method was used to determine the material composition and the nature of epigenetic changes and to obtain data on the void space as a result of the development of tectonic fracturing and diagenetic processes of leaching and secondary mineral formation in bedded carbonate reservoirs.

Transient conduit permeability controlled by a shift between compactant shear and dilatant rupture at Unzen volcano (Japan)

Abstract. The permeability of magma in volcanic conduits controls the fluid flow and pore pressure development that regulates gas emissions and the style of volcanic eruptions. The architecture of the permeable porous structure is subject to changes as magma deforms and outgasses during ascent. Here, we present a high-resolution study of the permeability distribution across two conduit shear zones (marginal and central) developed in the dacitic spine that extruded towards the closing stages of the 1991–1995 eruption at Unzen volcano, Japan. The marginal shear zone is approximately 3.2 m wide and exhibits a 2 m wide, moderate shear zone with porosity and permeability similar to the conduit core, transitioning into a ∼ 1 m wide, highly sheared region with relatively low porosity and permeability, as well as an outer 20 cm wide cataclastic fault zone. The low-porosity, highly sheared rock further exhibits an anisotropic permeability network, with slightly higher permeability along the shear plane (parallel to the conduit margin), and is locally overprinted by oblique dilational Riedel fractures. The central shear zone is defined by a 3 m long by ∼ 9 cm wide fracture ending bluntly and bordered by a 15–40 cm wide damage zone with permeability enhanced by ∼ 3 orders of magnitude; directional permeability and resultant anisotropy could not be measured from this exposure. We interpret the permeability and porosity of the marginal shear zone to reflect the evolution of compactional (i.e. ductile) shear during ascent up to the point of rupture, which was estimated by Umakoshi et al. (2008) at ∼ 500 m depth. At this point the compactional shear zone would have been locally overprinted by brittle rupture, promoting the development of a shear fault and dilational Riedel fractures during repeating phases of increased magma ascent rate, enhancing anisotropic permeability that channels fluid flow into and along the conduit margin. In contrast, we interpret the central shear zone as a shallow, late-stage dilational structure, which partially tore the core of the spine, leaving a slight permanent displacement. We explore constraints from monitored seismicity and stick-slip behaviour to evaluate the rheological controls, which accompanied the shift from compactional toward dilational shear as magma approached the surface, and discuss their importance in controlling the permeability development of magma evolving from overall ductile to increasingly brittle behaviour during ascent and eruption.

Petrological Architecture of a Magmatic Shear Zone: A Multidisciplinary Investigation of Strain Localisation During Magma Ascent at Unzen Volcano, Japan

Shearing of magma during ascent can promote strain localisation near the conduit margins. Any mechanical and thermal discontinuities associated with such events may alter the chemical, physical and rheological stability of the magma and thus its propensity to erupt. Lava spines can record such processes, preserving a range of macroscopic and microscopic deformation textures, attributed to shearing and friction, as magma ascends through the viscous-brittle transition. Here, we use a multidisciplinary approach combining petrology, microstructures, crystallography, magnetics and experimentation to assess the evidence, role and extent of shearing across a marginal shear zone of the 1994–1995 lava spine at Unzen volcano, Japan. Our results show that crystals can effectively monitor stress conditions during magma ascent, with viscous remobilisation, crystal plasticity and comminution all systematically increasing towards the spine margin. Accompanying this, we find an increase in mineral destabilisation in the form of pargasitic amphibole breakdown displaying textural variations across the shear zone, from symplectitic to granular rims towards the spine margin. In addition, the compaction of pores, chemical and textural alteration of interstitial glass and magnetic variations all change systematically with shear intensity. The strong correlation between the degree of shearing, crystal deformation and disequilibrium features, together with distinct magnetic properties, implies a localised thermal input due to shear and frictional processes near the conduit margin during magma ascent. This was accompanied by late-stage or post-emplacement fluid- and gas-induced alteration of the gouge, as well as oxidation and glass devitrification. Understanding and recognising evidence for strain localisation during magma ascent may, therefore, be vital when assessing factors that regulate the style of volcanic eruptions, which may provide insights into the cryptic shifts from effusive to explosive activity as observed at many active lava domes.

Different slip systems controlling crystallographic preferred orientation and intracrystalline deformation of amphibole in mylonites from the Neyriz mantle diapir, Iran

A deformed layered gabbro and a mylonitic gabbro sample from the marginal shear zone of the Neyriz mantle diapir in Iran were analyzed using electron backscatter diffraction (EBSD). Both samples have the common amphibole crystallographic preferred orientation (CPO) in which (100) lies perpendicular to foliation and <001> parallel to lineation. Amphibole grains in the layered gabbro sample have little internal deformation, whereas in the mylonitic gabbro sample the amphibole grains are strongly distorted and contain low angle grain boundaries. There is a subtle change in CPO as a function of grain size in the mylonitic gabbro. Coarse grains (porphyroclasts) have a (100) <001> CPO oriented with the main foliation reference frame whilst fine grains have a (100) <001> CPO oriented with the C′ shear bands. Detailed analysis of porphyroclast distortions and subgrain boundary trace analysis suggests that hard slip systems, most particularly (110) <1–10> control intracrystalline deformation. Schmid factor analysis suggest that these slip systems are not involved in foliation formation but are linked kinematically to C′ shear bands. It is unlikely that the slip systems that control intracrystalline deformation are important in CPO formation. We interpret that subgrain rotation recrystallization lead to grain size reduction and the elongate recrystallized grains were rotated towards the C′ shear bands by grain boundary sliding. This rigid body rotation, possibly in combination with easy slip on (100) <001> are considered the main cause of CPO formation. Amphibole zonation patterns in the layered gabbro sample suggest that oriented growth of amphibole may have contributed to CPO.

Crustal velocity structure of the Neoarchean convergence zone between the eastern and western blocks of Dharwar Craton, India from seismic wide-angle studies

The Dharwar Craton in the southern part of the Indian shield comprises two distinct blocks viz. the Meso-Neoarchean western and the predominantly Neoarchean eastern blocks, which are popularly referred to as Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC). The boundary between the WDC and EDC is marked by a prominent shear zone, the Chitradurga eastern margin shear zone (CSZ). Several studies suggested Neoarchean convergence between the EDC and WDC, although there is diversity of opinion on the tectonic process. We present here the results of a new seismic refraction/wide-angle reflection experiment along a 200km long profile across the WDC and EDC encompassing the CSZ to elucidate the crustal velocity structure of the convergence zone. Travel time inversion and amplitude modeling of the seismic data delineate a five-layered crustal velocity model for the Dharwar Craton with a 42km (minimum) thick crust in the WDC grading to a distinctly thinner crust up to 38km, in the EDC. The transect is also characterized by a high-velocity (7.0–7.1kms−1) mafic lower-crustal layer, but with distinctly variable thickness, 10km in WDC and 8km in EDC. The transition from thick to thinner crust coincides with the CSZ. The thick crust with a high-velocity basal layer is atypical of Archean cratons, but more akin to Proterozoic terrains. The crustal velocity structure across the EDC–WDC boundary zone is consistent with the Neoarchean plate convergence model that invoke tectonic scenarios, such as accretion of a hot orogen, the EDC and/or westward subduction of the latter beneath the WDC. Spatial variation in the high-velocity lower-crustal layer across the craton is ascribed to post-accretional mafic magmatism and underplating mainly during the Paleoproterozoic and Phanerozoic.

Pathways for degassing during the lava dome eruption of Mount St. Helens 2004–2008

The ability of volatiles to escape rising magma regulates the explosivity of a volcanic system. During silicic lava dome eruptions, strain localization at the conduit margin occurs during magma ascent, creating a damage halo with implications for gas escape. Here we report the first systematic study of permeability network anisotropy across the marginal shear zone of the A.D. 2004–2008 lava dome at Mount St. Helens (Washington State, USA). The results show increasingly large permeability anisotropy of as much as four orders of magnitude (over ∼4 m) moving from the interior of the spine through the damage halo. We find the permeability to be essentially isotropic in the spine interior but highly anisotropic in the damage zone and fault core. Our examination of the dome rocks reveals that the permeability anisotropy depends strongly on the presence of vertically oriented shear layers. Here we show that the rate of escape of volatiles will be several orders of magnitude higher vertically through a conduit margin shear zone than horizontally into the conduit wall.

Structural and geochronological constraints on the evolution of the eastern margin of the Tanzania Craton in the Mpwapwa area, central Tanzania

A study of the position, nature and geochronology of the eastern margin of the Tanzania Craton near Mpwapwa yields new constraints on Archaean to Neoproterozoic orogenesis of central Tanzania. The eastern part of the craton comprises typical Neoarchaean grey granodioritic orthogneisses dated with the LA-ICP-MS method by U–Pb zircon at ca. 2.7Ga. A gradual eastward increase in strain in these rocks culminates in a 1–2km wide, locally imbricated, north–south-trending, ductile thrust/shear zone with an oblique top-to-the-NW sense of movement. East of the craton-edge shear zone, high-grade supracrustal rocks are termed the “Mpwapwa Group” in view of uncertain regional correlations. There is an apparent lithological zonation of the Mpwapwa Group parallel to the craton margin shear zone. In the west, the group consists of typical “shelf facies” metasedimentary rocks (marbles, calc-silicates, quartzites, etc.). U–Pb dating of detrital zircons from two quartzites reveal only Archaean detritus, constraining their maximum depositional age to <ca. 2.6Ga. The shelf rocks pass eastwards into semi-pelitic gneisses interlayered with bimodal mafic-felsic gneisses, which may have volcanic protoliths and are intruded by multiple mafic sills. Dating of a felsic gneiss showed it to probably be a Neoarchaean rock which underwent Palaeoproterozoic metamorphism during the Usagaran event at ca. 1930Ma. This date likely represents initial juxtaposition of the (Archaean) Mpwapwa Group against the Tanzania Craton. Consequently the Group are not considered to be stratigraphic correlates of the Usagaran Konse Group or Isimani Suite seen further south, but rather form part of the Neoarchaean crust of the “Western Granulite” terrane of the East African Orogen. The Mpwapwa Group was intruded by weakly foliated biotite granite at 1873±31Ma. Zircons in the granite have metamorphic rims dated between 550 and 650Ma that grew during the East African orogenic event.Distinctive para- and orthogneisses, including charnockite dated at 2707±21Ma, outcrop in the southern part of the area. A sliver of southern tonalitic orthogneiss, entrained in the craton margin shear zone, is an igneous rock of probable early Palaeoproterozoic or Archaean age (>ca. 2300Ma) with a strong metamorphic overprint at ca. 1960Ma (zircon), confirmed by a metamorphic titanite age of ca. 1990Ma, again believed to date the initial phase of craton-margin shearing and juxtaposition of the Archaean crustal blocks. The role of the Neoproterozoic East African orogeny in the evolution of the craton margin is unclear, but the geometry of the shear zone, the presence of Neoproterozoic zircon rims in the Palaeoproterozoic granite and published studies from nearby, all suggest that the latest movements on the shear zone may be Neoproterozoic in age and that the structure may represent the local western front of the East African Orogeny.

Stratigraphic evidence for shear in structural development of the Triassic Levant margin: New borehole data on the epicontinental to deep marine transition in Israel

Triassic tectonic movements that include shear along a significant portion of the Levant margin, are evidenced from microfacies analysis of material recovered from the Asher-Atlit 1, Ga'ash-2, Meged-2, Devora-2 and Ramalla-1 onshore boreholes.Seismic studies showed that structural lineaments attributed to early Mesozoic extension underlie coastal and central Israel, trending NE-SW beneath southern Israel and sub-parallel to the then- extensional Palmyride basin of Syria. A more curved trend, approximately NNW–SSE, lies to the north beneath central Israel. Coast-parallel sedimentary domains formed above these lineaments, which constitute the newly formed passive margin of the Levant coast in the Triassic. The sedimentary history of these domains indicates that some were linked to each other, and others disconnected. The relative vertical movement between one domain and another is explained by activation of transpressional and trans-tensional shear movements on the underlying curved faults. These subordinate vertical movements also created the discontinuous barriers and small subsiding basins that separate the epicontinental sedimentary domain from the open Tethys, causing partial restriction necessary for the intermittent development of thick evaporites in the internal domains, alternating with a regionally suppressed carbonate system. This zone is here named the Onshore Levant Margin Shear Zone (OLMSZ).Shear was invoked theoretically in a number of models for the tectonic evolution of the eastern Mediterranean as part of a Neotethyan opening system. This paper provides rock-based evidence for such shear applicable to 20% of the eastern Levant margin in the Triassic.

A transpressional origin for the West Spitsbergen fold‐and‐thrust belt: Insight from analog modeling

The West Spitsbergen fold‐and‐thrust belt formed along the transform plate boundary between Greenland and the western Barents Sea during Paleocene‐Eocene breakup in the northern North Atlantic. Approximately 20–40 km margin‐perpendicular shortening accumulated in the belt has been attributed to transpression and strain partitioning in a restraining bend but also to head‐on collision. We have applied scaled analog tectonic modeling to test the former hypothesis. A pack of brittle quartz sand with a shallow thin layer of silicon putty was deformed by transpression at a 15° convergence angle by movement of a basal plate. The kinematics was quantified by means of digital particle image velocimetry. The result was a doubly vergent transpressional wedge, consisting of a steeply tapered retrowedge and a strongly internally deformed steeply tapered prowedge separated by a central strike‐slip zone and an adjacent low‐taper thin‐skinned fold‐and‐thrust belt. The doubly vergent wedge evolved through several kinematic phases. Three main stages were identified, namely, (1) initial distributed deformation, (2) development of an oblique doubly vergent wedge with progressive evolution of local strain partitioning along the marginal shear zones, and, finally, (3) a stage of full strain partitioning between a central strike‐slip zone and reverse displacement along marginal shear zones, with folding and thrusting in a thin‐skinned belt on the proside. The analog model convincingly reproduced the geometry and the kinematic evolution of the West Spitsbergen fold‐and‐thrust belt, supporting the hypothesis of its formation by strain partitioning in transpression with a small angle of convergence and significant lateral displacement.

Intrusion mechanisms in a turbidite sequence: the Voetspoor and Doros plutons in NW Namibia

Two syntectonic plutons of Cambrian age intruded Neoproterozoic metaturbidites in Namibia at the junction of the NS trending Kaoko and EW trending Damara belts. Sinistral transpression in the Kaoko Belt produced km-scale upright D1 folds overprinted by minor D2 folds. D3 is associated with N–S shortening in the Damara Belt. The plutons show two main pulses of intrusion: hornblende syenite intruded late during D1 or during D2 and biotite granite during D3. Each tectonic event produced a strain shadow defined by the shape of folds and the foliation trend around the plutons. The internal igneous fabric and the arrangement of wall rock xenoliths that locally make up 50% of the intrusion mass suggest that the plutons have a disk or wedge shape. A marginal shear zone indicates that one of the syenite intrusions descended during emplacement with respect to the wall rock. Emplacement is therefore inferred to be at least partly accommodated by descent of the intrusion floor. The biotite granite intruded into mega-strain shadows and tension gashes alongside and in the syenite during D3. The plutons show evidence of sinistral solid state rotation with respect to the wall rocks in response to D1–D2 transpression.

Origin of eclogitic metagabbro mass in the Sambagawa belt: Geological and geochemical constraints

Eclogite-bearing metagabbro masses in the high- P/ T Sambagawa metamorphic belt in SW Japan have long been regarded as tectonic blocks derived from the mantle wedge or hanging-wall side of the former Sambagawa subduction zone. The origin of one of the metagabbro masses (the Seba metagabbro) is reexamined by: (i) geological (chemico-structural) study focusing on the relationship between this body and the surrounding metasedimentary schists; and (ii) comparative major- and trace-element bulk-chemistry with other gabbros/metagabbros from the world. The geological studies reveal the presence of a marginal shear zone that: (i) is located between the enveloping pelitic schist and the central metagabbro; (ii) has a bulk chemical composition intermediate between the two; and (iii) shows up to 10 cm-scale prominent mafic–felsic banding. Based on the difficulty of forming such marginal shear zones by a tectonic block hypotheses, and key field observations in neighboring regions indicating formation of the marginal shear zone as sediments, the Seba metagabbro is concluded to have originated on the ocean floor as an olistolith. Comparative bulk-chemical studies show that Sr content of the Seba metagabbro is too high for gabbros derived from fracture zones associated with a spreading ridge. A comparison with other eclogitic metagabbro masses in the Sanbagawa belt reveals numerous similarities with the Seba metagabbro. This suggests all the metagabbro masses can be regarded as members of an ‘olistostrome complex’ that subducted and underwent the eclogite-facies metamorphism. The eclogitic complex includes the Western Iratsu mass that is considered to be an ancient seamount. This suggests that the metagabbro masses represent fragments of the ancient Western Iratsu seamount.

The Yanshanian Orogeny and Two Kinds of Yanshanides in Eastern-Central China

The Tan-Lu Fault was once a transform fault in the Paleotethys, west of which was the Qinling-Dabie Ocean separating the Yangtze Craton from the North China Craton, and east of which was the Su-Lu Ocean separating the Su-Wan Block from the Jiao-Liao Craton. The Qinling-Dabie Ocean closed in the Indosinian orogeny, which created the China-Southeast Asia Subcontinent, with the Tan-Lu Fault becoming a marginal shear zone along the newly-formed amalgamated subcontinent. The Su-Lu Ocean subducted partly in the Indosinian-orogeny, but not closed. In the Jurassic and Early Cretaceous, the Su-Wan Block drifted north wards with subduction of the Su-Lu Ocean and moved westwards to converge the subcontinent by sinistral shearing of the ENE-striking fractures. The Su-Lu Ocean finally closed and the Su-Wan Block collided with the Jiao-Liao Craton in the Early Cretaceous, which constituted a part of the magnificent interplate Yanshanides. The interplate orogeny rejuvenated the fossil sutures and deep fractures, as well as the Indosinian orogen, and the intraplate (intracontinental) Yanshanian orogeny occurred in the subcontinent. The East Asia Yanshanides, consisting of the interplate orogens in the outer side and the intraplate orogens in the inner side, collapsed quickly in the latest Early Cretaceous and Late Cretaceous. The eastern China area entered a tensile period from the Eogene, and the tectonic differentiation between the central and eastern China areas since the Jurassic was further strengthened.

The North Pole Dome: a non-diapiric dome in the Archaean Pilbara Craton, Western Australia

The northeastern part of the Archaean Pilbara Craton is characterised by an ovoid dome and basin pattern of domal granites separated by synformal greenstones (supracrustal rocks). This ovoid pattern is variably preserved at various structural and stratigraphic levels, with the most deeply eroded domes being granite dominated, and the least eroded domes being greenstone dominated. The North Pole Dome (NPD) is an example of a relatively high-level dome that has a flanking syncline preserving some of the youngest rocks of the craton (Fortescue Group) to be involved in the doming process. At the apex of the NPD is a small intrusive granite that was considered by some workers to be the top of a large underlying domal granite batholith. The NPD comprises mostly dense basaltic greenstones that range in age from approximately 3500 to 2700 Ma. The approximately 3460 Ma North Pole Monzogranite, a volumetrically insignificant intrusive granite body, intrudes the greenstones in the apex of the dome. No marginal shear zones occur around the intrusion. A new model, based on a detailed gravity traverse across the monzogranite, suggests that the granite intrusion is plug-like, up to 1.5 km thick and does not represent the exposed top of a larger underlying domal batholith. Results from potential field modelling show that the dome is relatively flat bottomed, with a base around 5.5–6.5 km deep. The NPD has no significant granitic material within the dome, but like all greenstones, is underlain by felsic crust (granite) below its base. The development of the NPD (and flanking syncline) was a multistage process. The first stage of doming involved relatively minor doming/tilting, possibly associated with the emplacement of the monzogranite, because palaeocurrents of synchronous volcanic rocks flowed radially outward from the dome. It is likely that this doming was minor as there are no recorded unconformities in the Warrawoona Group (in the NPD) above these volcanic rocks. A major dome-forming event (tilting >20°) occurred in the period between 3240 and 2772 Ma, and was unrelated to the emplacement of the small granite plug (diapirism). Regional folding and refolding from horizontal compression deformed the area into a domal shape (Type I fold interference pattern). Uplift and erosion of the dome was superseded by extension and deposition of flood basalts (Fortescue Group) that flowed towards the dome. Three further stages of shortening folded the regional unconformity and the underlying and overlying units, further amplified the underlying dome, developed the flanking Marble Bar Syncline, as well as fold interference patterns in the Fortescue Group. The NPD was developed over a 800 Ma time frame, ostensibly by a process of fold interference due to multiple stages of horizontal compression. This work shows that diapirism was not the cause of the development of the domal geometry of the NPD, and its flanking syncline, rather folding and refolding due to horizontal compression was the principal controlling factor.

Of isbræ and ice streams

AbstractFast-flowing ice streams and outlet glaciers provide the major avenues for ice flow from past and present ice sheets. These ice streams move faster than the surrounding ice sheet by a factor of 100 or more. Several mechanisms for fast ice-stream flow have been identified, leading to a spectrum of different ice-stream types. In this paper we discuss the two end members of this spectrum, which we term the “ice-stream” type (represented by the Siple Coast ice streams in West Antarctica) and the “isbræ” type (represented by Jakobshavn Isbræ in Greenland). The typical ice stream is wide, relatively shallow (∼1000 m), has a low surface slope and driving stress (∼10 kPa), and ice-stream location is not strongly controlled by bed topography. Fast flow is possible because the ice stream has a slippery bed, possibly underlain by weak, actively deforming sediments. The marginal shear zones are narrow and support most of the driving stress, and the ice deforms almost exclusively by transverse shear. The margins seem to be inherently unstable; they migrate, and there are plausible mechanisms for such ice streams to shut down. The isbræ type of ice stream is characterized by very high driving stresses, often exceeding 200 kPa. They flow through deep bedrock channels that are significantly deeper than the surrounding ice, and have steep surface slopes. Ice deformation includes vertical as well as lateral shear, and basal motion need not contribute significantly to the overall motion. The marginal shear zone stend to be wide relative to the isbræ width, and the location of isbræ and its margins is strongly controlled by bedrock topography. They are stable features, and can only shut down if the high ice flux cannot be supplied from the adjacent ice sheet. Isbræs occur in Greenland and East Antarctica, and possibly parts of Pine Island and Thwaites Glaciers, West Antarctica. In this paper, we compare and contrast the two types of ice streams, addressing questions such as ice deformation, basal motion, subglacial hydrology, seasonality of ice flow, and stability of the ice streams.

Ice-flow measurements and deformation at marginal shear zones on Sørsdal Glacier, Ingrid Christensen Coast, East Antarctica

AbstractObservations of crevasse and crevasse trace geometry recorded along marginal shear zones of Sørsdal Glacier, East Antarctica, are presented with fine-spatial-resolution measurements of ice flow collected at three selected study sites. Strain grids established at each site, comprising 254 points, were repeatedly surveyed during the summers 2000/01 and 2001/02 using a semi-kinematic global positioning system technique with relative coordinate precisions at the order of ±2 cm in horizontal and ±5 cm in vertical. The annual results show ice-flow velocities of 2–114 m a–1 across a 2.7 km section of the Sørsdal margin. Comparable ice-flow velocities were also obtained using a Doppler system DORIS. At one study site, covering a planar area of 0.04 km2, a preliminary comparison is made between the crevasse pattern and strain-rate tensors computed for 64 discrete points using the method developed by Nye (1959). The comparison indicates that the most recent crevasses are directly related to the principal strain rates in this zone of simple shear.

The thermal stealth flows of Santiaguito dome, Guatemala: Implications for the cooling and emplacement of dacitic block-lava flows

Research Article| May 01, 2002 The thermal stealth flows of Santiaguito dome, Guatemala: Implications for the cooling and emplacement of dacitic block-lava flows A.J.L. Harris; A.J.L. Harris 1Hawai‘i Institute of Geophysics and Planetology/School of Ocean and Earth Science and Technology (HIGP/SOEST), University of Hawai‘i, 2525 Correa Road, Honolulu, Hawai‘i 96822, USA Search for other works by this author on: GSW Google Scholar L.P. Flynn; L.P. Flynn 1Hawai‘i Institute of Geophysics and Planetology/School of Ocean and Earth Science and Technology (HIGP/SOEST), University of Hawai‘i, 2525 Correa Road, Honolulu, Hawai‘i 96822, USA Search for other works by this author on: GSW Google Scholar O. Matías; O. Matías 2Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hidrologia (INSIVUMEH), 7a Avenida 14-57, Zona 13, Guatemala City, Guatemala Search for other works by this author on: GSW Google Scholar W.I. Rose W.I. Rose 3Department of Geological Engineering and Sciences, Michigan Technological University, Houghton, Michigan 49931, USA Search for other works by this author on: GSW Google Scholar Author and Article Information A.J.L. Harris 1Hawai‘i Institute of Geophysics and Planetology/School of Ocean and Earth Science and Technology (HIGP/SOEST), University of Hawai‘i, 2525 Correa Road, Honolulu, Hawai‘i 96822, USA L.P. Flynn 1Hawai‘i Institute of Geophysics and Planetology/School of Ocean and Earth Science and Technology (HIGP/SOEST), University of Hawai‘i, 2525 Correa Road, Honolulu, Hawai‘i 96822, USA O. Matías 2Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hidrologia (INSIVUMEH), 7a Avenida 14-57, Zona 13, Guatemala City, Guatemala W.I. Rose 3Department of Geological Engineering and Sciences, Michigan Technological University, Houghton, Michigan 49931, USA Publisher: Geological Society of America Received: 18 Dec 2000 Revision Received: 05 Nov 2001 Accepted: 11 Dec 2001 First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2002) 114 (5): 533–546. https://doi.org/10.1130/0016-7606(2002)114<0533:TTSFOS>2.0.CO;2 Article history Received: 18 Dec 2000 Revision Received: 05 Nov 2001 Accepted: 11 Dec 2001 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A.J.L. Harris, L.P. Flynn, O. Matías, W.I. Rose; The thermal stealth flows of Santiaguito dome, Guatemala: Implications for the cooling and emplacement of dacitic block-lava flows. GSA Bulletin 2002;; 114 (5): 533–546. doi: https://doi.org/10.1130/0016-7606(2002)114<0533:TTSFOS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Thick, slow-moving block-lava flows are associated with extrusive activity in dacitic systems, where lava-core depressurization during flow-front collapse generates devastating block-and-ash flows. Dimensional and rare thermal data collected during January 2000 for an active dacitic block flow at Santiaguito (Guatemala) provide insight into cooling and emplacement mechanisms. Flow velocity was low (12.5 m·d−1), in spite of steep (≥10°) slopes, a result of the high viscosity (>4 × 109 Pa·s) that we calculate for this flow. The flow surface consisted of a thick (1.9–3.4 m), cool (40–111 °C) crust of meter-sized, subangular blocks. Extremely effective insulation by the thick crust results in model-derived core cooling of ≤0.08 °C·h−1. These low cooling rates make block flows the most thermally efficient of all styles of lava-flow emplacement, allowing cooling-limited flow lengths of several kilometers, in spite of low eruption rates (<0.5 m3·s−1). Flow-front observations along with a plug-flow model showed that collapse from the faster-moving flow top contributed to a caterpillar-track–type advance similar to that observed at basaltic ‘a‘a flows. Forward motion also caused toothpaste-like extrusions of the flow core through the frontal crust at basal and marginal shear zones. The axial part of the flow front was thicker than the marginal zones and was oversteepened. This geometry can be explained by a higher vertical velocity gradient in the axial zone, causing more frequent and larger-volume flow-front collapses. Axial-zone collapses also penetrate farther up flow, but not sufficiently to depressurize the flow core and generate a block-and-ash flow. For such a block-and- ash flow to occur, we calculate that an increase in velocity and/or thickness (due to increased slope or topographic confinement) must occur. Whereas low surface temperatures make block flows invisible to short-wave infrared sensors, the low velocity also contributes to the stealthy behavior of these flows. Their stealthy nature, however, masks the fact that they can extend many kilometers, moving block-and-ash flow sources closer to vulnerable communities. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Surface temperature measurements of active lava flows on Kilauea volcano, Hawai′i

Systematic hand-held radiometer measurements of lava surface temperatures in active flows and tubes on Kilauea volcano, Hawai′i reveal complexities that cannot be resolved in remotely sensed data from aircraft or satellites. Using portable infrared Minolta/Land Cyclops radiometers, we measured surface temperatures of flows at various distances from their sources and investigated cooling rates and the development of crust. Our measurements suggest that the upper surface of these lava flows can be split into a minimum of four thermal components: core (>1050°C), visco-elastic skin (750–900°C), rigid solid crust (<750°C), and flow margins (<175°C). For the ′a′a flows investigated, a cool rigid crust characteristically developed in the central part of channels within 30 m of the source vent and incandescent lava was exposed in the marginal shear zones of channels. This affects the heat loss and morphology of lava in active channels. Our investigations of temperature distributions on pahoehoe flow fields reveal temperature anomalies of up to 150°C above active tubes and tumuli.

Strike-slip and tectonics granitoid emplacement: an AMS fabric study from the Odenwald Crystalline Complex, SW Germany

AMS fabric studies supported by field and microscopic work were applied to identify the internal structure and possible emplacement processes of the Variscan late-tectonic granodiorite-granite intrusions of the Unit III in the Odenwald Crystalline Complex. This Unit is bounded towards NW and NNE by steeply inclined shear zones, the southern part is unexposed. The magnetic susceptibility ranges between 10−3 and 10−6 SI units and is caused by paramagnetic and subordinately by ferromagnetic components. AMS ellipsoids are typically oblate with gently plunging long axes (lineations). AMS foliations and lineations trend mainly WSW-ENE and NNW-SSE, parallel with the NNW and ENE trending marginal shear zones of Unit III, respectively. As revealed by microstructural studies, a penetrative foliation in the plutons is related to emplacement processes. Therefore the observed AMS foliation and lineation are also interpreted as the result of syn-emplacement deformation which is dominantly strike-slip. Weakly inclined foliations around pluton roof xenoliths point to a component of buoyant rise of magma. It is suggested that the granitoid magma was generated in a low level anatectic zone along a left-lateral transpressive shear zone during local extension at releasing bends. During successive fault movements magma ascended through extensional parts of the shear zone. Local normal faults and the Otzberg zone at the eastern margin of Unit III document mostly brittle extension, which overprinted the strike-slip fabrics after the emplacement of the plutons.

Ongoing margin migration of Ice Stream B, Antarctica

AbstractThe rate of margin migration of an ice stream can be determined using repeat measurements of the surface velocity profile within the marginal shear zone. The method relies on the assumption that the velocity profile is a characteristic feature of the margin, and that this profile is shifted laterally as the margin migrates. Application of the method to Ice Stream B, Antarctica, indicates that the southern margin is moving outward into the inland ice at a rate of at least 9.7 ± 1.1 m a−1.

Ongoing margin migration of Ice Stream B, Antarctica

AbstractThe rate of margin migration of an ice stream can be determined using repeat measurements of the surface velocity profile within the marginal shear zone. The method relies on the assumption that the velocity profile is a characteristic feature of the margin, and that this profile is shifted laterally as the margin migrates. Application of the method to Ice Stream B, Antarctica, indicates that the southern margin is moving outward into the inland ice at a rate of at least 9.7 ± 1.1 m a−1.