GP13A-01
Dynamically-induced structures formation in congested magma
Crystal fabrics preserved in igneous rocks offer a glimpse into the magma emplacement process. Detailed field mapping, in combination with AMS studies, seem to provide the best available data for unravelling intrusion architecture on the decimetre scale. However, a full and proper understanding of the fluid dynamics of congested fluid-particle mixtures during shear remains elusive. This is a shame as without recourse to such fundamental understanding, the interpretation of structural field data in the context of magma flow remains problematic. One way to gain insight into the process is to treat flowing magma as a dynamic material with a rheology similar to sheared, congested slurries. The fancy that dense magma equates to a high temperature slurry is an attractive one, and opens up a way to examine the emplacement process that does not rely exclusively on equilibrium thermodynamics as a final explanation of commonly observed igneous structures. Instead, using examples from mafic rocks where cooling has been rapid, the idea is put forward that in high Peclet number suspensions (where particle diffusion is negligible), shearing and non- Newtonian behaviour imparts a rich diversity of structures including layering, grading and flow segregation. Key to understanding the rheology, hence flow dynamics of congested magma, is the particle microstructure, a still poorly known essence of suspension flows. Where magma transport is continental in scale and long lived (e.g. Large Igneous Provinces), rotation of the earth may in theory endow a small but potentially measurable imprint on the preserved flow fabric.
GP13A-02
Lateral emplacement of the Western Mourne granite, N. Ireland, from AMS fabric data
Field observations and anisotropy of magnetic susceptibility (AMS) measurements of oriented blocks from the Palaeogene Western Mourne granite pluton indicate the presence of a weak fabric. Visible fabrics, determined from the preferred alignment of feldspar phenocryst long axes in outcrop, trend between NNE and north but it is unclear whether these are gently plunging lineations or the trace of dipping foliations. The texture of the granite shows little evidence for plastic strain suggesting that the fabric observed delineates magma flow. AMS fabrics are dominantly oblate defining sub-horizontal foliations, parallel to gently dipping margins and gently plunging lineations that trend SSW-NNE and diverge northward. These data so far point to an emplacement model for the Western Mourne granite that describes a laccolith, fed laterally from the SSW. This mirrors the NNE directed lateral emplacement of the adjacent Eastern Mourne granite (Stevenson et al. 2007, J Geol Soc Lond, 164, 99-110) suggesting that these two centres share a common feeder zone outside the Mourne area to the south, coincident with a 50mGal gravity anomaly close to the coast (GSNI data). Contemporaneous mafic dykes that crop out along this stretch of coast exhibit xenoliths of mafic cumulate that, together with the gravity anomaly, suggests there may be an unexposed mafic pluton in this area. Given the genetic links between mafic and felisic magmas in this region, the coincidence of the projected Mourne granite feeder zone and the possible buried mafic centre, leads to a model in which the Mourne granites where emplaced NNE as a gently dipping sheet, up dip, from this unexposed mafic centre. This model raises the possibility that the other Palaeogene igneous centres in NE Ireland (Slieve Gullion and Carlingford) may be laterally linked. The main implication of significant lateral movement of magma in the upper crust is that the location of the igneous centres in the upper crust or volcanic edifice at the surface may not reflect the point at the base of the crust where the magma was generated.
GP13A-03 INVITED
New Paleomagnetic Constraints on the Evolution of the NE Rift-zone and Associated Landslides, Tenerife, Spain
Tenerife Island is composed by the Teide Pico-Viejo complex and by three rift-zones organized in a triple armed pattern (NE, NW and NS). Each rift zone delimits major landslide embayments. Consequently, a link between rift zone and collapse formation has been strongly suggested by several authors. Here, we focus on the NE rift-zone (NERZ) to constrain its dynamic and evolution. The dikes constituting the rift zone are thought to be emplaced during Miocene/Pliocene/Pleistocene. Two major collapses are displayed on each side of the NERZ: La Orotava in the North and Guïmar at the South. Paleomagnetic sampling sites were established from 111 dikes along the rift. Presently 80 sites are fully demagnetized of which 10 have high site dispersion and do not yield interpretable results. Of the 70 interpretable sites 16 are of normal polarity and 54 of reversed polarity. After inverting the reverse polarity sites through the origin, the in situ group mean (calculated from 52 sites) yields a D=23.8, I=39.8, alpha95=3.7. These values are discordant to an expected Miocene to Pliocene field, with an inferred Rotation R=25.8 +/- 6.6 and Flattening F=0.9 +/- 5.4. The discordance can be explained by either a clockwise vertical axis rotation or a tilt. The results show that the rift is a dynamic feature, with a continuous activity punctuated by several linked collapse-intrusion events (cf. J.C. Carracedo et al. presentation). The discordant paleomagnetic data is interpreted to reflect tilting of the rift related to the several destabilization events. The rift, initially thought to be middle Pliocene/Pleistocene, is likely older with at least two previous stages, one Miocene and one Middle Pliocene. Our study reveals a long-lived, multi-faceted nature of the events that shaped the NERZ. Consequently, volcanic rift-zones are clearly not a "permanent" feature, but represent a dynamic and changing geological and geographical environment.
GP13A-04 INVITED
Plug Flow, Fracture, and Fast Emplacement of a Magma Sheet, Trachyte Mesa Laccolith, Henry Mts., USA.
Distinct magma sheets 0.5 to 3 m thick are well exposed on the NW margin of the Trachyte Mesa Laccolith, Henry Mts., USA. On the outer contact of the laccolith, magma sheet margins are defined by 2- to 3-cm-thick zones of intense cataclastic deformation of plagioclase phenocrysts. To determine if sheet margins extend away from the outer contact, three 1-m-deep cores were drilled into the top of the laccolith. Cataclastic zones are absent in the cores, but a sheet margin has been identified that is defined by consistent changes in the orientation of the magnetic foliation and by a large decrease in magnetic susceptibility. Anisotropy of magnetic susceptibility was measured on 122 specimens from the exposed outer surface of the intrusion and the cores. The magma sheet studied is foliated; at the top contact of the laccolith, foliation is sub-horizontal within a 2- to 3-cm-thick shear zone. At the base of this shear zone, foliation abruptly rotates into a subvertical orientation. The foliation continues to rotate toward the interior of the intrusion, defining an arc, with the base of the arc at 40 cm depth. The base of this arc coincides with a change in mean magnetic susceptibility. Crystal percentages were measured from the top 15 cm of the magma sheet in two locations. In both locations, there is a 22% decrease in the volumetric abundance of crystals within 1 cm of the shear zone. The crystal abundance increases toward the center of the magma sheet by approximately 22% in the next centimeter, suggesting that crystals were displaced toward the interior of the sheet by the high shear stress at the contact (Bagnold effect). Back-scattered electron images inside the shear zone reveal dramatic fragmentation and grain-size reduction of the plagioclase (An45) phenocrysts into micron-scale pieces which are elongated along the foliation. Where the matrix is visible between plagioclase clasts, it appears undeformed. The matrix is defined by abundant, ten-micron-sized quartz 'blebs' which overgrow and are adjacent to smaller euhedral microphenocrysts of plagioclase (An30). Cathodoluminescence images reveal two generations of quartz: older quartz blebs are cut by younger quartz microveins emanating from the cataclastic plagioclase. These microveins are the only feature to crosscut the matrix. We interpret the magnetic fabric, crystal percentage, and electron microscope data to indicate that plug flow (high shear strains near the margins with translation of the interior of the magma sheet) occurred. We infer that strain rate was high within the narrow shear zone at the top of the magma sheet, allowing the magma in the zone to cross the glass transition. In this scenario, both phenocrysts and glass matrix fracture. After the sheet was emplaced and deformation ceased, the magma returned to the liquid state and subsequently crystallized, leaving no trace of fracturing in the matrix.
GP13A-05
Sampling Strategies and the Anisotropy of Magnetic Susceptibility of Dykes.
Most studies of the anisotropy of magnetic susceptibility (AMS) of dykes have assumed that susceptibility axes should define a unique orientation relative to the dyke walls and magma flow direction. Theoretical considerations, however, predict systematic variations of AMS as a function of the amount of shear experienced by the flowing magma. Although this feature of AMS might seem undesirable at first sight, based in a theoretical model it has been suggested that actually we can take advantage of such variation in the orientation of the AMS axes to infer magma flow directions more confidently than until now if proper attention is given to the scheme followed during sample collection. In this work we report the results of a pilot study made to test those theoretical predictions. We resampled two dykes from the Ko'olau dyke complex in O'ahu and for which magma flow direction was previously inferred albeit with some uncertainty. The resampling was made following the guidelines suggested by the theoretical work as closely as possible (i.e., collecting more than one profile parallel to the dyke walls and across the dyke). The results of our measurements provide the first empirical evidence supporting the occurrence of a cyclic fabric acquisition during the emplacement of at least some dykes. The procedure described in this paper therefore constitutes a self-contained approach that can be followed to test in a case by case basis the occurrence of such fabric behavior. This in turn should provide enough elements to increase the reliability of the interpretation in the obtained results.
GP13A-06 INVITED
Episodic Assembly of Igneous Intrusions in the Shallow Crust at two Spatial and Temporal Scales, Henry Mountains, Utah
We describe episodic assembly of igneous intrusions in the shallow crust at two spatial and temporal scales, focusing on examples from the Henry Mountains (Utah, U.S.A.) intruded into flat-lying sandstones and shales. The lack of regional deformation during Oligocene assembly of these intrusions makes the Henry Mountains an excellent location to study intrusion processes without tectonic modification. At the scale of individual small (< 1 km3) intrusions we focus on three examples with exceptionally well exposed 3-D geometries. In order of increasing volume these include: (1) a lobate sill, (2) a laccolith, and (3) a bysmalith (a cylindrical, fault-bounded laccolith). Field evidence demonstrates that each intrusion was assembled from multiple horizontal sheet-like magma pulses. We suggest these intrusion geometries reflect the time evolution of an idealized upper crustal pluton as total magma volume increases. Thermal constraints for these intrusions require that assembly of each occurred in less than 100 years. Geochemical data indicate that (1) each intrusion is geochemically homogeneous, and (2) the sill and laccolith were fed by a magma batch distinct from that of the bysmalith. Episodic assembly is also apparent at the scale of a relatively large (~ 50 km3) laccolith-like intrusive center (Mt Hillers) that formed through amalgamation of dozens of component intrusions, including the three discussed above. Paleomagnetism demonstrates that early sills were uplifted and tilted by later underlying intrusions, and structural analysis allows us to establish the relative timing of other component intrusions. Geochemical data indicate that at least five separate magma batches contributed to assembly of the intrusive center. The duration of assembly is not yet well established, but is probably less than 1 M.y. Our data demonstrate that even small geometrically simple intrusions can have complex assembly histories, and that similar intrusive processes operate at both local and regional scales. Similar complex structural features and assembly histories have been inferred for larger syntectonic intrusions elsewhere, but unequivocal evidence of this complexity is rare. The interplay between deformation and magmatism commonly ensures that, as intrusions grow in size and duration of assembly, many of the features recording the details of such complex histories are modified or destroyed. Studying these processes in relatively simple locations like the Henry Mountains is therefore essential to unraveling assembly histories elsewhere.
GP13A-07
Volcanic facies from 1D to 3D: Using Wireline Logs and Terrestrial Laser Scanninng to Characterise the Internal Architecture of Flood Basalt Provinces
Recent research has shown that continental flood basalt provinces (CFBPs) are complex rock volumes with a great deal of internal heterogeneity. Quantitative data on this internal complexity is currently limited, so new methods presented here are important in understanding the internal architecture of a flood basalt province. This work introduces novel methods of data collection and analysis, and incorporates a facies classification scheme for flood basalts. Facies in this classification scheme are defined by their different physical characteristics, style and environment of emplacement, and include tabular lava flows, compound lava flows, hyaloclastites and dykes/sills. New analysis of wireline log data reveals that the different facies have a characteristic set of physical properties such as velocity and density distributions. As a result, it is possible to identify these onshore facies in offshore data, which is extremely useful where other data is limited. Volcanological facies can thus be identified from downhole wireline logs. New research is also presented on the outcrop characteristics of various facies types. Digital field mapping using terrestrial laser scanning equipment is an expanding area of Earth Sciences, and has proved extremely useful for the accurate 3D characterisation of complex outcrops. Here, it is applied to continental flood basalt lavas of the Faroe Islands Basalt Group, North Atlantic Igneous Province. Terrestrial laser scanning allows the rapid acquisition of accurate, robust data and as such has proved extremely useful in quantifying lava flow size, shape and core to crust ratio. Data are presented from fieldwork carried out in June 2007, capturing 3D outcrop models at a variety of scales to show the heterogeneity present within the various facies. The 3D outcrop models can be combined with the characteristic wireline log signatures of different facies to produce realistic geological models of physical properties. These integrated models will be used for geophysical modelling to improve the quality of sub-basalt seismic imaging. They also provide much quantitative information on, for example, lava flow thickness variations, the ratio of flow crust to flow core, and the amount and distribution of associated sediments. All of this information is vital in understanding the evolution of a CFBP.
GP13A-08
Sheet Swarms and Crustal Thickening at Icelandic Spreading Segments
Crustal accretion processes in Iceland provide subaerial analogues for mid-ocean ridge spreading systems. Deeply eroded Tertiary crust in northern Iceland exposes the internal architecture of numerous central volcanoes that formed at the centers of spreading segments similar to those on the East Pacific Rise (EPR) and Juan de Fuca Ridge (JDF). The 3-dimensional structure of abandoned spreading segments reveals details of crustal accretion processes, such as crustal thickening, which may also apply to EPR and JDF crust. In Icelandic segment centers, the intrusion of inclined sheet swarms contributes significantly to crustal thickening. Swarms of relatively thin, mafic to felsic sheets collectively add a thickness of 100's of meters to the crust. Structural analysis of several sheet swarms exposed northern Iceland defines their shape, thickness, kinematics magma transport and sequence of intrusive episodes. Inclined sheet swarms are often interpreted as 'cone sheets,' that is, assemblages of downward-converging, meter-scale intrusions that collectively define a conical shape. The sheets are assumed to intrude from a shallow-crustal magma chamber located beneath the segment center. Icelandic sheet swarms contain primarily basaltic sheets, with a range of textures and degrees of alteration. Sheets commonly dip between 45° and 65° towards the focus of local crustal thickening. Paleomagnetic data indicate rotation of individual sheets within a swarm is insignificant, while Anisotropy of Magnetic Susceptibility (AMS) reveals magnetic fabric parallel to the dip direction of the sheet, interpreted to result from up-dip magma flow. Spatial analyses of sheet swarms show a consistency of strike rather than radial symmetry, suggesting that the cone sheet classification may be an oversimplification. Cross-cutting relationships indicate multiple generations of sheets which steepen with progressive intrusion. Systematic compositional variations with time are not observed. The relatively subdued topography of the neovolcanic zone in Iceland indicates that focused subsidence rather than generation of higher topography must accommodate crustal thickening near segment centers. Subsidence in the middle crust must accommodate injection of sheet swarms as well as surface subsidence and thickening of lava units, which are both localized at central volcanoes. Similar processes may occur at EPR and JDF- type spreading segments where high magma budgets and persistent shallow crustal magma chambers are present.