Flow regimes during fluid-filled crack propagation in elastic media: Applications to volcanology and magma flow within dykes

Scaled analogue experiments were conducted to explore the effect of magma flow regimes, characterised by the Reynolds number (Re), on the transit of magma through the lithosphere via fractures. An elastic, transparent gelatine solid (the crust analogue) was injected by a fluid (magma analogue) to create a thin, vertical, and penny-shaped crack that is analogous to a magma-filled crack (dyke). A vertical laser sheet fluoresced passive-tracer particles suspended in the injected fluid, and particle image velocity (PIV) was used to map the location, magnitude, and direction of flow within the growing dyke from its inception to its surface rupture. Experiments were conducted using water, hydroxyethyl cellulose (HEC) or xanthan gum (XG) as the magma analogue. The results suggest that Re has significant impact on the direction of fluid flow within propagating dykes: Re > 0.1 (jet-flow) is characterised by a rapid central rising fluid jet and downflow at the dyke margin, whereas Re < 0.1 (creeping flow) is characterised by broadly uniform velocities across the dyke plane. Re may be underestimated by up to two orders of magnitude if tip velocity rather than internal fluid velocity is used. In nature, these different flow regimes would affect the petrological, geochemical, geophysical, and geodetic measurements of magma movement, key information upon which reconstructions of volcanic plumbing system architectures and their growth are based.

PARAMETER-PARAMETER FISIKA UNTUK MENGUNGKAP STRUKTUR STATIS BAWAH PERMUKAAN GUNUNGAPI

ABSTRAKParameter-parameter fisika gunungapi diungkap dengan metode geofisika. Survei kakas gravitasi dan magnetik yang menghasilkan anomali positive bagi medan gravitasi dan magnetiknya, mengungkap struktur statis bawah permukaannya. Analisis tremor volkanik mengungkap dinamika internalnya. Gerakan-gerakan (aliran) fluida magma di dalam gunungapi menjadi sumber getar yang memancarkan gelombang seismik yang di sebut tremor volkanik. Lokasi, migrasi, daya pancar, bentuk geometri sistem pipa-kantong magma, periodisasi, model matematis dan sebagainya. Gempa volkanik yang disebabkan aktivitas magma dapat dijadikan indikator. Hasil pengeplotan posisi hiposenter dan episenter terhadap gempa volkanik yang terjadi, juga dapat mengungkap struktur statis bawah permukaan gunungapi. Kata Kunci : parameter-parameter fisika gunungapi; struktur statis bawah permukaanbawah permukaan ABSTRACTUsing methods of geophysics, physical parameters of volcano are described. Gravity and magnetic surveys yield positive anomaly on their fields, which can be interpreted as an accumulated material beneath the surface with certain values of its mass density and magnetic susceptibility. Analysis of volcanic tremor at the volcano to the knowledge of its internal dynamics. Fluid magma movements inside a volcano acts as source of vibrations which radiate sesmic wave called volcanic tremor. Location, migration, radiation power, geometry of magma chamber-pipe system, periodicities, mathematical models, etc. Volcanic earthquakes caused by magma activity can also be used as indicators. The results of the hypocenter and epicenter position of the volcanic earthquake that occurred, can also reveal the subsurface static structure of the volcano. Keywords : physical parameters;subsurface static structure

Extensive Sills in the Continental Basement from Deep Seismic Reflection Profiling

Crustal seismic reflection profiling has revealed the presence of extensive, coherent reflections with anomalously high amplitudes in the crystalline crust at a number of locations around the world. In areas of active tectonic activity, these seismic “bright spots” have often been interpreted as fluid magma at depth. The focus in this report is high-amplitude reflections that have been identified or inferred to mark interfaces between solid mafic intrusions and felsic to intermediate country rock. These “frozen sills” most commonly appear as thin, subhorizontal sheets at middle to upper crustal depths, several of which can be traced for tens to hundreds of kilometers. Their frequency among seismic profiles suggest that they may be more common than widely realized. These intrusions constrain crustal rheology at the time of their emplacement, represent a significant mode of transfer of mantle material and heat into the crust, and some may constitute fingerprints of distant mantle plumes. These sills may have played important roles in overlying basin evolution and ore deposition.

Simultaneous Free Flow and Forcefully Driven Movement of Magma in Lamprophyre Dykes as Indicated by Magnetic Anisotropy: Case Study from the Central Bohemian Dyke Swarm, Czech Republic

A composite lamprophyre dyke from the Central Bohemian Dyke Swarm (Czech Republic) shows both indications of magma free flow (normal magnetic fabric with magnetic foliation and lineation parallel to the dyke plane) as well as those of forcefully driven magma movement (intermediate and inverse magnetic fabrics with magnetic foliation perpendicular to the dyke plane). The overall characteristics of the magnetic parameters across the dyke indicate the existence of at least two slightly differing parts that probably represent two magma pulses. The marginal part of the dyke is formed by kersantite, while toward the axial part, the composition gradually changes to spessartite, and obtains an increasing degree of amphibolization. The rocks of the dyke are inhomogeneous, both compositionally and structurally. It is likely that some portions of ascending magma were more viscous than the others, and the magnetic minerals in the more viscous magma portions may have oriented according to their longer dimensions perpendicular to the dyke, creating an inverse fabric. The lengthening perpendicular to the dyke was compensated by the vertical escape of neighboring more fluid magma, creating a normal magnetic fabric. The frequent oblique magnetic fabrics may represent transitions between the above two mechanisms.