http://jgs.geoscienceworld.org/cgi/content/abstract/162/1/1
Emplasemen gunung lumpur raksasa di CekunganKaspia Selatan:pencitraan seismik refleksi 3D pada zona akarnya
Emplacement of giant mud volcanoes in the South Caspian Basin: 3D seismic reflection imaging of their root zones
Richard J. Davies1 and Simon A. Stewart2
1 13DLab, School of Earth, Ocean and Planetary Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3YE, UK (e-mail: Richard.Davies@earth.cf.ac.uk)
2 2BP Azerbaijan, c/o Chertsey Road, Sunbury on Thames, Middlesex TW16 7LN, UK
Ditelaah dengan Kata Kunci Oleh: Prof. Dr. Hardi Prasetyo
Untuk LUSI LIBRARY:KNOWLEDE MANAGEMENT
Prof. Dr. R. Davies sebagai kontributor penting (important contributors) naskah-naskah ilmiah yang telah dipublikasikan (published manuscripts) untuk ditempatkan pada LUSI LIBRARY:KNOWLEDGE MANAGEMENT
SARI MAKALAH
Data baru 3-D dengan kualitas luar biasa sehingga dapat mencitra secara rinci arsitektur saluran dari mud volcano terbesar:
Data 3D (seismik refleksi) dengan kualitas yang luar biasa (exceptional quality 3D data) untuk gunung lumpur terbesar (largest mud volcano) yang belum banyak dijelaskan sebelumnya secara rinci.
Sehingga telah menyediakan hasil pencitraan yang pertamakalinya secara rinci (detailed imaging) dari suatu arsitektur saluran (plumbing architecture).
Sistem saluran ini menghubungkan suatu bangunan vulkanik utama (major volcano edifice) ke lapisan sumbernya di kedalaman (source layer at depth).
Anatomi gunung lumpur di Cekungan Kaspian Selatan sebagai ekstrusi kerucut ganda lumpur:
Gunung lumpur di Cekungan Kaspia Selatan terdiri dari ekstrusi kecurutganda lumpur di bawah dasar laut (extruded submarine mud bicone).
Dengan karekteristik umum yaitu: lebar 10 km dan tebal 1,4 km, di atasnya dibangun dengan sebuah kaldera berbentuk oval (oval caldera) lebar 1,2-1,6 km dan kedalaman 0,5 Km.
Arsitektur struktur sistem kaldera dicirikan oleh zona batuan induk yang runtuh dan hubungan dengan puncak lapisan sumber lumpur:
Kaldera menyempit ke bawah (caldera narrow downwards) ke zona batuan induk yang runtuh (zone of collapse country rock) ke bawah kerucut lonjong (tapering cone), tinggi 1 km.
Bagian simpul (the vertex) terletak dekat dengan bagian atas dari lapisan sumber lumpur (top of the mud source layer).
Hasil pencitraan elemen struktur secara rinci dapat memodel secara evolusi zona silinder sistem pengumpan yang padat, dianalogikan dengan kawanan greypons di daratan:
Pencitraan elemen-elemen struktur (imaged structural elements) memungkinkan dilakukan pemodelan evolusi (evolutionary model).
Sebuah pipa fluidisasi yang terjal mengumpan pelopor kerucut yang paling tua (steep fluidization pipe fed the oldest, pioneer cone).
Penulis (Davies) mengusulkan bahwa lebih banyak lagi pipa fluidisasi (fluidization pipes) tambahan yang menginjeksi batuan induk (country rock).
Membentuk suatu terobosan padat dari zona silinder (densely intruded, cylindrical zone), mirip kawanan Gryphon yang dapat diamati pada singkapan di daratan.
Proses pembentukan erosi dan kompasi dan runtuhnya kawah yang dihubungan keatas dengan patahan cincin sebagai pembatas tepian kaldera:
Erosi dan kompaksi pada dinding batuan (wall rock erosion and compaction) dari sona intrusi (intruded zone) menyebabkan runtuhnya ke arah bawah kerucut lonjong (collapse of the downward tapering cone).
Runtuhnya caldera (collapse caldera) dihubungkan ke arah atas dengan patahan cincin (ring fault) sebagai pembatas tepian kaldera (caldera margin).
Kelanjutan pengaliran pada tepian kerucut terpangkas, membentuk sistem kosentrik yang kecil, mempunyai analogi dengan kawah kaldera sistem gunung magmatik:
Selanjutnya aliran lumpur berfokus pada tepian-tepian kerucut terpangkas (sheared margins).
Volumetrik kontraksi dari kerucut volkanik yang ekstrusikan (extruded volcanic cone) menyebabkan suatu sistem konsentrik yang tidak biasanya berukuran kecil.
Kerucut konsentrik menghadap keluar terhadap patahan-patahan normal (outward-facing normal faults).
Model ini mempunyai banyak kesamaan pada sintesis terhadap sistem kaldera-diatrema (to syntheses of igneous maar–diatreme–caldera systems) dari gunung magmatik (igneous), yang mungkin dapat dianalogikan.
Kata Kunci (KEYWORDS): South Caspian Basin (Cekungan Kaspia Selatan), mud volcanoes (gunung lumpur), fluidization (fluidasasi), calderas (kaldera), seismic (seismik), reflection (refleksi atau pantul)
ABSTRACT
Exceptional quality 3D data for the largest mud volcano yet described provide the first detailed imaging of the plumbing architecture that connects a major volcanic edifice to its source layer at depth.
The volcano is in the South Caspian Basin and consists of an extruded submarine mud bicone, 10 km wide and 1.4 km thick, overlying an oval caldera 1.2–1.6 km in width and 0.5 km in depth.
The caldera narrows downwards into a zone of collapsed country rock forming a downward tapering cone, 1 km in height, the vertex of which is located close to the top of the mud source layer.
The imaged structural elements lead to an evolutionary model. A narrow, steep fluidization pipe fed the oldest, ‘pioneer’ cone. We propose that numerous additional fluidization pipes injected the country rock, forming a densely intruded, cylindrical zone, similar to ‘gryphon’ swarms observed at outcrop onshore.
Wall-rock erosion and compaction of the intruded zone led to collapse of the downward tapering cone that linked upwards into ring faults that define the caldera margins.
Later mud flowage focused on the conical sheared margins. Volumetric contraction of the extruded volcanic cone led to an unusual concentric system of minor, outward-facing normal faults. This model has many similarities to syntheses of igneous maar–diatreme–caldera systems, for which it may be analogous.
KEYWORDS: South Caspian Basin (Cekungan Kaspia Selatan), mud volcanoes (gunung lumpur), fluidization (fluidasasi), calderas (kaldera), seismic (seismic), reflection (refleksi)
Introduction
Mud volcanoes are an important global mechanism for degassing deeply buried sediments and it has been estimated that several thousand occur globally (Milkov 2000).
Various architectures connecting extrusive mud volcanic cones to their underlying source layer have been described, ranging from bulbous diapirs (Brown 1990) to steep diatremes (Robertson & Kopf 1998) and narrow vertical pipes (Graue 2000).
Most research, however, has been based on seismic data where the feeder system architecture is relatively poorly imaged, or on side-scan sonar data that reveals only the surface morphology of the volcanoes.
It is unclear whether current generic models for the structural roots of mud volcanoes are accurate or their relative simplicity reflects insufficient image resolution.
Here we report the results of mapping very large mud volcanoes in the South Caspian Basin using high-quality 3D reflection seismic data, which have imaged through the volcanic cones to reveal the architecture of their feeder systems emanating from the autochthonous mud source at depth.
We focus on a particularly large mud volcano that has a cone of up to 1.4 km in thickness and 10 km in diameter, and from which the feeder system can be traced vertically downwards over 2 km to the mud source.
We have used these observations to build a model of mud volcano and conduit evolution in the South Caspian Basin. This model has many similarities to published models of igneous feeder systems, which have traditionally been syntheses of a number of separate field exposures (e.g. Lorenz 1975). The direct imaging of an entire feeder system reported here could provide insights into volcanic system architecture.
The northern margin of the South Caspian Basin is marked by major NW-SE-striking Tertiary age folds that overlie an accrctionary prism and plate boundary (Jackson et al. 2002).
Our study area is on one fold within this trend, the Apsheron anticline, and includes four large (>2 km width) mud volcanoes (Figs 1 and 2a). Here we report detailed mapping focusing on one of these volcanoes, using a 3D pre-stack depth-migrated seismic reflection volume tied to 15 deep boreholes.
Lithological data from the boreholes show that the succession penetrated by the feeder systems is dominantly lacustrine and that the volcanic cones formed under shallow water (
Four seismic units (1-4) were delineated on the basis of discordant seismic reflection relationships, changes in seismic amplitude and structural characteristics (Fig. 2a-c). We first describe these units, and then analyse some structural elements in more detail and synthesize a model for the evolution of the conduit-edifice system.
(1) The succession from the sea bed to the top of the buried mud volcano constrains the time at which significant mud expulsion ceased as intra-Pliocene. Clear stratal onlap onto the cone defines its upper surface.
(2) The top and base mud volcano reflections define a mud volcanic pile that has the geometry of a bicone (two cones placed base-to-base; Fig. 2b and c). The bicone shows significant thickening into a central depression, where the base mud volcano surface is downfaultcd. We term this region a caldera. The bicone has undergone some flexure as a result of regional contractional folding, indicating that it formed prior to fold development.
(3) The base of the volcanic pile is separated from the mud source by c. 2 km of generally concordant Pliocene and Miocene strata that show no evidence of syndepositional compression or mud volcanism. The feeder system penetrates this interval.
(4) Below unit 3, an Oligocene succession of variable thickness (Maykop Formation) drapes a series of imbricate thrusts within Palacocene and older rocks (not intersected in Fig. 2b and c).
This is 10 km in diameter and 1.4 km in thickness at its centre, and consists of low-amplitude continuous reflections that converge radially away from the central biconic axis (indicated by broken black lines in Fig. 2b and c). The central axis is located above the caldera (Fig. 2d).
The volcanic pile is made up of at least five separate stacked cones that are not described in detail here. The lowermost cone marks the initiation of volcanism. It is 2 km in diameter and 200 m thick and is herein termed the 'pioneer cone'. It is located within c. 200 m of the margin of the caldera. The top and base surfaces of the bicone define the volume of extruded sediment, which is c. 22.5 km^sup 3^ (no correction for compaction).
Concentric faults. Fourteen concentric extensional ring faults cut the base mud volcano surface (Fig. 2a-c). In cross-section the faults are planar, have displacements of up to 30 m and dip at 45� relative to bedding; 12 of the 14 face away from the caldera (Fig. 2d). The fault tip lines are between 380 and 1100 m below the base mud volcano surface and less than 200 m above it (Fig. 2b and c). The outermost fault defines a ring of 9.1 km diameter (Fig. 2d). The rings are centred on the bicone axis.
Caldera.
The caldera is located at the base of the mud volcano and is oval in plan view, 1.2-1.6 km wide, c. 2 km^sup 2^ in area and up to 0.5 km deep (Fig. 2a-c). Fault displacement varies around the margin from a zone of c. 100 m fault offset in the SW to 400 m displacement in the SE. The bounding fault zone is steep (c. 75�). The caldera floor forms a concave-up dish shape (Fig. 2c).
Below the caldera is a downward tapering cone of country rock that is 1.6 km wide at its top and c. 100-300 m wide at its vertex. The vertex is c. 1.2 km below the floor of the caldera and c. 200 m above the Maykop Formation.
The cone contains continuous, concordant reflectors with concave-up geometry. Strata outside the cone are relatively undeformed; the stratal reflections terminate sharply against the conical fault system.
A number of high-amplitude reflections are located on the margins of the cone (marked V in Fig. 2c). These may be seismic artefacts or could be acoustic impedance contrasts across faults at the cone margin. Below the downward tapering cone, stratal geometries change to a dominantly convex-up form defining a minor, upwards pointing cone that is reminiscent of reactive diapir geometry (Fig. 2b and c; Vendeville & Jackson 1992).
A seismic coherency data volume (see Bahorich & Farmer 1995; Fig. 2e) across a downward tapering cone below another nearby caldera along the Apsheron anticline (Fig. 2a) reveals numerous seismic discontinuities (orange) that separate blocks of coherent seismic reflections (grey). NW-SE-trending extensional faults are also revealed by this seismic attribute (marked Y in Fig. 2c).
Model for feeder system evolution.
These seismic data are consistent with other examples from the South Caspian Basin in that they show no evidence for large balloon-like mud diapirs (Cooper 2001).
Instead, these data are more consistent with mud movement by fluidized mud flows through fractures (e.g. Morley 2003). For a hydro-fracture to form and propagate, fluid pressure must exceed horizontal stress plus the tensile strength of the overburden (Delaney et al. 1986).
Many factors that influence this relationship have varied through time in the study area, including disequilibrium compaction and tectonic shortening across the underlying plate boundary (Narimanov 1993; Jackson et al. 2002).
Once a hydro-fracture has formed the velocity of upward moving pore fluids can exceed the grain fall velocity, causing particles to become entrained as a fluidized sediment flow.
We propose that the pioneer cone was fed by a vertical, discrete and narrow fluidization pipe (Fig. 3a). Narrow vertical fluid and gas escape pipes are a feature of some igneous systems and have been linked to mud volcano evolution in other sedimentary basins (Lorenz 1975; Woolsey et al. 1975; Brown 1990; Graue 2000).
We assume that the volume of country rock replaced by the mud-filled pipe is lost to the surface by wall-rock erosion: mud flows at outcrop are often rich in wall-rock breccia (Kopf et al. 2003).
It is not clear whether subsequent pipe intrusions followed a similar course, but onshore outcrops often show a scattered array of small edifices (gryphons) demonstrating that there are several routes upwards, rather than a single one, within a kilometre-scale mud volcano (Hovland et al. 1997; Planke et al. 2003).
Applying this 'gryphon field' model to the period of extrusion of five main sequences mapped within the biconc, we suggest that numerous pipes repeatedly intruded the overburden at approximately the same location to feed the constructional edifice (Fig. 3b).
This model matches the seismic imaging: the orange zones of low seismic continuity in Fig. 2c are interpreted as faults and mud fluidization pipes that separate coherent blocks of country rock. Loss of mud volume from depth led to a localized basin or 'rim syncline', leading to the biconical form of the mud edifice.
This pipe system would represent a cylindrical zone of heavily intruded country rock or entirely of amalgamated mud pipes (Fig. 3b). We postulate that this cylindrical zone had a low mechanical strength in relation to the surrounding unintended country rock and underwent differential compaction, resulting in a downward tapering conical collapse (Fig. 3c).
Extensional faults within and surrounding the cone would have acted as inherent weaknesses localizing later fluidized flows, resulting in a geometry reminiscent of 'cone sheets' as described by Andcrson (1936).
The final result is that the internal structure of the downward tapering cone is a complex mosaic of faults, dykes and pipes feeding the volcanic edifice.
Pipe and dyke structures are not obvious in all the mapped calderas, suggesting either that the feeder pipes are below seismic resolution at this structural level, or that they are concentrated on caldera margins.
The concentric extensional fault set is almost perfectly circular in plan view. It is surprising that most of these faults face away from, rather than towards, the central depression as seen in examples that might be considered analogues (e.g.Maione 2001).
The kinematics are, however, consistent with volumetric contraction of the extrusive bicone (as a result of degassing and dcwatcring) inducing centripetal shear on its conical basal surface (Stewart & Argent 2000; Fig. 3c).
In addition to being the largest single mud volcano yet described, this case study provides new insights into the architecture of a feeder system that connects a major volcanic cone to its source layer several kilometres below.
These volcano and feeder systems have similarities to igneous centres, which also have cone-shaped edifices, calderas and downward tapering cones created during magma chamber collapse and ring-dyke intrusion (Francis 1970; Lorenz 1975; Branney 1995).
This similarity raises the possibility of mud volcano systems imaged on 3D seismic data shedding light on the structure of poorly exposed igneous architectures.
More accurate models of mud volcano and feeder geometry may constrain total mud volcano volume, which could serve as a basis for a novel method of estimating total methane emitted from these major structures into the atmosphere over geological time scales.
The opinions expressed here are solely the views of the authors and not necessarily the view of BP Azerbaijan or any of the AIOC partners. R.J.D. thanks Sehlumberger Tor use IESX interpretation software. M. Huuse, D. Hansen, P. Heinio and J. Frey Martinez are thanked for comments. C. Morley and U. Ring are thanked for reviews, and R. England for editorial remarks.
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Emplacement of giant mud volcanoes in the South Caspian Basin: 3D seismic reflection imaging of their root zones
Bibliography for: "Emplacement of giant mud volcanoes in the South Caspian Basin: 3D seismic reflection imaging of their root zones"
Davies, Richard J "Emplacement of giant mud volcanoes in the South Caspian Basin: 3D seismic reflection imaging of their root zones". Journal of the Geological Society. FindArticles.com. 23 Mar, 2011. http://findarticles.com/p/articles/mi_qa3721/is_200501/ai_n9484991/