Salt, Shale and Igneous Diapirs in and Around Europe

216 Pages·2000·19.59 MB·English

by  B. C. Vendeville

Salt, Shale and Igneous Diapirs in and around Europe
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VENDEVILLE, B., MART, Y. & VIGNERESSE, J.-L. (eds) 2000. Salt, Shale and Igneous
Diapirs in and around Europe. Geological Society, London, Special Publications, 174.
STEPHANESCU, M., DICEA, O. & TARI, G. 2000. Influence of extension and compres-
sion on salt diapirism in its type area, East Carphathian Bend area, Romania. In:
VENDEVILLE, B., MART, Y. & VIGNERESSE, J.-L. (eds) Salt, Shale and Igneous Diapirs
in and around Europe. Geological Society, London, Special Publications, 174, 131-
147.
GEOLOGICAL SOCIETY SPECIAL PUBLICATION NO. 174
Salt, Shale and Igneous Diapirs
in and around Europe
EDITED BY
BRUNO C. VENDEVILLE
University of Texas, USA
YOSSI MART
University of Haifa, Israel
and
JEAN-LOUIS VIGNERESSE
Universite Nancy, France
2000
Published by
The Geological Society
London
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Contents
Preface vii
Igneous intrusions
VIGNERESSE, J. L. & CLEMENS, J. D. Granitic magma ascent and emplacement:
neither diapirism nor neutral buoyancy 1
ROMAN-BERDIEL, T., ARANGUREN, A., CUEVAS, J., TUBIA, J. M., GAPAIS, D. &
BRUN, J.-P. Experiments on granite intrusion in transtension 21
MERLE, O. & DONNADIEU, F. Indentation of volcanic edifices by the
ascending magma 43
ROSSETTI, F., FACCENNA, C., ACOCELLA, V., FUNICIELLO, R., JOLIVET, L.
& SALVINI, F. Pluton emplacement in the Northern Tyrrhenian area, Italy 55
RABINOWITZ, N. & MART, Y. Seismic tomography of the Dead Sea region:
thinned crust, anomalous velocities and possible magmatic diapirism 79
Salt intrusions
TALBOT, C. J., MEDVEDEV, S., ALAVI, M., SHAHRIVAR, H. & HEIDARI, E. Salt
extrusion at Kuh-e-Jahani, Iran, from June 1994 to November 1997 93
GAULLIER, V. MART, Y., BALLAICHE, G., MASCLE, J., VENDEVILLE, B.C.,
ZITTER, T. & SECOND LEG PRISMED II sciENTific PARTY. Salt tectonics
in and around the Nile deep-sea fan: insights from the PRISMED II cruise 111
STEPHANESCU, M., DICEA, O. & TARI, G. Influence of extension and
compression on salt diapirism in its type area, East Carphathian Bend area,
Romania 131
MIRALLES L., SANS, M., PUEYO, J. J. & SANTANACH, P. Recrystallization
salt fabric in a shear zone (Cardona diapir, southern Pyrenees, Spain) 149
Shale intrusions
KOPF, A. & BEHRMANN, J. H. Extrusion dynamics of mud volcanoes on the
Mediterranean Ridge accretionary complex 169
Index 205
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Preface
Despite their differences in size and scale, shale, salt and igneous intrusions display
many similarities. For example, at the time of emplacement, the material forming
all three types of intrusions (viscous or not) is always much weaker than the country
rocks. Units of halite, granitic magma, or overpressured shale cannot sustain large
deviatoric stresses without deforming, which makes them highly mobile rocks, pro-
vided there is space available into which the intrusive material can flow. All intrusions
pierce, or appear to pierce, surrounding country rocks having a wide variety of lithol-
ogies. All three types of intrusions have experienced much more internal strain and
vertical movement than the country rocks. All have risen, at least partly, in the
upper continental crust. Another similarity these three types of intrusion have is
that their formation has traditionally been attributed to the same geological process,
Rayleigh-Taylor instabilities. This is the process in which a weak, viscous, less-dense
material (the intrusion) can spontaneously rise through and deform denser country
rocks that are assumed to behave viscously and to have negligible yield strength.
The latter assumption has been challenged by rock-mechanics data and geological
observations. Apart from magmas, evaporites, and overpressured shales, most
rocks in the upper continental crust exhibit a rheological behaviour controlled by
elastic and frictional properties. The strength of the country rocks is typically consid-
erably higher than (1) the strength of the intrusive rocks and (2) the deviatoric stress
generated by small instabilities at the interface between the intrusion's source layer
and the overlying rocks.
The three types of intrusions also differ. Evaporitic intrusions are fed typically by a
laterally continuous layer, and the rheological properties of the intrusive material do
not change drastically throughout the time of emplacement. In contrast, the material
forming mud volcanoes and igneous intrusions does not originate from a continuous
layer but is fed by more locally restricted sources, where thermal and mineralogical
conditions make the source rocks weak and mobile. Both magma and overpressured
shale tend to gain strength during their ascent, as the fluid pressure and temperature
decrease.
Most recent literature suggests that researchers working in all three fields no longer
support the traditional Rayleigh-Taylor instability mechanism and now favour a
mechanism in which the weak, intrusive material rises through much stronger country
rocks. Typically some amount of tectonic deformation is required to overcome the
strength of the country rocks. For example, normal faulting caused by regional exten-
sion thins and weakens the country rocks and creates the accommodation space in
which intrusions can rise. Regional contraction or transtension squeezes the weak,
mobile intrusive material and thereby provides the additional pressure required for
the intrusion to lift and deform the overlying country rocks.
Most of the articles herein illustrate how emplacement of intrusions is associated
with regional or local tectonics. Vigneresse & Clement discuss arguments in favour
of various mechanisms of granitic magma emplacement and emphasizes the role of
regional tectonics in allowing igneous intrusions to rise. Roman-Berdiel et al. illus-
trate, by means of experimental models and field observations in the Spanish Variscan
belt, how transtensional tectonics can control the mode of emplacement of granitic
plutons. Merle & Donnadieu address the combined influence of thin-skinned
viii PREFACE
(gravity-driven) and thick-skinned (tectonic) deformation on the geometry and kine-
matics of faulting above magmatic intrusions. Rossetti et al. demonstrate how regio-
nal, post-orogenic extension has controlled the emplacement of monzogranitic
intrusions in the northern Tyrrhenian region. Rabinowitz & Mart present seismic
tomography data from the Dead Sea rift that suggest the presence of a magmatic
intrusion at depth caused by transtension. Talbot et al. provide new data on the
rates at which salt diapirs in the Zagros region rise and spread. Gaullier et al. illustrate
the different styles of salt structures in and around the Nile deep-sea fan, where
regional thick-skinned tectonics interact with thin-skinned, gravity-driven tectonics.
Stephanescu et al. present seismic-reflection data from Romania, the area where the
term diapir was first defined, and demonstrate how regional tectonics has triggered
and controlled salt-diapir rise and evolution. Miralles et al. describe the internal
geometry of evaporitic diapirs in the south Pyrenees. The last article, by Kopf &
Behrmann, shows that mud diapirism in the Mediterranean Ridge was a rapid but
episodic process related to tectonic-plate convergence.
We hope that the several examples provided in this publication will help future
studies focus on the role of regional tectonics in controlling intrusion emplacement,
a process that was previously thought to depend solely on the internal properties of
source and country rocks. We also hope that the many similar characteristics of var-
ious types of intrusions illustrated herein will provide an impetus for further, multi-
disciplinary approaches in all three fields.
The content of this Special Publication was derived from a symposium entitled
'From the Arctic to the Mediterranean: Salt, Shale and Igneous Diapirs in and
around Europe'. The symposium was convened in 1998 by Yossi Mart and Bruno
Vendeville at the 23rd General Assembly of the European Geophysical Society in
Nice, France.
We would like to thank Angharad Hills and Bob Holdsworth for their help in
organizing this book and for their patience. We also thank A. W. Bally, K. Benn,
B. C. Burchfield, J. P. Burg, A. Castro, A. Cruden, G. Eisenstadt, D. Grujic, M. P.
A. Jackson, F. Kockel, R. Nelson, K. T. Nilsen, F. Odonne, M. G. Rowan, M. de
Saint-Blanquat, H. Schmeling, D. D. Schultz-Ela, W. M. Schwerdtner, J. Simmons,
R. Weijermars, and other anonymous reviewers for their careful reading of the manu-
scripts.
Bruno Vendeville
Yossi Mart
Jean-Louis Vigneresse
Granitic magma ascent and emplacement:
neither diapirism nor neutral buoyancy
J. L. VIGNERESSE1 & J. D. CLEMENS2
1CREGU, UMR CNRS 7566 G2R, BP 23, 54501 Vandoeuvre
Cedex, France (e-mail [email protected]
2School of Geological Sciences, CEESR, Kingston University,
Penrhyn Road, Kingston-upon-Thames, KT1 2EE, UK
Abstract: It is probable that granitic magma ascent does not result from the intrin-
sic properties of the magmas. Within the uppermost crust, neither the reduced
viscosity nor the density contrast between magma and surroundings are them-
selves sufficient to induce either low-inertia flow (diapirism) or fracture-induced
magma propagation (dyking). Igneous diapirism is intrinsically restricted to the
lower, ductile crust. Dyking is therefore the most probable ascent mechanism
for granitic magmas that reach shallow crustal levels. A neutral buoyancy level
in the crust, at which magma ascent should stall, is never observed. This is demon-
strated by coeval emplacement of magmas with different compositions and densi-
ties, and the negative gravity anomalies measured over many granitic plutons. We
suggest that deformation, through strain partitioning, is necessary to magma
ascent. Pluton formation is controlled by local structures and rock types rather
than by intrinsic magma properties. As a result of its intermittent character,
deformation (both local and regional) induces magma pulses, and this may have
important consequences for the chemical homogeneity of intruded magmas.
Ascent of granitic (leucogranitic to tonalitic) magma in the continental lithosphere is
the most potent means of mass transfer and chemical segregation between the lower
and upper crust. Granitic magmatism contributes to crustal recycling and, to a lesser
extent, continental growth. In rapidly accreted crust, granitic rocks can occupy up to
one-third of the volume of the middle crust (Meissner 1986). Together with shear
zones, they are the most conspicuous effects of plate convergence and collision in
orogenic zones. Consequently, they are a key element in understanding the evolution
of Earth's mechanical workings.
As most felsic magmas are emplaced in a liquid, or near-liquid state, at temperatures
over 800°C, a major heat source must be involved in their genesis. The existence of such
a heat source also has consequences for the deformation regime of the crust. Com-
monly, a large temperature rise in the lithosphere will also increase the capacity of
crustal material to respond to stress (by deformation). Such a thermal anomaly will
contribute to the dissipation of energy and thus buffer the temperature effects. The
stress regime will be imposed externally by the tectonic processes operating at the
time. As a reaction to a significant increase in crustal heat flow, the rheological capacity
of rocks to sustain stresses decreases, and this activates deformation. Therefore, most
departures from a 'normal' heat distribution in the lithosphere will trigger deformation
(e.g. formation of extensional rift zones, or shear zones in convergent situations).
From: VENDEVILLE, B., MART, Y. & VIGNERESSE, J.-L. (eds) Salt, Shale and Igneous Diapirs in and around
Europe. Geological Society, London, Special Publications, 174, 1-19. 1-86239-066-5/00/S15.00
© The Geological Society of London 2000.