Problems and Methods for Lithospheric Exploration

222 Pages·1984·7.579 MB·English

by  Roberto Cassinis (auth.), Roberto Cassinis (eds.)

Problems and Methods
for Lithospheric
Exploration
ETTORE MAJORANA INTERNATIONAL SCIENCE SERIES
Series Editor:
Antonino Zichichi
European Physical Society
Geneva, Switzerland
(PHYSICAL SCIENCES)
Recent volumes in the series:
Volume 9 ENERGY DEMAND AND EFFICIENT USE
Edited by Fernando Amman and Richard Wilson
Volume 10 INTERACTING BOSE-FERMI SYSTEMS IN NUCLEI
Edited by F. lac hello
THE SOLUTION OF THE INVERSE PROBLEM
IN GEOPHYSICAL INTERPRETATION
Edited by R. Cassinis
Volume 12 NEUTRINO PHYSICS AND ASTROPHYSICS
Edited by EUore Fiorini
Volume 13 UNCONVENTIONAL APPROACHES TO FUSION
Edited by B. Brunelli and G. G. Leotta
Volume 14 MINIATURIZATION OF HIGH-ENERGY PHYSICS
DETECTORS
Edited by A. Stefanini
Volume 15 UNIFICATION OF THE FUNDAMENTAL
PARTICLE INTERACTIONS II
Edited by John Ellis and Sergio Ferrara
Volume 16 THE SEARCH FOR CHARM, BEAUTY,
AND TRUTH AT HIGH ENERGIES
Edited by G. Bellini and S. C. C. Ting
Volume 17 PHYSICS AT LEAR WITH LOW-ENERGY
COOLED ANTIPROTONS
Edited by Ugo Gastaldi and Robert Klapisch
Volume 18 FREE ELECTRON LASERS
Edited by S. Martellucci and Arthur N. Chester
Volume 19 PROBLEMS AND METHODS FOR LITHOSPHERIC
EXPLORATIONS
Edited by Roberto Cassinis
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Problems and Methods
for Uthospheric
Exploration
Edited by
Roberto Cassinis
University of Milan
Milan, Italy
Plenum Press • New York and London
Library of Congress Cataloging in Publication Data
Main entry under title:
Problems and methods for lithospheric exploration.
(Ettore Majorana international science series; v. 19)
"Proceedings of the fourth course of the School of Applied Geophysics on the in
tegration of geophysical and geological data, held March 16, 1982, at the Ettore
Majorana Center, in Erice, Sicily, ltaly"-T.p. verso.
Includes bibliographical references and index.
1. Geology, Structurai-Congresses. 2. Geophysics-Congresses. 3. Seismology
Congresses. I. Cassinis, R. (Roberto) II. International School of Applied Geophysics.
III. Series.
QE601.P715 1984 551.8. 84-9817
ISBN-13: 978-1-4612-9451-1 e-ISBN-13: 978-1-4613-2373-0
DOl: 10.1007/978-1-4613-2373-0
Proceedings of the Fourth Course of the School of Applied Geophysics on the
Integration of Geophysical and Geological Data, held March 16, 1982, at the Ettore
Majorana Center, in Erice, Sicily, Italy
© 1984 Plenum Press, New York
Softcover reprint of the hardcover 1s t edition 1984
A Division of Plenum Publishing Corporation
233 Spring Street, New York, N.Y. 10013
All rights reserved
No part of this book may be reproduced, stored in a retrieval system, or transmitted
in any form or by any means, electronic, mechanical, photocopying, microfilming,
recording, or otherwise, without written permission from the Publisher
PREFACE
This volume contains several contributions among those held at the
fourth Course of the International School of Applied Geophysics,
Erice, March 15-24, 1982.
The content has been arranged according to the three main topics
covered during the course: the goedynamic models, the contribution
of geophysics to their construction and the optimization and
constrains of the exploration methods.
At the end of the volume, four short notes are published, written
by participants to the course and related to the general subject.
It is regrettable that not all the lectures and discussions are
available for publication in this volume. However, it is hoped that
the papers presented here will stimulate further discussions and
suggest new topics for future meetings.
R.Cassinis
v
CONTENTS
1
Introductory note
R Cassinis
Global seismology and the investigation of deep
5
continental structure
G Nolet
The relation between superficial and deep structure of
23
the Caucasus region and possible geodynamic model
V N Sholpo
Seismicity and crustal structure in the Italian region:
53
a preliminary zoning
R Cassinis
93
Structure seismology
I P Kosminskaya
Main features of crustal structures in mediterranean
113
collisional zones
P Giese
Potentia~ and constraints of near vertical reflection
133
seismics in non-sedimentary deep formations
K Helbig and J Schmoll
Geoelectrical deep explorations by means of direct
currents
L Alfano
Reflection profiling of the continental crust 179
R Phinney
Evidence for a strong crust-mantle boundary undulation
in the TYrrhenian sea from a refraction seismic
survey 183
C B!)ttcher
vii
A bending model for the Calabrian arc 191
G Gaudiosi, G Luongo, G P Ricciardi
A method of hypocentral location by means of an earthquake's
first motion 201
M Gasperini and B Alessandrini
Seismic wave attenuation in southern Italy crust: scattering
from random heterogeneities mechanism or intrinsic
Q variations with the depth? 207
A Rovelli
INDEX 213
viii
INTRODUCTORY NOTE
Roberto Cassinis
Institute of Geophysics
University of Milan
Via L. Cicognara 7, 20129 Milano
The basic discoveries made in the sixties by geophysical methods
gave origin to the hypothesis of plate tectonics. Then, the increased
intensity of the surveys revealed details that could not be explained
by the general scheme. This is especially true for the continental
lithosphere, where the investigation has proved to be more difficult
than in oceanic areas, owing mainly to the occurrence of stronger
lateral variations. This type of development is common to all
branches of science.
Geology also builds up its models on schematic general hypoth
eses; when new data seem to contradict the model, they are considered,
at least during a first phase, as "disturbing factors" and every ef
fort is made to accommodate the evidence to the scheme instead of
using it as proof against the hypothesis. The abundance of new in
formation gathered has now started a second phase of knowledge: the
evidence brought by independent information must be used to correct
the model and to discard the over-generalized assumptions.
The improvement of geophysical methods, especially during the
last decade, has revealed subtleties that may not have been appreci
ated in the previous research. Even more fundamental progress has
been the use of integrated data by structural geologists having a
deep knowledge both of the potential and of the constraints of the
geophysical methods as well as of the other tools helping the synoptic
evaluation of an area, e.g., the images taken from the orbiting Earth
satellites. On the other hand one must be aware of the limitations
of the exploration methods and try to optimize their use by the best
possible statement on the objectives.
The IV Course of the School of Applied Geophysics at the
Majorana Center was originally intended to deal with the particular
models of "collisional" zones as well as with the problems involved
in their exploration. During the Course, discussions arose on the
term "collisional" itself; moreover, the subjects treated showed
that the problems of the exploration of the lithosphere must be con
sidered on a global basis, the comparison between the types of crustal
and upper mantle structure being essential for a correct application
of geophysical methods. For this reason, the title of this volume
has been modified.
Of course, the models discussed and the results especially deal
with transitional zones that sometimes can be defined as "collisional".
These areas, the unstable or mobile belts of the world, are those
where the internal life of our Planet is most clearly shown. The
importance to investigate these areas and to understand their dynamic
processes has been stressed recently by the evidence that goals of
immense economic value are involved both in a positive and in a nega
tive sense (like the survey for new mineral resources or the miti
gation of earthquake hazards).
Three fundamental topics have been treated in the lectures:
geodynamic models, geophysical data as well as the iterative process
among them; and, last but not least, the available methods for the
exploration, their constraints, their optimization and their future
developments.
GEODYNAMIC MODELS AND GEOPHYSICAL RESULTS
The Alpine orogenesys and the Mediterranean region have been
the preferred topics (Giese, Sholpo, Wezel). The comparison between
the Caucasus and the Alps seems particularly stimulating. The Medi
terranean is one of the regions of the world where the schematism of
the plate tectonics finds more difficulties to fit the complexity of
the structure. Probably, a deeper knowledge of the present structure
of this area will give a major input to the necessary refinement of
the "global tectonics".
The transition from the oceanic to the marginal lithosphere and
then to the continental lithosphere as well as the accompanying
lateral variations of the asthenosphere have been described by
Kosmiskaya and Nolet and illustrated by the interpretation of long
lithospheric profiles (active and passive seismology). The results
do not fully comply with the essential postulate of global tectonics,
viz the spreading of the oceanic floor as the only mechanism respon
sible for the evolution and for the horizontal movement of continental
masses.
2
Cassinis, Giese and Eva presented studies on particular problems.
The seismogenesis in Italy is complicated by the complexity of the
geological history and by the consequent change of stress direction
at different levels in the crust. Integration of methods is compul
sory while dealing with this type of survey as well as for the in
terpretation of the geothermal anomaly in Tuscany, the largest in
Europe, where resistivity and velocity measurements help to make a
choice between the possible thermal models. Seismicity of a particu
lar portion of the Indonesian arc has been investigated, showing that
even the typical island arcs are divided into compartments of dif
ferent geophysical character by transverse shifts.
GEOPHYSICAL METHODS
Seismic exploration has been the method most extensively treated.
The lectures on this subject have been started by a review paper by
Helbig and Schmoll who examined the potential and the constraints of
near vertical reflection seismics, that, at least theoretically, is
the only method able to reveal the fine structure of the crust.
Phinney continued on this topic describing the field and processing
methods employed by COCORP and showing some of the results obtained
along the continental profiles. A particular and difficult problem
due to the type of the recorded events, is migration. The conclusion
is that the features which are most easily seen are layered in some
sense (deep sedimentary basins, layered crystalline complexes, low
angle thrust faults). The significance of the signals coming from
a depth compatible with the "M" transition is still debated.
Kosmiskaya, Scarascia and Giese discussed the potential and
constraints of the more widely used technique for deep exploration,
the wide or critical angle refraction, also named "deep seismic
soundings". Especially in mobile, orogenic belts, problems arise
on the reliability of data: refinements of field and interpretation
techniques are needed. In spite of their lower resolution and their
sensitiveness to lateral variations, the deep seismic soundings still
are the only means to obtain the velocity distribution beneath the
sedimentary cover.
Nolet described the advanced tools of passive seismology, using
both surface and body waves of teleseismic events, to obtain a de
tailed upper mantle velocity model to a depth of 700 km under a
region of the size of western Europe.
Alfano dealt with the electromagnetic methods. Active geo
electrical methods, using suitable techniques (dipole-dipole) are
preferable to study the resistivity distribution in the upper crust
to about 10-15 km, especially where a complicated structure exists.
For larger depths natural fields must be measured. The factors
limiting the reliability of the latter type of investigation are
3