Journal of Conference Abstracts

Session G01:3A

G01 : 3A/09 : G3

New Constraints on Subduction and Collision Processes in the Central Andes from Integration of the P-To-S Converted Phases and Deep Reflection Profiling

S. V. Sobolev (stephan@gfz-potsdam.de)¹,

X. Yuan (yoan@gfz-potsdam.de)¹,

R. Kind (kind@gfz-potsdam.de)¹,

G. Zandt (zandt@geo.arizona.edu)²,

O. Oncken (oncken@gfz-potsdam.de)¹,

G. Bock (bock@gfz-potsdam.de)¹ &

G. Asch (asch@gfz-potsdam.de)¹

¹ GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473, Germany

² University of Arizona,Tucson, Arizona 85721-0077, USA

The Central Andes represent a unique combination of active subduction and plateau formation processes which were addressed by several geophysical and geological surveys during the last decade. In this study we integrate available observations on P-to-S converted waves, vertical incidence reflections and seismic tomography to obtain additional constraints on these processes. The key observations employed are teleseismic P-to-S converted waves from seven seismological networks with a total of 225 three component stations that were temporarily operated in the Central Andes between 1994 and 1997. Another dataset is the vertical incidence reflection survey ANCORP96 which imaged a strong reflector dipping parallel to the Wadati-Benioff zone at depths of 40-80 km (Nazca reflector) under the Central Andean forearc [ANCORP working Group, Nature in press].

Several strong P-to-S converting boundaries can be traced over hundreds of kilometers. The 660 km seismic discontinuity is dipping by 30-40 km in the high velocity mantle domain (subducting Nazca plate) identified by P-wave tomography [Engdahl et al., 1995, GRL, 22, 2317-2320]. Both, subsidence of the 660 km phase boundary and high seismic velocity, can be explained by 400-500 K temperature decrease in the Nazca slab relative to the surrounding mantle. A strong positive converting boundary dipping parallel to the W-B zone can be traced from the trench down to about 120 km. It is followed by a negative converted phase. Corresponding wave forms can be explained by a 5-10 km thick dipping layer which has 15% S-velocity deficit, i. e. by subducting oceanic crust not entirely converted to eclogite down to a depth of 120 km. Most of the intermediate depth earthquakes are located within this layer with notable exceptions when earthquake clusters are extended by 20 km below it. The Nazca reflector is located just above the oceanic crust imaged by long-period converted waves but it is not visible by long-period converted waves itself. In contrast, the continental Moho is clearly imaged at 40 to 70 km depth by converted waves but has almost no (or limited) expression in short-period reflected waves. Converted waves image a more than 350 km long intracrustal boundary which is dipping to the West across the entire Altiplano plateau from 20 km depth below the Eastern Cordillera to 40 km depth below the Western Cordillera and Precordillera. This boundary is interpreted as evidence of the underthrusting of the Brazilian shield crust, reaching substantially further west than previously thought and thereby explaining formation of the thick crust under the Altiplano plateau entirely by crustal stacking.

G01 : 3A/10 : G3

Insights on Lithospheric Structure of the Central Andean Subduction Zone from Seismological and Magnetotelluric Investigations

Stefan Lueth (stefan@geophysik.fu-berlin.de)¹,

Heinrich Brasse

(hbrasse@geophysik.fu-berlin.de),

Peter Giese (giese@geophysik.fu-berlin.de),

Christian Haberland

(haber@geophysik.fu-berlin.de),

Ewald Lueschen (lueschen@bavaria.geophysik.uni-muenchen.de)² &

Andreas Rietbrock (andreas@egmdpri01.dpri.kyoto-u.ac.jp)³

¹ FU Berlin, FR Geophysik, Malteserstr. 74-100, Hs. D, Berlin, Germany

² Institut fuer Allgemeine und Angewandte Geophysik, U Muenchen, Theresienstr. 41, D-80333 Muenchen, Germany

³ Desaster Prevention Research Institute, Kyoto, Japan

A variety of active and passive seismological investigations has been executed in the Central Andes in the past two decades, most of them by the Collaborative Research Center "Deformation Processes in the Andes" (SFB 267). In addition, other geophysical methods (MT, gravity, thermal heat flow) were applied, completing a multidisciplinary dataset containing constraints on the physical state of the lithosphere which one method alone cannot realize.

Attention will be focussed on a transsect at 21°S, where, mainly based on multiple coverage seismic data, densest information is available. Within the continental crust of the forearc region, constrained by modelling of wide-angle-reflections, vertical incidence reflections with anomalously high amplitudes may give hints on subduction-related processes. Their high amplitude probably point to high fluid concentrations near the subducting oceanic crust and in the middle crust of the Precordillera. An offset of at least 10 km between the main reflector from the CDP-data and the main cluster of earthquake locations derived from a passive seismological network seems to indicate different factors causing reflectivity and seismicity.

Going farther east, from the vulcanic arc up to the eastern margin of the Altiplano, seismic information grows poorer. Wide-angle measurements only resolve upper portions of the crust, reflection data show rather diffuse patterns. However, here passive seismology (Q-tomography) and inversion of magnetotelluric data reveal an anomalously attenuative (Q less than 200) and conductive (<rho> less than 1 Ohm m) lower crust. Obviously, whereas crustal thickness of the Eastern Cordillera (65 to 70 km after refraction seismic data) can be explained by underthrusting of the brasilian shield under the andean crust, under the Altiplano existence of near solidus state material with atypical properties must be assumed.

G01 : 3A/11 : G3

Fluids in the Lithosphere beneath the Western Andean Crust

Frank R. Schilling (fsch@gfz-potsdam.de)¹ &

Peter Giese (giese@geophysik.fu-berlin.de)²

¹ GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany

² Freie Universität Berlin, Malteserstraße 77-100, D-12249 Berlin, Germany

Considering geophysical, petrologic-geochemical observations and petrophysical estimations the crustal structure indicate the existence of fluids and melts within the lithosphere of the southern Central Andes (21-23^oS). A pronounced variation in crustal parameters is observed over the entire Andean orogen. The orogen is characterized by a crustal thickness of up to 70 km under the magmatic arc and backarc, strongly reduced seismic velocities (< 6.3 km/s), high electrical conductivity (up to 1 S/m), elevations up to 6000 m, high heat flow values (> 100 mW/m²) and a negative Bouguer anomaly of -450 mGal. High v_P/v_S-ratio and low Q-values (< 200) are observed in 3D tomographic sections beneath the Western Cordillera. Model calculations indicate that 15 - 20 vol.% of melt at depth below 20 km are necessary to explain the observed geophysical behavior. High heat flow values support the idea that large areas of the deeper Andean crust are strongly weakened by the presence of partially molten rocks. Thus the Western Cordillera, the active volcanic arc of the Andean mountain range act as ductile buffer between the two more rigid crustal blocks of the Forearc and Backarc regions. High v_P/v_S-ratios (approx. 1.85) are observed beneath the Forearc and are attributed to a hydrated mantle wedge. Low temperatures due to the cold descending Nazca-plate and rising fluids lead to free fluids and Serpentinite/Amphibolite in the Forearc. Fluids are released from the down going Nazca-plate from pores and through dehydration reactions. Geoelectric observations suggest the presence of free fluids due to high conductivity values, concentrated in deep reaching fault systems. Mineralogical observations support the suggested high amount of fluids through fluid related mineralizations (e.g. copper mines) and an ignimbritic volcanism. Geophysical monitoring and their petrophysical consequences, geodynamic interactions, petrologic and geological aspects will be outlined to differentiate between various fluid-rock interactions within the Andean crust.

G01 : 3A/12 : G3

The Continental Crust of the Central Andes (21°-26°S) ­ The Petrological-Geochemical Perspective

Gerhard Franz (Gerhard.Franz@tu-berlin.de)¹,

Friedrich Lucassen

(luca0938@mailszrz.zrz.tu-berlin.de) &

Robert Trumbull (bobby@gfz-potsdam.de)²

¹ Technische Universität Berlin, Sekr. EB 15, Germany

² GeoForschungsZentrum, Telegrafenberg, 14407 Potsdam, Germany

The lithospheric segment between 21° and 26° S on the Andean continental margin was studied by geophysical, petrological and geochemical methods, with emphasis on crustal evolution, the material properties resulting from this evolution, and the Recent processes revealed by geophysical measurements. This segment is an essentially homogeneous block; its basement was consolidated by an orogenic event (high-T, medium-P metamorphism, typical for upper to middle crustal level) mainly in the Early Paleozoic between 550 and 500 Ma ("Brasiliano Mobile Belt") with final exhumation at = 400 Ma. The bulk of this material has upper crustal geochemical-isotopic signatures, and the dominant process of formation is recycling of Proterozoic protoliths with crustal residence model ages (T DM Sm/Nd) of 1.6 to 2 Ga. Precambrian crustal consolidation model ages (Grenville) are restricted to north of the segment at Arequipa-Berenguela-Uyarani (S-Peru, Bolivia). The basement experienced another stage of intense crustal recycling without major juvenile additions during the Late Paleozoic (= 300 Ma granitic magmatism; 270 Ma high-T/high-P metamorphism; = 240 Ma exhumation). Significant crustal growth is first recorded in the Jurassic, documented by large amounts of mantle derived igneous crust which now constitutes the Chilean Coastal Cordillera. Crustal growth continued during the Cretaceous and Tertiary magmatic arc evolution, but modified only a small part of the continental crust in the Chilean Precordillera. The bulk of the crust must be quartz-feldspar dominated, and there are no indications for a mafic lower crust. This is confirmed by the isotopic-geochemical composition of large-volume Cenozoic ignimbrites, which appear to be largely crustal melts of the above-described basement. Though garnet can be present in the lower crust, where the appropriate P-T conditions are reached, the bulk composition of the basement (low Fe, Al) precludes the presence of large amounts of garnet. An eclogitic garnet-clinopyroxene dominated lower crust is improbable. Seismic, gravimetric and magnetotelluric studies reveal two different types of crust, to the east and west of the present magmatic arc. The geophysical data support the petrological-geochemical evidence.

G01 : 3A/13 : G3

Evidence for a Discontinuous Moho and Metamorphism of Continental Crust Subducted beneath the Southern Uralides Inferred from Wide-Angle Seismic Data

Frederic Masson

(fmasson@dstu.univ-montp2.fr)¹,

Friedemann Wenzel² &

Hakon Austrheim³

¹ CNRS - Universite Montpellier II, Laboratoire de Geophysique et Tectonique, Montpellier, France

² Geophysical Institute, University Karlsruhe, Germany

³ Mineralogisk-Geologisk Museum, Oslo, Norway

A reinterpretation of the wide-angle seismic data of the URSEIS experiment providesa new lithospheric model of the Southern Urals. This model confirms the well-known bivergent collisional orogen structure of the crust and the presence of a thick crustal root. In contrast to other models it shows the subduction of the lower crust of the East European Craton beneath the central axis of the orogen. In addition, it clarifies the structure of the Moho as first order discontinuity in the external parts of the orogen and a progressive transition from crustal to mantle velocities in the central part beneath the Magnitogorsk zone. We interpret the apparent discrepancy between dipping structures as seen in the refraction and near-verticalreflection data and the layer-cake velocity structure as result of a late- or post-orogenic metamorphic overprint of the deep crust.

G01 : 3A/14 : G3

Deep Structure and Evolution for the Pre-Caspian Basin

Sergey L. Kostyuchenko,

Dmitry L. Fedorow,

Leonid N. Solodilov,

Anatoly V. Egorkin &

Evgeny E. Zolotov

GEON Center, Moscow, Russia

Based on the results of deep seismic refraction and wide-angle reflection profiles, the depth of the Pre-Caspian basin reaches 20-22 km, and the crystalline crust thins to 14-16 km. The Moho depth in central zone of the basin is 32-36 km and deepens 42 km and more towards its edges.The Manash river-Karachaganak DSS profile, carried out by GEON Center in 1987, displays an extension of the crust that is observed over about 150-180 km. In marginal sectors of Pre-Caspian basin, the upper crust shows P wave velocities of 6.1-6.2 km/s and those of S wave of 3.4-3.5 km/s. Within the flanks of extension zone, the upper crust exhibits P wave velocities of 6.4-6.6 km/s and S velocities of 3.6-3.8 km/s. The P velocity of the middle crust increases from 6.5 km/s, in the flanks, to 6.8 km/s, in the central zone of the basin. In contrast to high velocity values of P wave (7.0-7.2 km/s) and S wave (3.9-4.25 km/s) in the lower crust of the edges, the velocities in the extension zone do not exceed 6.9 km/s and 3.95 km/s respectively. Composition of the crust inferred from Vp and Vs values and from ratios between P and S velocities shows that the crust of marginal sectors of the basin is continental: upper layers have acidic composition, middle crust is composed of unseparated felsic rocks, and the lower crust consists of intermediate and basic rocks. There is no evidence for an felsic crust exist in the central portion of the Pre-Caspian basin. That crystalline crust is oceanic (Kostyuchenko et al., 1998). There is a low velocity zone (8.2 km/s) in the upper mantle between 60 and 75 km of depth, and a mantle plume, presumably Riphean in age, may be proposed for the evolution of the Pre-Caspian region based on P wave velocity values of 8.6-8.7 km/s in the local upper mantle block between 45 and 60 km beneath the Aralsor rift (Egorkin et al., 1980). A tomographic cross section of the lithosphere along Manash river-Karachaganak profile shows that an active asthenosphere occur beneath the Pre-Caspian basin. The following main stages of extensional activity can be proposed: (1) a phase of Riphean rifting based on McKenzie (1978) model when the mantle diapir ascended, and (2) large-scale sliding apart of lithospheric plates in the Middle Devonian (Zonenshain et al., 1990) or in the Middle-Devonian-Early Carboniferous times (Lobkovsky et al., 1996) along the low velocity zone observed at depths of 60-75 km. The final rifting in the Pre-Caspian region approached the stage of the development of oceanic crust. The Middle Carboniferous-Triassic post-rift rapid subsidence events probably related to a thermal activity of the upper mantle.

Egorkin AV, Razinkova MI, In Monograph. Nauk. Mos, 90-95, (1980).

Kostyuchenko SL, Fedorov DL, Nedra Povolzhiya, 16, 6-10, (1998).

Lobkovsky LI, Cloeting S, Nikishin AM, et al, Tectonophys, 266, 251-285, (1996).

M^cKenzie D, P, Earth Planet. Sci. Lett, 40, 25-32, (1978).

Zonenschain LP, Kuzmin MI, Natapov L, M, Monograph. Nedr. Mosc, 1, 1-328, (1990).

Session G01:3B

G01 : 3B/25 : G3

Seismic and Related Results from LITHOPROBE'S SNORCLE Transect ­ 4 B.y. of Earth History in Northwestern Canada

Ron M. Clowes (clowes@lithoprobe.ubc.ca)

LITHOPROBE, Univ. of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada

LITHOPROBE's Slave NORthern Cordillera Lithospheric Evolution (SNORCLE) transect is a multidisciplinary investigation of the structure and evolution of continental lithosphere in northwestern Canada, one of the few regions on Earth where the rock record spans 90% of Earth history. From east to west, the transect crosses a series of orogens: the Archean (4.0-2.5 Ga) Slave Province, one of the principal cratonic elements of Laurentia; the Early Proterozoic (2.1-1.8 Ga) Wopmay Orogen, comprising a series of N-S oriented magmatic arcs and accreted terranes; and the northern Canadian Cordillera, which records a complex history of crustal extension (1.5-0.37 Ga) and crustal accretion (0.37 Ga- present). A 725 km long reflection survey, recorded across the Precambrian domains of the southwestern Northwest Territories, shows spectacular evidence for delamination structures at the crustal scale and, for the first time, at lithospheric scale (to 100 km depth). A wide-angle seismic survey, with profiles over all tectonic elements of the transect, provides crustal velocity information; the profile coincident with the reflection survey shows results consistent with geometry from the reflection image, including the mantle structure. In the Slave craton, teleseismic receiver function studies delineate equivalent crustal and upper mantle structures. Deep penetrating electromagnetic studies indicate a thick (~250 km) "electrical" lithosphere below the Slave craton with dramatic thinning (to ~150 km) below the Proterozoic domains to the west. Petrological, chronological and other geological studies provide important complementary information.

G01 : 3B/26 : G3

Seismic Images of a Late-Archaean Crustal Collision Zone

John Ludden (ludden@crpg.cnrs-nancy.fr)

CRPG-CNRS, Vandoeuvre-lès-Nancy, 54501, France

The LITHOPROBE project completed a comprehensive study along a corridor across southeastern Superior Province. The results provide a unique image of late-Archean crust. A suture-zone, imaged by seismic reflection, penetrates to mantle depths of 70 km or more and defines a zone along which the granite-greenstone belt override and underthrust a plutonic belt that can be traced across the Superior Province (Calvert et al., 1995). This suture can be traced as a lower crustal detachment below the greenstone belt. Correlation of the seismic images with an exposed crustal section exposed in the Kapuskasing uplift show that the crust is divided into three main units: upper crust comprising supracrustal assemblages which are largely allochthonous, mid-crust comprising tonalite gneiss and paragneiss, and a layered mafic lower crust. Metasedimentary series can be traced into imbricated structures underneath the supracrustal rocks, implying that these sequences were part of the accretionary process.

Accretion in the southern Superior province was rapid, and followed by a period of up to 300 Ma of conversion into continental crust. The lowermost crust units, in the plutonic belt north of the mantle suture-zone, show high reflectivity and deformational structures which were acquired in the during accretion at 2.7 to 2.65 Ga. In contrast, although less reflective, structures defined by reflectivity in lowermost crust of the greenstone belt overprint older structures indicating a younger, probable post-accretion age, which is supported by studies of crustal xenoliths (Moser and Heamann, 1997) that indicate a late thermal or magmatic overprint in the lower crust at about 2.4 Ga.

The Superior province preserves the largest proportion of crust accreted on Earth and the LITHOPROBE results support a model involving plate subduction and accretion of crust. In the late Archean, extensive melting and accretion of hot, immature, thick mafic crust may have resulted in conditions favourable for its preservation that have only occurred on rare occasions since this period.

Calvert, A, Sawyer, EW, Davis, WJ, & Ludden, JN, Nature, 375, 670-674, (1995).

Moser W & Heamann L, Contrib. Min. Petrol , 128, 164-175, (1997).

G01 : 3B/27 : G3

Special Seismic Images of the Lithosphere of the Western Trans-Hudson Orogen

Zoli Hajnal (zoltan.hajnal@sask.usask.ca)¹,

Balazs Nemeth (balazs.nemeth@sask.usask.ca)¹,

John Lewry (jlewry@eagle.wbm.ca)²,

Steve Lucas (slucas@cc2smtp.emr.ca)³,

Don White (white@cg.emr.ca)³ &

Ron Clowes (clowes@lithoprobe.ubc.ca)⁴

¹ Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK, Canada

² Department of Geology, University of Regina, Regina, SK, Canada

³ Geological Survey of Canada, 601 Booth, Street, Ottawa, ON., Canada

⁴ Department of Earth & Ocean Sciences, University of British Columbia, Vancouver, BC, Canada

Five spatially distributed, but closely linked, seismic reflections profiles and three large offset (~ 750 km) refraction surveys, investigated the internides of the Paleoproterozoic Trans-Hudson Orogen, as one of the transects of the Canadian National LITHOPROBE program. The integrated compilation of these data sets reveals a number of regionally comparable seismic images of the tectonic architecture of the collisional belt. There are comprehensive evidences for the intricate involvement of the lithosphere in the tectonic evolution of the area. Within the western part of the juvenile terrane the structurally disturbed Moho is always a strong reflector, characterized by distinct but simple wave forms. Under the craton, with the exception of Line S2B, the same mantle-crust contact, is recognized by weak, marginal reflections and associated complex seismic signatures. The amplitudes of seismic signals, throughout the lower crust, are high and marked by strong subparallel, laterally well traceable zones of reflection patterns. The exclusion is the southern part of Line S₂B where the comparable lower crustal signatures are altered by a gentle southeastern dip Within the upper crust, short intricate patterns of reflectivity suggest complex brittle deformation. Interconnected seismic lines, starting from a well defined Archean window allow tracing of the sole thrust (Pelican Thrust) of the accreted terrane through the western flank of the orogenic belt The regionally distinct nature of these seismically defined structures implies occurrence of variable collisional processes along a north-south strike of the of the western orogen. P-wave signatures outline significant velocity anomalies in the mantle beneath the central region of the orogenic belt. Position and horizontal extent of these anomalous zone suggest that they may represent a preserved mantle suture structure that juxtaposed two Archean cratons. The velocity variations extend from the base of the crust to great depth and were formed by alterations within the lithosphere during the later stages of the converges. Isotopic studies of diamond impurities, from recently discovered kimberlite cluster of the region, further support this hypothesis.

G01 : 3B/28 : G3

LITHOPROBE Investigations in the Paleoproterozoic Eastern Trans-Hudson Orogen, Canada: What Have we Learned About Lithospheric Architecture?

Don White (white@cg.nrcan.gc.ca)¹,

Stephen Lucas (slucas@gsc.nrcan.gc.ca)¹,

Alan Jones (jones@cg.nrcan.gc.ca)¹,

Zoltan Hajnal (zoltan.hajnal@sask.usask.ca)²,

Herman Zwanzig³,

Balazs Nemeth (balazs.nemeth@usask.ca)² &

Wouter Bleeker¹

¹ Geological Survey of Canada, 615 Booth St., Ottawa, Ont., K1A 0E9, CANADA

² Dept. of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 0W0, CANADA

³ Manitoba Energy and Mines, 1395 Ellice Ave., Winnipeg, Manitoba, R3G 3P2, CANADA

The eastern Trans-Hudson Orogen records 200 m.y. (1.91-1.72 Ga) of Paleoproterozoic lithospheric assembly and cratonization in which a collage of juvenile intra-oceanic terranes (LaRonge-Lynn Lake arc, Kisseynew marginal basin and Flin Flon accretionary collage) weretrapped and amalgamated during collision of converging Archean cratons (Hearne, Superior and Sask cratons). LITHOPROBE multidisciplinary studies focused on this region from 1991-1996 and included a network of regional seismic reflection, seismic refraction and deep sounding magnetotelluric profiles which have provided a framework for understanding the regional lithospheric architecture resulting from this collisional process.

Key results from the interpretation of the geophysical data address both the internal structure of the juvenile collage (Reindeer Zone) and the structure along the boundary of the adjacent Archean (Superior) craton. Contraction of the intraoceanic domain (Reindeer Zone) led to terminal collision with the Hearne (ca. 1.85 Ga) and Sask (ca. 1.83 Ga) cratons prompting collapse and inversion of the marginal basin (Kisseynew domain) separating the Flin Flon and La Ronge-Lynn Lake arcs and production of the thickest crust (up to 17 s or > 50 km) preserved within the orogen. Collapse of the basin occurred asymmetrically with thrusting and recumbent folding of Kisseynew rocks over the Flin Flon arc collage and underthrusting beneath the La Ronge-Lynn Lake arc rocks where a crustal suture marks the end result of northward subduction of oceanic crust. Terminal collision of the Reindeer Zone with Superior craton along the eastern margin of the orogen at ca. 1.82 Ga marked a transition in the nature of the Superior boundaryzone from a lower plate collisional thrust belt setting at 1.88-1.82 Ga, through lithospheric delamination at 1.82-1.80 Ga to a steep transpressive plate boundary at 1.80-1.72 Ga. A longstrike-variations of this boundary zone were controlled by the pre-collisional geometry of the Superior craton margin. The older (> 1.82 Ga) east-verging fold and thrust belt is preserved within a reentrant, whereas in the vicinity of a major promontory deeper crustal levels are exposed and are dominated by younger west-verging transpressional structures.

G01 : 3B/29 : G3

Mantle Microgeodynamics

Geoffrey Lloyd (g.lloyd@earth.leeds.ac.uk)

School of Earth Sciences, The University, Leeds LS2 9JT, UK

Traditional structural geological studies of crustal deformation have long recognised the link between microscopic and macroscopic processes. It is also well known that crustal deformation is usually partitioned between zones of high and low strain accommodation. Although the scale of mantle processes is potentially larger than in the crust, the relationship between microscopic and macroscopic behaviours and the tendency towards strain localisation are likely to be maintained. It is essential therefore to integrate observations at all scales into a composite view of mantle geodynamics. However, whereas it has become relatively easy to investigate macrogeodynamic processes via various seismic monitoring techniques, microgeodynamic observations ideally rely on the availability of the real material. Unfortunately, sampling efficiency tends to decrease with depth, such that many microgeodynamical investigations depend upon experimental investigations of (uncharacteristic?) synthetic or analogue samples and/or various theoretical modelling concepts or assumptions. Nevertheless, specimens that have sampled progressively deeper levels of the mantle are now being recognised. This contribution considers some recent developments in microgeodynamic analysis and assesses their implications for macrogeodynamic processes that impact on both the lithosphere and biosphere.

G01 : 3B/30 : G3

Crustal Composition beneath the Dabie Mountains (China): Evidence from Laboratory Seismic Measurements on Exposed UHP Rocks

Hartmut Kern (kern@min-uni-kiel.de)¹,

Shan Gao (gaoshan@dns.cug.edu.cn)²,

Zhengmin Jin,

Till Popp (tp@min.uni-kiel.de)¹ &

Shuyan Jin²

¹ Inst. für Geowissenschaften, Olshausenstr. 40, 24098 Kiel, Germany

² Faculty of Earth Sciences, China University of Geosciences, 430074 Wuhan, P.R. China

The Dabie-Sulu ultra-high pressure (UHP) metamorphic belt of Central China represents a zone in which upper and lower continental crust has been subducted and then exhumed rapidly from mantle depth back to crustal levels. Recently, the Dabie-Sulu complex has been chosen as the preferential site for the first well of the CCDP, under the framework of the International Continental Drilling Program (ICDP). Knowledge of the in-situ physical properties of the deeply derived rocks is critical for the interpretation of the lithospheric structure and composition. About 30 UHP rock samples representing major lithologies were collected from surface exposures. Rock physical properties were measured in the laboratory at pressures up to 600 MPa (room temperature) and temperatures up to 600°C (600 MPa confining pressure), including compressional (Vp) and shear wave velocities (Vs), velocity anisotropy (shear wave splitting), density, and intrinsic pressure and temperature derivatives of Vp and Vs. Velocities generally increase with increasing densities: they are highest in the eclogites and lowest in the quartzite, granites and felsic granulites. The experimental results clearly demonstrate that mineralogy, which is a function of both chemical composition and metamorphic grade, is the most significant parameter influencing intrinsic wave velocities. On the basis of a regional geotherm, the in-situ velocities (Vp and Vp/Vs) were calculated for the different lithologies and compared with seismic data. The seismic velocity profiles reveal a four-layer structure (upper, middle, upper lower and lowermost crust) with an average thickness of 34 km. From the experimental results we infer that a mixture of about 90% felsic gneiss with variable amounts of high Vp amphi-bolite/gabbro constitute the middle crust. Intermediate granulite and mafic granulite fit both Vp and Poisson's ratio of the upper lower and lowermost crust, respectively. The combined P-wave velocities and Poisson's ratios suggest that eclogite is not a volumetrically important constituent of the present-day deep crust in the Dabie Mountains. The low average crustal thickness of about 34 km in the Dabie Sulu belt and the relatively low velocities of the total lower crust compared to global averages, as well the observed negative Europium anomaly (Gao et al., 1998) give evidence for delamination of large proportions of the mafic (eclogitic) lower crust.

Gao S, Zhang BR, Jin ZM, Kern H, Luo TC & Zhao ZD, EPSL, 161, 101-117, (1998).

G01 : 3B/33 : G3

High Resolution Deep Seismic Images of the Baltic Shield and its Southwestern Margin and Tectonic Interpretations

Niels Balling (geofnba@aau.dk)

Geophysical Laboratory, University of Aarhus, Finlandsgade 6-8, DK-8200 Aarhus N., Denmark

Over the past 10 years high resolution deep seismic reflection profiling has beeen carried out across principal tectonic units of the Baltic Shield and its southwestern margin. Projects such as BABEL in the Gulf of Bothnia and the Baltic Sea, MOBIL SEARCH in the Skagerrak and MONA LISA in the North Sea have provided seismic images of unique resolution of structural details at crustal and locally even lithospheric depth scale. Seismic observations and integrated geophysical and geological interpretations are presented from tectonic key areas.

From the Gulf of Bothnia, reflectivity structures in the crust and uppermost mantle resolved to a depth of 70-80 km are interpreted as representing a 1.9 Ga fossil subduction and continent-arc collision structure. Similar features are observed in the Southern Baltic Sea indicating the existence of a 1.8 - 1.7 Ga old active margin and convergence structure suggested to form the southeastern part of the Transscandinavian Igneous Belt. In the Skagerrak, 1.1 Ga old Sveconorwegian continental collision structures can be traced from present day surface to the uppermost mantle at 50 km depth. Along the southwestern margin of Baltica in the North Sea, dipping seismic reflectors in the crust and mantle lithosphere observed to a depth of 90 km, close to the bottom of the lithosphere, are interpreted in terms of a Caledonian (ca. 430 Ma) subduction and suture zone originating from the closure of the Tornquist Sea between Baltica and Avalonia. In this area seismic structures are observed which reflect subsequent Late Palaeozoic and Mesozoic crustal attenuation and extension resulting in formation of deep basins. Results are shown which significantly contribute to our understanding of the nature and age of the crust-mantle interface and structures within the mantle lithosphere.

G01 : 3B/34 : G3

Deep Lithosphere Differences Across the Trans-European Suture Zone

Soren Gregersen (srg@kms.dk) &

The Tor Working Group

KMS, Rentemestervej 8, DK-2400 Copenhagen NV, Denmark

Very significant deep lithosphere differences can be distinguished in the data of the Tor project, of the largest seismic antenna ever in Europe. These large differences are observed in preliminary interpretations of the teleseismic tomography project in Germany, Denmark and Sweden. The Tor project has a horizontal resolution of 20-30 km compared to more than 100 km in previous studies. The investigation includes P-wave teleseismic travel-time tomography plus S-wave tomography, receiver functions, anisotropy and many inversion methods. The Tor line goes along a well studied crustal profile of an earlier project, so that the sediments and crustal structure are assumed known, and the inversion efforts are concentrated on the deep lithosphere and asthenosphere differences to depths around 300 km. The investigations can be called two-and-a-half dimensional, being a 900 km profile with 100 km width plus a few seismographs off the profile. The field work lasted nearly a year in 1996-1997. Travel time anomalies of many data examples have been plotted in maps. These maps have been compared with computed travel time maps based on the best existing 3D model of the crust and upper mantle in the area. We see clear differences from one end of Tor to the other, but the tomographic inversions are still in preparation. We can distinguish, that parts of the travel time differences are caused by deep lithosphere differences, accounting for one-half-to-one second of travel time differences, and that this transition is gradual.

G01 : 3B/35 : G3

Crustal Structure of the Svecofennian Orogen - Images of the BABEL Profiles

Pekka J. Heikkinen (Pekka.Heikkinen@seismo.helsinki.fi) &

Annakaisa Korja (Annakaisa.Korja@seismo.helsink.fi)

Institute of Seismology, P.O.Box 26, FIN-00014 University of Helsinki, Finland

The BABEL reflection profiles in the Bothnian Sea and Bothnian Bay reveal the tectonic history of the Palaeoproterozoic Svecofennian orogeny. The Svecofennian orogeny took place, when the continental Karelian plate in the east collided with a newly formed Central Finland island arc plate in the west. Later Southern Finland Island Arc Complex amalgamated to this newly formed continent from the south. In the Subjotnian, 200 million years later, an aborted rift was formed within the Southern Finland Island Arc Complex. BABEL line 1 images the architecture of the Southern Finland Arc Complex. When combined with geological, gravimetric and seismic wide angle data, the varying reflectivity patterns can be understood in relation with the major processes forming the Svecofennian crust in the region. The unusually thick crust (55-60 km) is cored by unreflective, high density gabbroic intrusions (200 km x 50 km x 20 km), interpreted as a magmatic core of a primitive island arc complex. A highly reflective ramp anticline structure developed against the island arc in the south. The crust was thickened via a sequence of ramp anticlines comprised of the Southern Svecofennian migmatite, schist and volcanic belts. Mantle plume magmatism in the Subjotnian resulted in extensional environment that was expressed by crustal thinning via reflective listric shear zones, that most probably are inverted thrust faults, and via unreflective rapakivi granite magmatism. Crustal underplating and intraplating are seen as highly reflective, high velocity lower crust underlain by a 5 km thick, unreflective lower crust, which in turn is cut by a new sharply reflective Moho. BABEL lines 3&4 image reflective NE-dipping suture of the Southern Finland Island arc complex against the Central Finland Arc Complex and the collisional structures of the deformed supracrustal rocks. Ladoga- Bothnian Bay transform fault zone that developed at the Svecofennian Karelian boundary zone is imaged as a 30 km wide, subvertical unreflective band cutting through the crust.

G01 : 3B/36 : G3

Kola Super Deep Borehole (KSDB-3): Elastic Anisotropy of Crystalline Rocks in the Middle Crust

Felix Gorbatsevich (gorichs@gorichs.murmansk.su)¹ &

Yuri Smirnov²

¹ Geological Institute kola Science Centre RAS, 14, Fersman Str., Apatity, Murmansk region, 184200, Russia

² Scientific-Industrial Centre Kola Super-Deep Well, 17, Yubileinaya Str., Murmansk region, 184415, Russia

According to geophysical investigation results (DSS, VSP etc.) the surroundings of the KSDB-3 - the Pechenga Block of the Baltic Shield - are characterised by a four-layered crust, the actual total thickness of which is about 40 km. The main borehole (and several complementary ones) intersected the entire sedimentary-volcanic sequence of the Lower Proterozoic Pechenga Formation (0-6,842 m) and a considerable part (6,842-12,261 m) of the Archean granitic-metamorphic complex of the basement. A study of elastic anisotropy of rocks along the KSDB-3 section by the acoustopolariscopy method has made it possible to subdivide the section into two main different zones on the basis of the anisotropy index B. One zone extends from the surface down to the depth of 4.43 km and consists of low-anisotropic and nearly isotropic rocks, such as diabase, peridotite, phyllite, tuff, sandstone, and siltstone (B=0.08). This zone contains copper-nickel mineralization accompanied by high anisotropy at a depth interval from 1.8 to 1.9 km. A sharp boundary going along the Luchlompolo fault is recorded at a depth of 4.43 km. This fault coincides with the inclined seismic boundary interpreted previously as a contact between the Proterozoic and the Archean. Rocks occurring below (mostly, these are amphibolite, schist, gneiss, granite and migmatite) are high-anisotropic on average. Rocks of orthorhombic symmetry with oblique arrangement of symmetry planes to the borehole axis prevail. In some depth intervals the B index is more than 0.4. As judged by the maximum B values, the rock elastic anisotropy is monotonously decreasing below the depth of 8.3 km. A previously unknown effect of linear acoustic anisotropy of absorption has been revealed in 90% of rocks of the Archean section. Ten structural-anisotropic floors have been distinguished by anisotropy parameters from the surface to 12.26 km. The stages differ in dip angles, a trend azimuth of anisotropy plane, the value of anisotropy index. The boundaries between the structural-anisotropic stages and the suite and stratum contacts within Proterozoic and Archean complexes, as a rule do not coincide. The current status of theory and practice of geophysical observations, carried out on the earth's surface, does not yet allow one to specify a symmetry type and other data on not exposed rocks with certainty. Thus, scientific programs, using deep and super-deep wells, are necessary to develop algorithms for processing geophysical work results (DSS, VSP etc.) in the regions of anisotropic rocks occurrence.

G01 : 3B/37 : G3

Lithospheric Structure of the Australian Continent from Surface-Waveform Tomography

Frederik J Simons (fjsimons@mit.edu),

Rob D van der Hilst &

Alet Zielhuis

MIT Rm 54-517A, Cambridge 02139, USA

Globally, higher-than-average seismic velocities persist to large depths under the old and stable parts of continents, in a manner suggestive that the thickness of tectosphere, the continental thermo-chemical boundary layer, correlates positively with crustal age. However, significant crustal variations occur at length scales not resolvable by global inversions, and it is unclear how they affect the seismic signature of the tectosphere.

Australia is very well suited to address this issue: it is favorably located with respect to zones of active seismicity which provide ample sources for seismic tomographic imaging, and the makeup of the continent is extremely varied: ages of formation and lithospheric stabilization range from Archaean to Phanerozoic.

We present a model of 3D variations of shear-wave speed in the Australian upper mantle, obtained with Partitioned Waveform Inversion. PWI uses path-averaged velocity profiles (obtained by fitting waveforms) in a tomographic inversion. The data are from the portable arrays of the SKIPPPY project and from permanent stations (from AGSO, IRIS and GEOSCOPE). Both the fundamental and higher modes of Rayleigh waves of ±1600 source-receiver combinations were used. This produces a lateral resolution of ±250 km, with good sensitivity throughout the upper mantle and most of the transition zone.

At large wavelengths, the velocity anomalies corroborate the globally observed pattern. However, there are variations on smaller scales that suggest a complex relation between lithospheric thickness and crustalage. We explore this relation by determining the average wave speed in several tectonic units, and by investigating how this seismic signature changes with increasing depth.

Until 150 km depth, wave speed deviations vary in agreement with the surface pattern of lithospheric formation ages, i.e. fast wave propagation in the Precambrian and slow in the Phanerozoic. Some Proterozoic tectonic subregions present pronounced velocity highs to depths exceeding 300 km. Surprisingly, the Archaean units do not seem to penetrate that far down: beneath 250 km depth, the wave speeds of the Yilgarn and Pilbara blocks are, in fact, similar to the average wave, speeds beneath the Phanerozoic. The Tasman Line, an outcrop division between the Proterozoic Central cratons and the Phanerozoic East, cannot be associated unequivocally with a contrast in seismic properties. The lateral extent of the tectosphere at depth sometimes differs from its surface expression: to depths of about 150 km, high wave, speeds continue well east of the Tasman line.

More parameters than formation age are important for the stability and survival of a tectosphere. Our results suggest that cratons that have been disrupted tectonically or rifted along passive margins may lose their deep seismic expression, and that the relation between wave speed and crustalage needs to be studied on a smaller scale than is possible with global seismic studies.

G01 : 3B/38 : G3

Deep Seismic Profiling Through Au-As Hydrothermal Paleofields in the South-Limousin, French Massif Central (GéoFrance3D)

Adnand Bitri, (a.bitri@brgm.fr)

Catherine Truffert (c.truffert@brgm.fr),

Jean-Yves Roig (jy.roig@brgm.fr),

Jean-Philippe Bellot (jp.bellot@brgm.fr),

Vincent Bouchot (v.bouchot@brgm.fr),

Jean-Pierre Milesi (jp.milesi@brgm.fr) &

Patrick Ledru (p.ledru@brgm.fr)

¹ BRGM, 3 av. C. Guillemin, BP 6009 45060 Orléans cedex 2, France

The South-Limousin area is located on the SW border of the French Massif Central. Geological field observations show that this area consists of a nappes stacking, namely from top to the bottom, the Upper Gneiss Unit (UGU), the Lower Gneiss Unit (LGU) which encloses the gold bearing veins of the St-Yrieix district, and the Parautochthonous Micashist Unit (PMU). The "Lauriéras" deep seismic reflection profile has been shot from the TPU through the Saint-Yrieix district, where a 10-km long gold bearing structure is at present being mined by Cogéma, and end in the UGU to the North.The seismic data were recorded on a polyseis telemetric system with 4 ms sampling rate and 20 sec. record length. Sources, 10 to 15 kg dynamite within single hole (around 15 m depth), had 220 m spacing whereas 180 geophone groups were positioned every 55 m. A symmetric split-spread geometry was used along most of the profile, completed with a and-on geometry at the edges to allow better coverage. The coverage is around 22 fold along the profile. The Thiviers-Payzac Unit (TPU) cropping out in the southernmost part of the study area, has a geometry with respect to the others structural units still debated. The South Limousin wrench Fault (SLF) at the contact between the UGU and the TPU has obliterated the primary relationships. South of the SLF, beneath a seismic transparent medium attributed to TPU, well coherent reflectors dipping south may correspond to the PMU or the LGU unit. The anticline structure, pointed out by geological field observations, is observed on the seismic line, emphasised by continuous reflectors from south to north of the line. The Moho, corresponding to the end of the reflective lower crust, is observed around 10.5 sec. TWT. namely around 32 km depth. This seismic experiment, combined with magnetic, electromagnetic and gravity data, allow us to give new geometrical constraints for crustal scale Variscan structures, subsequent gold-bearing hydrothermal palaeofields and their relationships with the lower crust and the Moho.

Session G01:3P

G01 : 3P/01 : PO

Orogenic Wedging and Lithospheric Crocodiles in Thickened Previously-Rifted Continental Regions

Karel Schulmann (schulman@illite.u-strasbg.fr)¹,

Alan Bruce Thompson (alan@erdw.ethz.ch)² &

Joseph Jezek (jezek@prfdec.natur.cuni.cz)³

¹ EOST,1 rue Blessig 67084 cedex Strasbourg, upsg, charles university, albertov 6, praha, 12843, czech rep.

² Erdwissenschaften, ETH Zurich, CH -8092, Switzerland

³ UPSG, Charles University, Albertov 6, Praha, 12843, Czech Republic

We have considered thickening of continental lithosphere previously thinned (b = 2) in a proto-rift by homogeneous pure shear. We have examined crusts of different lithologies and layer thicknesses with different geotherms, for the cases of coupled and uncoupled lithological and thermal structures. If collision occurs immediately or shortly after rifting, then the still softened sub-rift material will be thickened and sub-rift mantle will occur as a rigid layer in the depth interval between 17.5 to 30 km. For layered crustal lithologies there will be hard layers separating soft material that deforms ductilely at elevated temperature. If hard layers are transposed against softer layers then orogenic wedging of sub-rift into shoulder, or the reverse, can occur. During the thermal history of convergence of previously softened crust, inversions in direction of wedging can occur due to layer hardening and relative opposition of harder and softer layers. Systematic migration of detachment layers occurs during the convergence history. Major differences result for felsic versus mafic lower crust, for different layer thicknesses, and with thermal age of cooled rift lithosphere. The thermal age before onset of convergence is a leading factor determining whether double or single wedge orogens develop.After 30 to 70 Ma of thermal relaxation of the rifted domain, the integrated strength of this region increases. If the collision starts in this time interval then the double wedge structure changes, because the lower crust of the rift domain becomes stronger than adjacent shoulder upper crust. This change in strength profile is responsible for migration of decoupling horizons in rifted domains to the base of the rift upper crust. After 70 Ma of cooling the rift mantle becomes stronger than shoulder lower crust - leading to migration of the lower decoupling horizon to the boundary between shoulder mantle and lower crust. The last scheme is typical for a combined lithopheric subduction-orogenic wedging model.

G01 : 3P/02 : PO

Crustal Structure Across the Cordilleran Orogen of Northwest British Columbia from LITHOPROBE and ACCRETE Seismic Data

Philip Hammer (hammer@eos.ubc.ca),

Ron Clowes &

Bob Ellis

University of British Columbia, Dept. of Earth and Ocean Sciences, Vancouver, BC, V6T 1Z4, Canada

The northwestern Cordillera of North America (British Columbia and SE Alaska) is comprised of a complex assemblage of terranes accreted to North America during two distinct periods. In the early Jurassic, the Intermontane superterrane (Stikinia, Cache Creek, Quesnellia, Slide Mountain and Cassiarterranes) was thrust eastward over North America. During the mid-Cretaceous, the exotic Insular superterrane (Alexander and Wrangellia terranes) collided with North America, further deforming the Intermontane terranes and producing the Coast orogen. The history and 3-D geometry of many of these terranes are poorly understood. The LITHOPROBE Slave-Northern Cordilleran Evolution (SNorCLE) and ACCRETE transects target this geotectonic environment to investigate the growth of continents by magmatic and terrane accretion.

The SNorCLE Refraction Experiment (SNoRE97) included four distinct refraction/wide-angle reflection profiles. One of these, Line 22, extends southwest from the Tintina Fault, across the Intermontane terranes and into the Coast orogen suture zone. Nine explosive shots (1000 - 3000 kg) were detonated along the 500 km profile. Recording was accomplished using 450 seismographs positioned every 1 km with the one-component (PRS and SGR) and three-component (REFTEK) instruments interspersed. The ACCRETE seismic experiment utilized the coastal fjords to profile crustal structure from the Pacific Plate through the Coast orogen. A132 l airgun array, shooting along a 200 km long fjord, provided the seismic source for 60 REFTEK seismographs, seventeen of which were deployed northeast from the head of the fjord, thereby directly linking ACCRETE with SNoRE97 Line 22.

The two resulting wide-angle data sets provide a densely sampled, 2-D profile. Data quality from both experiments are excellent; numerous reflected and refracted P- and S-phases are identifiable and are observed to large offsets. Data analyses of the SNoRE97 dataset include initial 1-D velocity modelling using Tau-P analysis and subsequent 2-D velocity modelling using travel time inversion and ray amplitude forward modelling. SNoRE97 Line 22 velocity structure and wide-angle reflection models are integrated with the ACCRETE P-velocity, S-velocity and Poisson's ratio models. Coordinated multidisciplinary research in the transect region is ongoing and will include coincident near -vertical incidence reflection data to be collected in late 1999. Geological and geochemical studies permit a fully integrated interpretation of the geophysical data.

G01 : 3P/03 : PO

S-N-O-R-C-L-E

Frederick A. Cook (cook@litho.ucalgary.ca),

Arie J. van der Velden (arie@litho.ucalgary.ca),

Kevin W. Hall (hall@litho.ucalgary.caS-N-O) &

Brian J. Roberts (roberts@crust.cg.NRCan.gc.ca)

Department of Geology and Geophysics, University of Calgary, Calgary, Alberta, Canada

Seismic reflection profiling of the Precambrian lithosphere in northwestern Canada by LITHOPROBE has produced images of delamination structures (tectonic wedges) at crustal scale (to about 30 km depth) and, for the first time, at lithospheric scale (to about 100 km depth). It has further delineated subcrustal structures that were likely caused by imbrication of subducted lithosphere. These features are delineated along a transect that begins in the Precambrian Slave Province, where the oldest known rocks (> 4.0 Ga) are found, crosses a Paleoproterozoic orogen (Wopmay orogen, ca. 2.1-1.84 Ga), and ends near the eastern edge of the Mesozoic-Cenozoic northern Canadian Cordillera. This, the first phase of the Slave-NORthern Cordillera Lithospheric Evolution (SNORCLE) transect, addresses accretionary processes during the Archean and Proterozoic assembly of northwestern North America. The final phase of reflection profiling in late 1998 or 1999 will extend the profile across the northern Cordillera and will link to the U. S. ACCRETE transect in the Pacific ocean near southern Alaska to complete a ca. 2000 km transect from the oldest rocks of an Archean craton to the modern Pacific ocean.

G01 : 3P/04 : PO

New Moho and Basement Maps from BIRPS Data

Richard England (england@esc.cam.ac.uk)

Department of Earth Sciences, University of Cambridge, UK

The BIRPS deep seismic reflection dataset covers the shallow water continental shelves of the UK and Ireland and adjacent parts of the North Sea. The dataset represents the densest coverage of any deep seismic dataset over a comparable area of the Earth's continental lithosphere. The unmigrated data have all been interpreted and digitised to produce a set of maps of two-way-travel-time to top basement (Rotleigendes), Moho and major chronostratigraphically defined units. These maps can be depth converted using appropriate velocities obtained from the limited wide-angle/refraction data available and by integrating the results of potential field modelling. Maps of thickness and distribution of selected stratigraphic units have also been produced. The maps confirm that the most significant changes in crustal thickness are due to Mesozoic rifting. The roots of the Variscan and Caledonian orogens have been almost completely removed. There are no regional trends observable in the basement thickness, other than those associated with rifting. The average thickness of the crust is around 30 km, which is considerably thinner than averages obtained from compilations of wide-angle/refraction data which use data from a global dataset. In this respect, the crust of NW Europe is uncharacteristic of the continental crust. This observation is important if we consider that many models for the kinematics and dynamics of deformation processes are based on observations of crustal structure made in NW Europe. Styles of faulting within the North Sea rift system can be shown to vary with changes in the thickness of the basement along the rift axis. Large faults with km's of displacement dominate where the crust is thicker, beneath the Mid-North Sea - Ringkobing-Fyn High. Where the crust is thinner, smaller faults with correspondingly smaller displacements are observed. This suggests that the surface geometry of rifts is controlled by variations in properties of the continental crust.

G01 : 3P/05 : PO

Earthen Shell Axial Symmetric Structure

G. F. Makarenko (Knst@kapella.gpi.ru)

General Physics Institute, Rus.Ac.Sci.

Fig. For the N hemisphere shows tectonic position of early-Mz (P/T and T/I) cover basalts - traps and ocean lavas. Basalts embrace the rears of geosyncline (GS) fold-thrust zones of hercinian and late-hercinian cycles (folding C/P and P/T). Rear lavas are final-GS, postorogenic magmatites. They are appearing with the submergenses after the filling of frontal foredeeps and they are preceding to graben volcanites (T/I and I/Cr correspondingly) (Makarenko, 1983). Post-hercinian lavas have weakened T°lenses on the deeps 450 km (Dzievonski, 1989). Fig shows Urals and Cordilleras (60°E-120°W) on vertical line. One can also see other structures-geographical twins (crossed lines): E. Pacific and W. Indic, Atlantic and Palau ridges, Corner-Miln rise and Japan Is. forms et.al. Geological formations for hercinides are as follows. Urals: E - lavas, W - orogenic debris; Appalachians with Ouachita: SE - lavas, NW - debris; Cordilleras: W - lavas, E - debris. Later in the Cordilleras rear post-hercinian basalts become initial basites of nevadian and laramian GS cycles. Parts of post-hercinian lavas of Pra-Himalayas and of Sikkan-Yunnan later become initial-GS basites for modern Himalayan and Arakan-Yoma chains. Final rear basalts of the cycles change into initial basites if the GS activity is conserved. The isolating of GS troughs agrees with taphrogenesis on the lavas of stable zones. The rears of the cimmerides: cover lavas I/Cr + grabens Cr/Pg + T°lenses 250 km; nevadides: Cr/Pg + Pg/N + T(lenses 100 km; laramides: Pg/N + N/Q + T°lenses 50 km. Nevadides, closed by their fronts with their rear lavas (Cr/Pg, see DSDP data) and with their common taphrogenic rifts, create mid-ocean ridges. Young fragments of the ridges (N. Atlantic, E. Pacific) confirm to closed laramides. Cimmerides, closed by their fronts, create huge basalt ocean provinces with ancient ridges - volcanic mountains. Thus the author's conception for the first time gives the chance to visualize global continent-ocean asynchronous GS zones. They are in different evolution stages now. The zones produce easily verifiable planetary net (Makarenko, 1993). This net is stable. The forms of continents and oceans are changing all the time. T°lenses descend only when the lavas get cold, this circumstance rejects convection in the mantle. Lava fields of different ages and their T°lenses can overlap one another: Westward of Cordilleras there are Pg/N lavas too, Eastward of N Africa there are Cr/Pg lavas too (these fields are absent on Fig). The mobilistic models are dealing with some other object, than the real Earth. Earthen shell has axial symmetric structuralism, which is evident now.

Makarenko G F, Nauka, Moscow, 208 p.(1983). Dziewonski A.M, The Encyclopedia of solid Earth.. NY, 331-359 (1989).

Makarenko, Cosmoinform, Moscow, 280 p.(1993).