Upper mantle structure in the ocean-continent transition zone: Kamchatka

ISSN 0742�0463, Journal of Volcanology and Seismology, 2010, Vol. 4, No. 4, pp. 232–247. © Pleiades Publishing, Ltd., 2010.Original Russian Text © L.I. Gontovaya, S.V. Popruzhenko, I.V. Nizkous, 2010, published in Vulkanologiya i Seismologiya, 2010, No. 4, pp. 13–29.

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INTRODUCTION

The Kamchatka Peninsula is a tectonically unique partof the Pacific–Asia transition zone. The southern part ofthe peninsula, which is structurally related to the Kurilisland arc (IA), is characterized by similar features ofdeep�seated tectonics. In the north the peninsula is situ�ated at the junction zone between the Kuril–Kamchatkaand the Aleutian island arc along with the Emperor volca�nic zone and Emperor Fracture, whose northern branchis the closest to the structures of the Kronotskii Peninsulaand the Kamchatskii Bay. The interaction between thesestructures at depth manifests itself in a high level of seis�micity and in the variation of Benioff zone parametersalong the peninsula [27], as well as in the positions of vol�canic belts, in a volcanism that is unique by the volumeand composition of its lavas [1, 7, 8, 26, 31, etc.], in thediscordance between dynamic processes at different levelsin the lithosphere [16, 25, etc.], in the coexistence of twotectonic regimes (mountain building and rifting) [12, 13],and in other characteristics. This situation has given rise todifferent, occasionally radically opposed points of view onreconstructing the tectonic evolution of the region [1, 2,10, 13, 21, 23−35, etc.]. The problem is also compounded

by poor knowledge of the deeper structure of the uppermantle (UM), since data of this kind are practicallyunavailable for the western and central parts of the penin�sula, in particular, for the Sredinnyi Range area.

Since the 1970s, extensive publications have beendevoted to the study of the upper mantle structure inKamchatka. These studies were mostly concerned withinterpretations of kinematic and dynamic characteristicsof seismic waves that are excited by explosions andteleseismic earthquakes and they threw some light on themore general features of the deep lithosphere structurebeneath eastern Kamchatka [3, 6, 11, 15, 22, 28–30, 37,etc.]. In recent years, a 3D�velocity model for the crustand upper mantle has been developed using various mod�ifications of seismic tomography [9, 19, 32, 33, 36, etc.]. Itshould be noted that tomographic images criticallydepend on the amount and (at least as importantly) thequality of the data used, the initial velocity model, theassumptions about crustal structure, and the interpreta�tion method. For this reason the results from modeling ofvelocity images, while having similar general features,such as increased velocities within the Benioff layer andlower velocities in the mantle wedge beneath the volcanic

Upper Mantle Structure in the Ocean–ContinentTransition Zone: Kamchatka

L. I. Gontovayaa, S. V. Popruzhenkob, and I. V. Nizkousc

a Institute of Volcanology and Seismology, Far East Division,Russian Academy of Sciences, Petropavlovsk�Kamchatskii,

683006 Russiab OAO LukinCholot, Petropavlovsk�Kamchatskii, 683016 Russia

c Data Service Department, Data and Consulting Section, Schumberger Logelko, Moscow, 109147 RussiaReceived March 15, 2008

Abstract—We consider results from modeling the crustal and upper mantle velocity structure in Kamchatkaby seismic tomography and compare these with gravity data and present�day tectonics. We found a well�pro�nounced (in the physical fields) vertical and lateral variation for the upper mantle and found that it is con�trolled by fault tectonics. Not only are individual lithosphere blocks moving along faults, but also parts of theBenioff zone. The East Kamchatka volcanic belt (EKVB) is confined to the asthenospheric layer (theasthenosphere lens) at a depth of 70–80 km; this lens is 10–20 km thick and seismic velocity in it is lower by2–4%. The top of the asthenosphere lens has the shape of a dome uplift beneath the Klyuchevskoi group ofvolcanoes and its thickness is appreciably greater; overall, the upper mantle in this region is appreciably strat�ified. A low�velocity heterogeneity (asthenolith) at least 100 km thick has been identified beneath the CentralKamchatka depression; we have determined its extent in the upper mantle and how it is related to the EKVBheterogeneities. Gravity data suggest the development of a rift structure under the Sredinnyi Range volcanicbelt. The Benioff zone was found to exhibit velocity inhomogeneity; the anomalous zones that have beenidentified within it are related to asthenosphere inhomogeneities in the continental and oceanic blocks of themantle.

DOI: 10.1134/S0742046310040020

JOURNAL OF VOLCANOLOGY AND SEISMOLOGY Vol. 4 No. 4 2010

UPPER MANTLE STRUCTURE IN THE OCEAN–CONTINENT 233

belts, still differ significantly among themselves in thedetail that is available in the resulting velocity models forthese structural features. The present study examines thedeep velocity model of the lithosphere down to about a200 km depth that was developed by the techniquedescribed in [34, 36] and discuses how the model is con�sistent with gravity data and the present�day tectonics ofthe peninsula.

A BRIEF OVERVIEW OF KAMCHATKA’S TECTONICS AND CRUSTAL STRUCTURE

Extensive data exist, which throw light on variousaspects of Kamchatka’s recent history. The tectonicregionalization map that is given here, which also incor�

porates results from deep geophysical studies 1, highlights

major blocks and folded zones separating them as themain structural units (Fig. 1). We classify the SredinnyiKamchatka massif and structures with continental–sub�continental crust, viz., the Bol’sheretsk plate and theUkelayat–Sredinnyi block, as structures with typical con�tinental crust. To the east and southeast of these are theEast Kamchatka and South Kamchatka structural forma�tion zones, which are separated by the Ganaly–Petropav�lovsk block (which is also called the Malki–Petropavlovskor Nachiki zone of transverse dislocations) and the Pacificzone, which includes a system of block uplifts in the east�ern peninsulas of Kamchatka. The present�day structureof the peninsula is largely controlled by numerous Ceno�zoic downwarps, which are inherited and superimposed(East Kamchatka and South Kamchatka downwarps) anddepressions (the Central Kamchatka depression and oth�ers), as well as by superimposed volcanic belts—the Cen�tral Kamchatka volcanic belt (CKVB) and the East Kam�chatka volcanic belt (EKVB). The former formed duringPaleogene–Quaternary time and the latter during Neo�gene–Quaternary time. Several investigators of the EKVBsubdivide it into the South Kamchatka and the East Kam�chatka volcanic zones and the Central Kamchatkadepression (CKD). The northern, broadest part of theCKD contains the Klyuchevskoi group of volcanoes, oneof the largest in the world; it can boast of the highest rateof discharge of volcanic material and the composition ofits material is unique [8, 26, 31, etc.]. We define the Cen�tral Kamchatka rifting zone (CKRZ) in the map in theCKD area, including its western periphery, where volca�nic sheet complexes are abundant (CKVB, the CentralKamchatka volcanic belt); the rifting zone seems toinclude a system of linear features striking northeast, viz.,

1 Deep Geological and Geophysical Investigations by SeismicExploration Techniques (Reflected Earthquake Waves) and byMagnetic and Electrotelluric Methods on Regional Profiles (Eisk–Stavropol’–Neftekumsk–Caspian Sea 850 km Long and Korf–Pezhino 500 km Long). The Analysis and Generalization of Datafrom Deep Geophysical Investigations Conducted in the NorthCaucasus, Kamchatka, and Southern Siberia. A report. 2006. Stateregistration number 1�03�149/4, vols. 2–3, 490 pp.).

two inherited depression zones and two (mostly buried)uplifts.

The crust of the region is characterized by a well�pro�nounced zonal (block–banded) structure in plan. Thereare three zones in southern Kamchatka and six northeastof the Ganaly–Petropavlovsk block that differ both inthickness and in crustal structure. These zones strikenortheast in southern Kamchatka, while further north theprevailing directions are nearly north–south and north�east. One peculiar feature of this map is the fact that thearea that encloses the CKRZ has a relatively thinner crust(28–30 km) (Fig. 1b).

As matters stand today, there is hardly any doubt as tothe controlling influence of the upper mantle structureand properties in shaping the history and present�daydirection of the Kamchatka tectonic evolution. For thisreason it seems important to study the velocity structure ofthe lithosphere, the depth to and features of the asthenos�phere, including the Benioff zone, and the relationships ofthe mantle and crustal heterogeneities to seismicity fordifferent tectonic areas of Kamchatka.

VELOCITY MODEL OF THE LITHOSPHERE

We consider a 3D�velocity model of the transitionalzone lithosphere beneath Kamchatka, which was calcu�lated using seismic tomography from wave arrival times oflocal Kamchatka earthquakes that were recorded by thenetwork of stations operated by the Kamchatka Branch ofthe Geophysical Service (KB GS) of the Russian Acad�emy of Sciences (RAS). The approaches we used to selectinput data and calculation techniques significantlyenhanced the resolution of the 3D constructions as com�pared with the previous results for Kamchatka [9, 32, 33].

The velocity fields of compressional (VP) and shear(VS) waves were calculated in several steps. The first step

was to determine a 1D�optimal�velocity model2, which

together with station corrections gives the minimum traveltime rms error for all earthquakes selected from the cata�log [36]. This model, which is based on P and S wave data,was used to revise the hypocenters and for further model�ing of the 3D crustal and upper mantle velocity structure.The seismicity and the configuration of the seismic net�work are such as to allow seismotomographic construc�tion for eastern Kamchatka only, the water areas of theeastern bays inclusive. The results are presented as hori�zontal and vertical cross sections of the 3D�crustal andupper mantle velocity model. One can use these results toinfer the distribution of mantle inhomogeneities overdepth and laterally, their relationships to surface features,and their limitations. The earthquakes used for the calcu�lation were projected onto all profiles and maps shown in

2 The calculated optimal model is appreciably different from thevelocity curve that is now used for the location of Kamchatkaearthquakes [15], as well as from the one found in [32].

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the figures (the Ks ≥ 11 events only are shown). The bandfrom which these events were selected for each verticalcross section is ±10 km wide, while for the horizontal crosssections they were projected from depth intervals of±5 km. The configuration of the Kamchatka seismic net�work and the geometry of local seismicity enabled us toreconstruct a 3D�velocity model down to depths of 200–250 km; the parameterization of the model was 30 × 30 ×10 km for the crust and 30 × 30 × 20 km for the uppermantle.

Figure 2 illustrates the lithosphere velocity structurealong vertical cross sections oriented in accordance withthe “Kamchatkan” trend, southwest–northeast. Figure 2bshows the calculated 1D�optimal�velocity (VP) model andcomparison of this to I.P. Kuzin’s model [15]; the latter isused at present in routine earthquake location work for theKamchatka regional catalog. Profile 1 runs along the east�ern peninsulas along the line of maximum seismicity andthrough the Kamchatskii Bay; profile 2 traverses theEKVB as far as the bend in the Benioff zone and furthernorth it is closest to the Vostochnyi Range; profile 3 is sit�uated in the central part of the CKD and traverses the Kly�uchevskoi group of volcanoes in the northeast. The pro�files clearly reveal a lateral inhomogeneity of the lithos�phere–asthenosphere in the differences between thecontours and the degree of anomaly contrast, which pro�duce an overall “image” of the lithosphere velocity struc�ture. The structure is characterized by vertical stratifica�tion of the crust and upper mantle with extended and well�pronounced interfaces; some of these have absolute valuesof VP in the figures. The best�pronounced features in thevelocity model are the upper and lower crust, whose veloc�ity properties differ appreciably between different struc�tural zones of eastern Kamchatka, the crust–mantleinterface (this can be tentatively related in the cross sec�tions to the velocity isoline VP = 7.5 km/s; or to the depth30 km in the average velocity curve), a dipping high veloc�

ity mantle inhomogeneity3 to which the Benioff zone is

related (we shall refer to it in what follows as the focal layer,combining its velocity properties and seismicity), and

3 The hypothetical origin of this anomaly, which differs dependingon the particular geotectonic hypothesis assumed, is not consid�ered in this paper in our analysis of factual material.

asthenospheric mantle inclusions beneath the volcanicbelt and the CKD.

The thickness and velocity structure of this focal layershow considerable variations over depth and laterally. Theanomaly pattern reflects the geometry of the layer and therelated seismicity, and is in general agreement with well�known data [27, etc.]. However, the higher resolution ofour velocity model revealed certain new relationshipsbetween the details in the velocity structure and the focallayer seismicity. The velocity anomalies with the highestgradient are observed along profile 1 within the layer; hereone also has the highest seismicity level. The character ofseismicity changes along the profile: from south to north,as far as the bend of the Benioff zone, the earthquakes aremostly confined to the depth range 20–40 km, that is, tothe lower crust, the Moho interface. The seismicity in theKamchatskii Bay (the northern part of the profile) has adifferent seismicity: the earthquakes occur in the uppercrust and are probably related to different tectonic pro�cesses.

Beneath the EKVB the focal layer is expressed in thevelocity field as poorly definable positive anomalies (nomore than 2–3%) as far as the bend of the Benioff zone(Fig. 2, profile 2). In the Kronotskii Peninsula area andnorth along the profile, the velocities VP in the focal layerare much higher (as high as 8.5–8.7 km/s), which agreeswith the strong velocity anisotropy identified in this region[5, 6]. Beneath the CKD the focal layer is seen as poorlypronounced anomaly “patches” with associated seismic�ity increases, the most significant among these being thatbeneath the northern part of the profile in the area of theKlyuchevskoi group of volcanoes.

The top of the mantle velocity inversion zones(asthenoliths, inclusions, and asthenosphere lenses)beneath the volcanic belt is confined to the depth range70–100 km; the isoline VP = 8.0 km/s corresponds to thetop in the figure (profile 2). The velocity within thesezones is lower by 0.2–0.4 km/s. The asthenospheric inho�mogeneities have varying depths and thicknesses beneathdifferent Kamchatka structures. Profile 2 in the EKVBshows an extended layer (a lens) about 20 km thick, whichcan be clearly followed throughout the entire belt of volca�noes approximately as far as the latitude of the KronotskiiVolcano, that is, before the westward displacement of the

Fig. 1. Part of a tectonic regionalization map for the Koryak–Kamchatka folded area (a) and a map of crustal thickness (b): (1) boundariesof structural formation zones, massifs, plates, and blocks, (2) inferred eastern boundary of the Central Kamchatka rifting zone and struc�tural material complexes of doubtful origin (volcanogenic cherty formations of the Khairyuzovo horst–anticlinorium), (3) other majorfaults inferred from geological and geophysical data, (4) axes of major anticlinal features as inferred from geological and geophysical data(including buried features), (5) Cenozoic downwarps and basins, (6) ellipses with alphabetical designations of faults (the longer axes areoriented in accordance with fault strikes). Folded and folded block zones: P–W–Kz Penzhina zone (Penzhina–West�Kamchatka zone),E�Kz East Kamchatka zone (Olyutorka–East�Kamchatka zone), S Southern Kamchatka zone, PACz Pacific zone of eastern peninsulas,C�Krz Central Kamchatka rifting zone, U–Sb Ukelayat–Sredinnyi block, Bp Bol’sheretsk plate, M�Km Median Kamchatka massif,G–Pb Ganaly–Petropavlovsk block. Fault zones: C�K Central Kamchatka zone, M–P Malki–Petropavlovsk zone, Vl Vilyuchinsk zone,K–A Kronotskii–Andrianov zone, K�T Kronotskii–Tigil zone, U�K Ust’�Kamchatsk zone, E�K East Kamchatka zone, P�C Palana–Commander Is. zone. The inset shows (in grey) the positions of the Pliocene–Quaternary island�arc volcanic belts: EK East Kamchatka,SK South Kamchatka, CKD Central Kamchatka depression, SR Sredinnyi Range. The lines of equal crustal thickness are in km.

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volcanic belt. The asthenosphere lens is thicker beneaththe Klyuchevskoi group of volcanoes and finds a betterexpression in the velocity anomaly pattern; overall, theasthenosphere is considerably stratified there (profile 3).The asthenospheric inhomogeneity (asthenolith) is bestexpressed in the central part of the CKD in the form of amajor low�velocity anomaly; characteristically, theasthenolith top corresponds to the geometric features ofthe Benioff zone (profile 3). At the surface, the southwest�ern boundary of the asthenoliths is controlled by the Malkitransverse zone of dislocations and the northeastern by theKronotskii–Andrianov fracture zone.

Figure 3 shows vertical cross sections oriented acrossthe Benioff zone. These cross sections traverse the Kam�chatka lithosphere approximately at the latitudes of theMutnovskii and Avacha groups of volcanoes (profiles 4and 5, respectively), of the Karymskii volcanic center(profile 6), of Kronotskii Volcano (profile 7), and of theKGV (profiles 8 and 9). Figures 4 and 5 illustrate blockdiagrams of mutual intersections between the cross sec�tions discussed here, profile 3 with profile 7 (Fig. 4) andwith profile 9 (Fig. 5). These figures afford a clearer idea ofthe 3D structure of the lithosphere–asthenosphere vol�ume studied here and of the relationships betweenasthenolith inclusions beneath various tectonic features.The pattern of velocity anomalies reveals considerablevertical stratification of the crust and upper mantlebeneath eastern Kamchatka and their lateral variations,showing interrelationships between the velocity features inthe mantle wedge and the focal layer structure. The char�acter of these interrelationships suggests horizontal move�ments (flows) of crustal and upper mantle material alongweakened zones; the movements involve the layers of thecontinental block and the focal layer. The best�expressedasthenospheric inhomogeneity in the velocity field is thatwhich can be followed north of the latitude of the Avachavolcanoes at a depth of about 150 km. At the surface theinhomogeneity corresponds to the CKD; the anomalypattern helps determine its boundaries in the southern,northern, and western directions. An analysis of theseblock diagrams suggests interrelationships between theasthenospheric inhomogeneities beneath the EKVB andthe CKD. For example, Fig. 4 shows that the “roots” ofthe asthenoliths at a depth of about 200 km are confinedto a narrow linear zone, probably the Central Kamchatkadeep�seated fault. In the east, this anomaly is bounded bythe Benioff zone and is possibly connected along a faultsystem to the asthenospheric layer beneath the EKVB.

According to the east–west cross sections, the KGV isabove the asthenolith layer whose top is uplifted to reach adepth of about 80 km. The velocity pattern clearly revealshigh�gradient steeply dipping zones (faults that reach intothe mantle), which find their expression in near�surfacetectonics (Fig. 5). These zones seem to delimit an exten�sive (of radius at least 50 km) downwarp zone beneath theKlyuchevskoi group of volcanoes. The deep�seated

magma supply zone can be followed as a narrow (within20–50 km) low�velocity anomaly. This anomaly shows ahigher contrast in the VS field, where it can be trackeddown to at least 200 km depth; that zone traverses thefocal layer and has a well�defined connection to oceanicmantle (Fig. 3, profile 9). The anomaly that is identifiedbeneath the KGV is probably due to increased tempera�ture and the supply of juvenile fluid and melt. Thus, theupper mantle in the area shows such high contrasts in theelastic parameters that one can even detect a channel pro�viding “communications” between the KGV and theasthenospheric inhomogeneity at a depth of at least 150–200 km; here also, one observes considerable changes inthe Benioff zone geometry.

The upper mantle in the Kamchatskii Bay area hasabnormally low velocities; at a depth of 80 km one findsthe asthenospheric layer, which is evidently responsible forthe tectonic and magmatic activity of the area. Thishypothesis is supported by increased heat flow and thematerial composition of the volcanic ejecta [8, 21, 26,etc.]. The anomalous properties of the upper mantle in thearea may be due to the influence of the Emperor FractureZone (or the extension of the latter into the KamchatskiiBay and further beneath Kamchatka, where it traversesthe CKD structure).

The horizontal cross sections of the velocity modelafford a control on the boundaries of the anomalous man�tle zones that were previously identified (Fig. 6). A highvelocity focal layer is clearly visible at a depth of about200 km. The layer is divided by low velocity anomaliesacross the Benioff zone, which are probably connected toa narrow, linear inhomogeneity in this depth rangebeneath the CKD (probably corresponding to the mantleroots of the central Kamchatka deep�seated fault).Themost significant of these transverse anomalies, whichstrikes northwest, corresponds to the Malki zone of trans�verse dislocations at the surface. This (transform) faultmay have been the location where crustal and upper man�tle blocks were displaced. The velocity pattern of the focallayer (at a depth of 120–140 km) shows a discontinuity inthe high velocity anomaly and a displacement; higher inthe section (at the approximate latitude of Cape Shipun�skii) one notes that the focal layer changes its strike, turn�ing toward the northwest. The northern block of Kam�chatka very clearly shows a linear anomalous structurestriking along the Aleutians; this begins in the Kam�chatskii Bay and can be followed as far as the KGV onland; it possibly includes an extension of the Emperor Vol�canic Zone. A similar linear structure was previously iden�tified on land from electrically conductivity anomalies[17].

In the lithosphere the focal layer loses well�definedcontours, its structure becomes more complex and itsinhomogeneity increases; the layer is a wide band there,consisting of a set of velocity anomalies; the overall “pat�tern” is high velocity. It is only in the 40–60�km layer (in

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the range of the crust–upper mantle interface) that thefocal layer pattern gives a clear�cut expression in the veloc�ity field. The layer can be tracked along the eastern penin�sulas as a narrow band of high velocity anomalies sepa�rated by east–west, relatively low�velocity ones. Thevelocity field pattern shows interrelationships between on�land anomalies and their offshore extensions. The 40–60�km velocity layer shows the highest contrast in theanomalous velocity field. Here one clearly sees the zonewhere oceanic crust (the high velocity linear anomaly) issubducted under continental crust. Deeper than that, at60–80 km, the contrast is lost, and the focal layer is poorlyexpressed in velocity.

Overall, the velocity model of the lithosphere–asthenosphere block under study seems to be an exten�sional rift structure, with its greatest depth being at least200 km in the CKD area. The character of interrelation�ships between velocity inhomogeneities suggests consider�able horizontal and vertical displacements of crustal andupper mantle material along weakened zones and faultzones, which also involve the Benioff zone.

COMPARING THE VELOCITY MODEL WITH GRAVITY DATA

The area of Kamchatka has been almost entirely cov�ered by high precision gravity surveying, the resulting datais used to deal with a wide range of regional and prospect�ing problems. However, the use of gravity data alone with�out any additional information cannot yield any reliableresults for crustal geologic density models. For example, itwas shown that variations in crustal thickness, topography,and the density of crust and upper mantle compensateeach other so as to give a nearly zero resulting field [14]. Agreat influence on the morphology of the gravity field isexerted by a correct estimate of the depth and structuralfeatures of the Moho interface, whose gravity effect iscomparable with the influence of the most “gravity�active” sedimentary layer (which is a Cenozoic volcano�genic one in Kamchatka), especially when one deals withextensive basins and downwarps, which in fact produceregional anomalies due to their dimensions. Thus, there isa danger of mistaking the gravity effect due to near�surfacestructural material rock complexes with the effects ofdeeper horizons. For this reason additional information isrequired in the interpretation of gravity data to restrict therange of possible inversion results and this additionalinformation can be obtained by seismic studies. Oncethese combined methods (seismic and gravity techniques)have identified a crustal inhomogeneity, the latter can beextended to areas where there are no seismic data.

In our analysis of gravity data we considered the rela�tionships between the morphology of the gravity field andits frequency components on the one hand and the crustalfeatures obtained by analyzing geological and geophysicaldata [4, 11, 20, etc.] on the other. Geologic density mod�

eling showed that agreement with an observed Bougueranomaly can be reached only by introducing density inho�mogeneities in the upper mantle. We used data on crustalthickness and density, as well as on crustal velocity struc�ture in eastern Kamchatka, to calculate a geologic densitymodel of the upper mantle and to show that the observedanomalies are consistent with theoretical ones. Figure 7shows the results of modeling along the profile thattraverses the Benioff zone at the latitude of the Klyuchev�skoi group of volcanoes (Fig. 3, profile 9), demonstratingthat the velocity and density anomalies are in agreementwith the observed gravity field. Considering that the veloc�ity data are in rather good agreement with the results of thegeologic density modeling, as well as estimates of thedepths and dimensions of velocity inhomogeneities, wecarried out frequency filtering of the gravity field. It wasfound that relative velocity lows are observed in areas withlower values of the velocity field in the depth rangebetween 50 and approximately 150 km. The pattern of thetransformed gravity fields suggests that the zones with adecreased field correlate with the areas of maximum Neo�gene–Quaternary volcanism in the CKVB and in theCKD (Fig. 8). We think that it is especially important thatthese zones are largely discordant with respect to theregional tectonic features of Kamchatka and to the Mohomorphology. At the same time, we also find agreementbetween the major geological and geomorphic features ofKamchatka and the deep geophysical data. In particular,it may be hypothesized that the generation of the CentralKamchatka central�type morphostructure [18] wasrelated to the crustal geophysical inhomogeneities thathave been identified.

RELATIONSHIPS BETWEEN GEOPHYSICAL MODELS AND TECTONICS

The cross sections presented above and the maps ofvelocity and gravity anomalies provide convincing evi�dence that the Kamchatka lithosphere involves lateral andvertical geophysical variations. Thus, one naturally askswhat the relationship is between the deep crustal structureand the upper mantle structure. That relationship seemsnot to be very obvious, but it can be inferred from the posi�tions of the major morphotectonic structures of the pen�insula, the systems of fault zones, including neotectonicblock uplift and subsidences of the crust, the extent andpositions of the volcanic belt, and the composition of vol�canic ejecta.

Let us consider in more detail the character andprobable causes of some of these relationships. Firstof all, we wish to emphasize that the fault zones showvery high structural contrasts and that the velocityfields are largely controlled by fault tectonics.Indeed, an analysis of velocity “patches” (anomalies)at different depths in the upper mantle shows thatseveral deep�seated fault systems, such as the north�

242


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west trending Ptr and K–T, the east–west K–A, thenorthwest trending U–K and several others (Fig. 1a) areof fundamental importance for shaping the present�daytectonics of the peninsula. The greatest contrast is shownby the system of northwest faults situated in the Petropav�lovsk�Kamchatskii area, which bounds the Malki–Petro�pavlovsk zone of transverse dislocations (the Ganalyblock) on the northeast (Figs. 1, 6). This zone is related tochanges in the velocity structure of the focal layer andprobably to considerable horizontal displacements inupper mantle. It may be supposed that the intersection ofthis zone and the CKD (with the Central Kamchatka rift)is responsible for the most significant (within the depthrange studied) low�velocity zone, whose probable exten�sion is beneath the Sredinnyi Range. Lastly, it is on thisfault system that the stratification in the upper mantlevelocity structure occurs with the asthenosphere lensidentified in the CKD area wedging out, which mayexplain a weaker manifestation of Quaternary volcanismin the Malki–Petropavlovsk zone.

We think it to be a pattern that the zonal crustal struc�ture (and thickness) in map view is clearly in agreementwith the distribution of velocity anomalies in the 80–140�km depth range, that is, exactly with the mantle layerwhere the asthenospheric “bodies” have been identified.Obviously, the thermodynamic processes in the asthenos�phere control the geodynamic evolution of the lithosphere

as a whole, including its uppermost layer, the crust. Oneunambiguous confirmation of this hypothesis is the distri�bution of the velocity inhomogeneities along the verticalcross sections (Figs. 2, 3). One clearly sees that the areas ofrelatively uplifted top of the asthenosphere correspond toareas with relatively thinner crust, which may reflect theoccurrence of upwelling there.

Unfortunately, there are practically no geophysicaldata on the deep lithosphere structure beneath the Sredin�nyi Range. However, judging from the character of thetransformed gravity field, heat flow, and recent move�ments, one may hypothesize that the movements arecaused by active processes that are still operative in theinhomogeneous upper mantle. From the velocity model itfollows that there is an asthenospheric inclusion (astheno�liths) at depths of 80–140 km in the area between theMalki–Petropavlovsk and the Palana–Commander Is.zones of transverse dislocations beneath the central, wid�est part of Kamchatka. The geodynamic processes that aregoing on within that earth volume probably cause a gen�eral uplifting of it and the generation of an anticline. Theuplift has a rather complex structure that results from vol�canism and compensation processes that appeared andcontinued in the Late Cenozoic. Indeed, when one com�pares the asthenolith area and the character of theobserved gravity anomaly with the general morphostruc�tural features of Kamchatka as obtained from interpreta�

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tion of small�scale space imagery [18], it turns out that theCentral Kamchatka ring depression formed above thatearth volume upon the background of a general uplifting.Its tectonic periphery includes several horst–anticlinaluplifts of the eastern Kamchatka ranges in eastern andwestern Kamchatka (Fig. 1a). Since Late Cenozoic volca�nic rocks are rather abundant in the Central Kamchatkaring depression, the depression can be viewed as compen�satory. These processes seem to also be parageneticallyrelated to the mantle velocity structure, which reflects itsthermodynamic, petrophysical and other properties. Itfollows from the results of geologic density modeling thata density deficit, and thus negative anomalies of seismicwave velocities, can also be hypothesized for the westernarea of the Sredinnyi Range volcanic region that has notyet been probed by seismic tomography. We note that thelarge active Ichinskii volcano is situated in this area andcontinues the evolution of long�lived (Neogene) volcaniccomplexes (centers).

Overall, according to the distribution of seismic andgravity anomalies, the velocity structure of the uppermantle is in general agreement with the regional tectonics.If the hypothesis of a certain central�type symmetry in theupper mantle is correct (which, for example, is largelyreflected in Figs. 2, 4, and 6), then with the identifiedweakened zones (“channels”) that penetrate through theBenioff zone, one may hypothesize the existence of adeep�seated source of material for the asthenolith region(Fig. 1a) beneath the CKD (possibly a deep�seated man�tle diapir). The development of such a feature may berelated also to the changed geometry of the Benioff zone.Such a hypothesis is certainly not entirely indisputable inconnection with the asymmetry of the overall seismicitypattern, which is poorly seen within central Kamchatka.However, it can be supposed that the latter is a conse�quence of a more fundamental and deep�seated inhomo�geneity of the upper mantle.

CONCLUSIONS

Summarizing the above discussion, several conclu�sions can be drawn to varying degrees of certainty.

(1) Results of geophysical studies, in particular, by themethod of seismic tomography, show that the lithosphereand asthenosphere beneath Kamchatka down to depth200 km, as studied thus far, involve considerable verticaland lateral velocity inhomogeneities; a layered blockstructure is clearly expressed in the velocity field. Thegreatest contrast and persistency in the lithosphere areshown by the interface between the lower and upper crust,the crust–mantle interface, the asthenosphere top, andthe dipping high velocity focal layer that separates the oce�anic and the continental block in the upper mantle.

(2) Asthenospheric inhomogeneities have been identi�fied in the upper mantle and their depths and positionswere determined. The East Kamchatka volcanic belt is

confined to the asthenosphere lens at a depth of 70–80 km, its thickness is approximately 10–20 km there,with the velocity in the lens being lower by 2–4%. Theasthenosphere lens is thicker in the area of the Klyuchev�skoi group of volcanoes, the depth to its top is appreciablysmaller beneath the volcanoes, forming the shape of amantle dome uplift. Shear velocity anomalies helped thetracking of deep, and probably “thermal,” roots beneaththe Klyuchevskoi volcanoes down to the depth wherethese intersect the focal layer and even deeper.

For the first time we have identified a low velocityinhomogeneity (asthenolith) in the upper mantle beneaththe Central Kamchatka depression, whose thickness is atleast 100 km and have determined its dimensions andboundaries (the eastern, southern, and northern ones).The mantle “roots” of the asthenolith are probably con�fined to depths greater than those thus far studied andintersect with the focal layer at a depth of about 200 km.This inhomogeneity is probably due to a deeper mantlesource, which is indirectly related to the present�day vol�canism of the northern part of the East Kamchatka volca�nic belt; gravity data suggest an extension of this featureunder the Sredinnyi Range volcanic belt.

(3) We have found considerable lateral velocity varia�tion in the focal layer and interrelationships of the layercharacteristics (layer geometry, velocity structure, andseismicity features) with velocity features in the continen�tal and oceanic upper mantle blocks. The pattern of veloc�ity anomalies suggests horizontal eastward displacementsof lithosphere blocks along weakened layers in the crustand upper mantle, with the displacements also involvingthe Benioff zone.

(4) The prevailing feature at several depth levels in theupper mantle is the control of the upper�mantle structureby fault tectonics. The velocity structure shows fault zonesthat are oriented along and across the Benioff zone. Of thelatter, the one that is particularly well seen in the velocityfield pattern is the fault zone in the area of Petropavlovsk�Kamchatskii. The zone has deep (at least 150 km deep)mantle roots and is expressed in the structure (horizontaldisplacement) of the focal layer. The neotectonic mag�matic activity in the area between the Kamchatskii Bayand the Klyuchevskoi group of volcanoes is probablyrelated to the transverse fault zone, which has been iden�tified from the long intensive low velocity anomaly in thatarea in the depth range of 60–120 km. This zone was pre�viously identified from anomalous resistivity measured onland.

(5) This combined analysis of the velocity structure,the transformed gravity field, and other data suggests thepresence of a low�velocity mantle inhomogeneity in thewestern (not probed by seismic tomography) area of theSredinnyi Range volcanic zone.

Since anomalous velocity fields provide indirect evi�dence for the geodynamic setting, these new data on the

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deep velocity structure of the upper mantle beneath Kam�chatka can be used for geodynamic analyses.

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Upper mantle structure in the ocean-continent transition zone: Kamchatka