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AGU Geodynamics Series Volume 30, PLATE BOUNDARY ZONES Edited by Seth Stein and Jeffrey T. Freymueller, p. 295-324 1

A Dangling Slab, Amplified Arc Volcanism, Mantle Flow and Seismic Anisotropy in the Kamchatka Plate Corner

Jeffrey Park,1 Yadim Levin,1 Mark Brandon,1 Jonathan Lees,2 Valerie Peyton,3 Evgenii

Gordeev )4 Alexei Ozerov ,4

Book chapter in press with "Plate Boundary Zones," edited by Seth Stein and Jeffrey Freymuller

Abstract

The in Russian East Asia lies at the junction of a transcurrent plate boundary, aligned with the western , and a steeply-dipping zone with near-normal convergence. Seismicity patterns and P-wave tomography argue that subducting Pacific terminates at the Aleutian junction, and that the downdip extension (>150km depth) of the slab edge is missing. Seismic observables of elastic anisotropy (SKS splitting and Love-Rayleigh scattering) are consistent \Vith asthenospheric strain that rotates from -parallel beneath the descending slab to trench-normal beyond its edge. Present-day arc volcanism is concentrated near the slab edge, in the Klyuchevskoy and Sheveluch eruptive centers. Loss of the downdip slab edge, whether from thermo-convective or ductile instability, and subsequent "slab-window" mantle return flow is indicated by widespread Quaternary volcanism in the Sredinny Range inland of Klyuchevskoy and Sheveluch, as well as the inferred Quaternary uplift of the central Kamchatka depression. The slab beneath Klyuchevskoy has shallower dip (35°) than the subduction zone farther south (55°) suggesting a transient lofting of the slab edge, either from asthenospheric flow or the loss of downdip load. Such lofting may induce pressure-release melting to supply the Klyuchevskoy and Sheveluch eruptive centers. Petrologic indicators of high -peridotite equilibrium temperatures, long residence times for the hydrous arc-volcanic component, and weak expression of subducted sediment flux support the lofting hypothesis, and discourage an alternate interpretation in terms of accelerated slab rollback and/or a heightened influx of subducted volatiles. Over the late Cenozoic, the Komandorsky Basin subducted beneath northern Kamchatka and produced arc volcanics in the Sredinny Range. Several lines of evidence suggest the northeast migration of a plate (North America/Pacific/Komandorsky) along the eastern Kamchatka coast in Oligocene-Miocene times. Three "Cape " (Shipunsky, Kronotsky, Kamchatka) along the coastline are exotic, with geologic similarities to present-day western Aleutian islands, and may have accreted in a "caulking-gun" process as the triple junction migrated NE. The late Cenozoic transfer of arc volcanism from the Sredinny Range to the eastern volcanic front of Kamchatka may have been facilitated by the progressive replacement of a shallow-dipping Komandorsky slab with a steeply-dipping Pacific slab. 2

1. INTRODUCTION 1998] or an independent rigid-plate fragment [e.g., Cook et al., 1986; Riegel et al., 1993; Konstanti­ The Kamchatka peninsula marks the northeast­ novskaia, 2001]. On a smaller spatial scale, south­ ern edge of the . The many volcanoes ern Kamchatka is split by an axial depression that of the peninsula glow brightly in the Pacific "ring separates the peninsular topography into an eastern of fire." The modern plate kinematics of the Kam­ volcanic chain and the western Sredinny Range [ Fedo­ chatka region are deceptively simple (Figure 1). The tov and Masurenkov, 1991] (Figure 3). The Sredinny Aleutian Islands of Alaska trend SE-NW along the Range is now largely inactive both volcanically and strike-slip Bering zone between the Pacific and tectonically. North American plates [Geist and Scholl, 1994; Seliv­ Kamchatka is one of the few places in the world erstov, 1997; 1998], terminating against Cape Kam­ where land-based observations can probe the upper chatka at 56°N. South of the Bering fault, a subduc­ mantle at and beyond the terminus of a mature sub­ tion zone with rapid normal convergence (80 mm/yr duction zone. Near the Aleutian-Kamchatka cor­ according to �VEL-1 [DeMets, 1992a]) lies offshore ner, the subduction zone lacks deep , and southern Kamchatka, trending to the SW in a gen­ Benioff-zone dip decreases from 55° to 35° [ Go1·batov tle arc towards the T

ics, and suggest dynamic models for the Kamchatka tonics is motivated by seismic data from the "Side region. Tomography can detect lateral variation of Edge of Kamchatka Slab" (SEKS) deployment of 15 fast and slow velocity anomalies in the mantle, and broadband portable seismometers in 1998-1999 [Lees suggest the presence or absence of cold (fast) slab and et al., 2000]. Section 2 discusses the tectonic setting warm (slow) asthenosphere. Mantle fl.ow may cause of the Kamchatka region, including geologic elastic anisotropy in strained peridotite [Christensen, identification, geodetic constraints and plate-tectonic 1984; Ribe, 1992; Zhang and Karnto, 1995; Zhang et reconstructions. Section 3 discusses earthquakes and al., 2000], and can be detected by shear-wave splitting seismic velocity structure in the region, with a focus [ Vinnik et al., 1984; Silver· and Chan, 1991], Ps con­ on slab location and structure. Section 4 discusses verted phases in body-wave receiver functions [Levin seismic indicators of mantle fl.ow, both near the slab and Pa1·k, 1997; Bostock, 1998], and Love-to-Rayleigh edge and in the mantle wedge. Section 5 discusses ig­ surface-wave scattering [Yu and Park, 1994]. neous processes, both primary characteristics and Mantle dynamics in the Kamchatka region hinge indicators from mantle xenoliths, the latter of which on tectonic mechanisms whose application to the re­ are unusually common in the region. In Section 6 we gion is still largely hypothetical. Several scenarios discuss several scenarios for mantle kinematics in the are plausible. When oceanic lithosphere subducts Kamchatka region. Many predictions of these scenar­ into the mantle, it may undergo trench-axis rollbad ios are poorly constrained at present; we suggest how [Dewey, 1980; Kincaid and Olson, 1987; Otsuki, 1989; the constraints might be improved. Royden and Burchfiel, 1989], in which the astheno­ sphere under the slab is pushed either downward or 2. TECTONIC SETTING along the trench towards the ''free" end of the subduc­ tion zone. Flow in the mantle wedge is often concep­ From its southern tip to the Koryak highlands, the tualized in terms of a 2-D corner flow driven by shear­ Kamchatka peninsula extends more than 1600 km, coupling to the downgoing slab [Ida, 1983; Peacock similar to the distance between the cities of Los An­ and Wang, 1999]. Trench-parallel flow on the sea­ geles and Seattle along the western margin of North ward side of the trench has been proposed for a variety America. The peninsula varies in width from 100 of convergent settings [Alvarez, 1982; Kaneshima and km in its northern isthmus to 400 km at its widest Silver, 1992; Russo and Silver, 1994; 1996; Yu and in the south {Figure 2). Subduction has shaped the Park, 1994], and simulated in physical analog exper­ entire eastern coastline of Kamchatka, but is active iments [Buttles and Olson, 1998]. In some situations only beneath the southern half of the peninsula, ex­ the subducting slab is thought susceptible to ther­ tending roughly 700 km between its junctions with mal instabilities [Davaille and Lees, 2002] or ductile the Kurile and Aleutian island chains. Subduction of necking and detachment [Davies and von Blanken­ the Pacific plate is near-normal to the plate bound­ bu1'[), 1995]. Such processes may help explain why the ary, which turns at a right angle at Cape Kamchatka Wadati-Benioff zone under central Kamchatka lacks to become transcurrent. The inactive volcanic is­ deep seismkity. lands of the western Aleutians lie along the transcur­ In the former USSR, the Institute of Volcanol­ rent boundary, which also clivides geographically the ogy and the Experimental-Methodical Seismic De­ Pacific Ocean from the . Past subduc­ partment (KEMSD) coordinated geophysical research tion in the north is indicated by an inactive chain aimed at the active volcanoes and zones of volcanic mountains along the spine of the isthmus of Kamchatka. In the 1990s these institutes reorgan­ [Hochstaedler et al., 1994; Kepezliinskas et al., 1997], ised within the Russian Academy of Sciences, and uplifted foredeep sediments e..xposed offshore onKara­ there have been many collaborations between Rus­ ginsky Island [Kovalenko et al., 1999], and late Ceno­ sian and outside scientists to study volcanic processes zoic seafl.oor spreacling in the Komandorsky Basin of and petrology, to measure geodetic movements, and the western Bering Sea [Seliverstov, 1997; 1998]. On to probe the seismic structure of the region. This its western side, Kamchatkais separated from Eurasia paper attempts a synthesis of recent geological in­ by the Okhotsk Sea. vestigations of the Kamchatka plate boundaries, and The volcanic Sredinny Range, now inactive, runs so highlights a range of tectonic scenarios that may along the northern isthmus and the center of south­ be tested with future field observations and studies ern Kamchatka {Figure 3). From the Kuriles to Cape of existing data. A re-evaluation of Kamchatka tee- Kamchatka, an Eastern Volcanic Front (EVF) is pop- 4

ulated by numerous active and inactive volcanic cen­ plate motion, as recorded by the bend in the Hawaii­ ters. Between the two highland ranges lies the Cen­ Emperor chain, but no causal connection has tral Kamchatka Depression (CKD) extending from yet been proposed. In the northwest Pacific at this inland of Petropavlovsk-Kamchatsky to the Bering time, Pacific plate motion switched from northward Sea. Within the CKD lie the northernmost active to west-northwest. Subduction and volcanism in the volcanoes of the western Pacific plate boundary, dom­ western Aleutians ceased in favor of right-lateral tran­ inated by Sheveluch and the group of volcanic cen­ scurrent motion, which has continued into the present ters surrounding Klyuchevskoy. From its southern [ Yogodzinsky et al., 1993). The collision of the Olyu­ extreme, the Kamchatka River runs northeast along torsky terrane with Eurasia may have influenced the the CKD, turning seaward between Klyuchevskoy and Pacific plate-velocity change, but a detailed assess­ Sheveluch volcanoes to a mouth at the Pacific just ment of this relationship remains to be made. south of Cape Kamchatka. Shapfro [1995) argues, from an inferred shortage of coeval arc volcanism along the Eurasian margin, 2.1. Accreted Terranes and the Origin of that prior to collision the Olyutorsky arc lay over a Kamchatka southeast-dipping subduction zone. In this scenario, the arc overrode the Eurasian dur­ Russian East Asia is composed of ter­ ing collision. In the Koryak highlands of northern ranes that accreted to Eurasia [Watson and Fujita, Kamchatka, Garver et al. (2000) report a continu­ 1985; Stavsl..11 et al., 1990; Zonenshain et al., 1990; ous supply of detrital zircons to the Ukelayet Bysch Bogdanov et al., 1990; Wormll, 1991; Til'mann and from the erosion of a nearby during Dogdanov, 1992; Zinkevich and Tsukanov, 1993; Geist the Campanian through Eocene, which may weaken anti Scholl, 1994; Geist et al., 1994; Shapiro, 1995; the assertion by Shapfro (1995) that subduction ceased Seliverstov, 1998; Shapfro et al., 1997; Pechersky et along the Eurasian margin in the latest . al., 1997; Konstantinovskaya, 1999; Nokleberg et al. Assuming southeast-dipping subduction under the 1997; 2001) as recently as the mid-Eocene ( ...... 43 Ma) Olyutorsky terrane, K onstantinovskaya (1999) argues [Garver et al., 2000; Soloviev et al., 2001). Kamchatka that subduction polarity switched after collision ac­ was formed by of island-arc terranes to the cording to the model of Chemenda et al. (1995). In Okhotsk-Chukotka volcanic belt of eastern Siberia, this scenario, the Gana! and Sredinny metamorphic which was active from the mid- to late Cretaceous terranes in south Kamchatka represents burial, heat­ (Figure 4). A complex succession of terranes con­ ing and later exhumation of partially subducted sedi­ nects the main peninsula to the Eurasian mainland. ments. Konstantinovskaia [2001) notes, however, that. This zone is termed the Koryak system by Zonen­ no exhumed high-pressure/low-temperature metamor­ shain et al. [1990), and includes numerous fiysch units phic rocks have been found along the trace of the po­ and [Alekseyev, 1987). These units attest to tential subduction-polarity reversal. Chemenda et al. the accumulation of thick terrestrial sediments on the [1995) predict such "blueschist" metamorphism to be continental margin, and lo the consumption of ocean exhumed in their model. basins as accretion proceeded. Since the Olyutorsky emplacement, three exotic Thrnst over the so-called Ukelayet fiysch in spec­ terranes have accreted to the Pacific coast of Kam­ tacular exposures in the Koryak highlands of north­ chatka. From south to north, these form the coastal ern Kamchatka, the Olyutorsky terrane {termed the peninsulas of Cape Shiµunsky, Cape Kronotsky, and Achaivayam-Valaginsky arc {AVA) by Shapiro [1995), Cape Kamchatka [Konstantinovskaya, 1999). The last Shapiro et al. (1997] and Konstantinovskaya [1999; of these is identified by Shapiro [1980) and Geist et al. 2001)) was the last major terrane to attach itself to (1994) as a former volcanic island of the Aleutian arc, the peninsula [Fhjita and Newberry, 1983; Geist et al., presently thrusting onto Eurasia in a broad crustal 1994), although minor terranes have since accreted deformation zone. Paleomagnetic data have demon­ below). The Olyutorsky terrane extends along (see strated that Cape Kamchatka was far south of Kam­ nearly the full length of the Kamchatka peninsula, chatka in the early Tertiary {66-36 Ma) [Bazhenov and may include the Shirshov and Bowers Ridges of et al., 1991; Pechersky et al., 1997). Transcurrent the Bering Sea, both now submerged. The emplace­ motion along the western Aleutians appears to be ment of the Olyutorsky terrane is argued by Garver distributed between right-lateral strike-slip faults on et al. (2000] to have occurred near 43 Ma. This date both sides of the island chain [Ekstrom and Eng- coincides with the sharp mid-Eocene shift in Pacific 5 dahl, 1989; Geist and Scholl, 1992; Gaedicke et al., ity lack strong subduction trace-element signatures, 2000]. The western Aleutian arc apparentlyconverges which suggests that Komandorsky-plate subduction against Kamchatka as a result of shear-coupling with in northernKamchatka may have weakened or stalled the Pacificplate, albeit at a slower rate. after 10 Ma.

2.2. Cenozoic Kinematics of the 2.3. The Okhotsk Sea: Komandorsky Basin Independent Plate or Extended Continent? In recent plate reconstructions, the Komandorsky Basin originated as a remnant patch of the Pacific To the west of Kamchatka lies the Okhotsk Sea plate, adjacent to the Olyutorsky terrane, and was (Figure 1). Some researchers hypothesize the ex­ later isolated by trench-parallel motion of the west­ istence of an Okhotsk microplate, bordered on the ern Aleutian arc [Seliverstov, 1998; Nokleberg et al., west by a transcurrent shear zone along Is­ 2001). Northeast of the western Aleutians in the land, and on the south and east by the convergent Bering Sea, the Komandorsky Basin underwent an Kui-U-Kamchatka subduction zones [Cook et al., 1986; episode of margin-parallel spreading in the late Ceno­ Riegel et al., 1993; Konstantinovskaia, 2001]. Plate zoic [Baranov et al., 1991]. Margin-parallel backarc reconstructions for Eastern Siberia commonly invoke spreading is uncommon, but consistent with the ob­ the accretion of an Okhotsk terrane, at or near the servation by Jurdy [1979] that spreading directions in emplacement time of the Olyutorsky terrane [e.g. Zo­ Pacific marginal basins tend to align with the mo­ nenshain et al., 1990]. The northern boundary of a tion of the downgoing plate. Jurdy and Stefanick hypothetical Okhotsk plate would be a belt of diffuse [1983) modelled backarc spreading as a response of seismicity within the Cherskii along the the overriding plate to a transient stress increase in­ northern coast of the Okhotsk Sea [Fuj ita et al., 1990; duced by a velocity change in the downgoing slab. A Riegel et al. 1993; Imaev et al. 2000]. Evidence for a portion of the Pacificlithosphere likely subducted be­ plate boundary across the isthmus of northern Kam­ neath the western Aleutians in the mid-Eocene, and chatka is wanting, as no active tectonic structures dis­ later shifted its motion to (largely) trench-parallel, place the Sredinny Range significantly. Because its forming a transcurrent plate boundary. In the Ju­ interior is covered with water, the argument for an in­ ,·dy [1979] scenario, stresses transmitted through the dependent Okhotsk plate depends on kinematic data mantle wedge by this change in slab motion would from its seismogenic boundaries [Seno et al., 1996]. have induced back-arc spreading. Jurdy [1979] in­ These data are subject to different interpretations. terprets magnetic data from marginal seas to argue DeMets [1992ab] analysed earthquake focal mecha­ that backarc spreading staits roughly 5 Ma after a nisms in the I

Constraints on past plate kinematics in the Kam­ have opposite character at depths > 61 km, suggest­ chatka region from surface geology are complemented ing an "unbending" of the slab as it descends below a by constraints of present-day plate dynamics from strongly-coupled zone of shallow thrust earthquakes. seismology. Earthquakes are a direct indicator of North of 54°N, the slab dip shallowsto ,...., 35°. The plate interaction, and seismotectonic diagrams can transition in apparent slab dip occurs over a narrow trace the path of brittle lithospheric material through (,...., 30 km) interval of slab length along the trench, 7 and is marked by an unusual concentration of small native, based on Davi.es and von Blankenburg (1995], earthquakes at 60-100 km depth, where the dips be­ namely, that asthenospheric flow near the slab edge gin to diverge [ Gorbatov et al., 1997). A source m�­ focuses tensional stress on it, and that a slab frag­ anism for one of the larger recorded earthquakes m ment bas detached and sunk into the lower mantle as the contorted zone (Mw=5.8, 22 l\Iarch 1980) shows a result. a stress pattern consistent with local plate torsion i.e. a trench-parallel bending stress. Gorbatov et al. 3.2. Seismic Velocities and Slab Dynamics [1997] notes that, with few exceptions, active volca­ Aseismic slabs can be traced through the man­ noes in Kamchatka Jay above the 90-150-km depth tle with "fast" anomalies in seismic velocity, caused range of Benioff-zone seismicity, so that the change in by their residual "cold" thermal anomalies. Using slab dip correlates with an inland shift of volcanic ac­ residual-sphere analysis, a method that maps tele­ tivity from the Eastern Volcanic Front to the Central seisrnic traveltime anomalies into the near-source re­ Kamchatka Depression. gion, Boyd and Creager [1991] suggested that "fast" Go1·batov et al. (1997] find deep-focus seismicity velocities extended beyond the seismogenic zone in to depart from simple geometry near the slab edge. the western Aleutians. Although the events analysed Deep-focus seismicity is less common in the Kam­ were few and restricted to the western Aleutians, the chatka Wadati-Benioff zone than beneath e.g., Japan, residual-sphere analysis favored a "folded tablecloth" Fiji, and (Figure 5). However, south of 53°N subduction geometry, in which the Pacific slab draped scattered earthquakes occur deeper than 400 km. The over the Aleutian-Kamchatka junction. Because sub­ maximum depth of seismicity shallows to 200-300 km duct.ion in the western Aleut.ians is inact.ive, C1·eage1· within the contortion zone near 54°:'.\, and shallows and Boyd (1991] proposes that.Pacific lithosphere sub­ further to ,.,.,100km beneath the vigorous volcanic cen­ ducted at an oblique angle in the central Aleutians ters of Klyuchevskoy and Sheveluch. In the vicinity and then translated a.seismically towards Kamchatka. of its Aleutian terminus, therefore, the Pacific slab A regional P-wave tomographic study of the north­ loses its defining characteristics at an unusually shal­ west Pacific subduction zones casts serious doubt on low depth. the "folded tablecloth" model [Gorbatov et al., 2000]. Is the slab edge rnissing, or has the slab changed Fast anomalies that would be expected to dip north­ character? Shallow and intermediate-depth seismic east from the western Aleutians are not found, and activity in the Wadati-Benioff zone defines the vol­ the upper mantle beneath the seismogenic slab edge ume of downgoing lithosphere that suffers brittle frac­ is anomalously slow ("'1%). The velocity images fa­ ture. The seismogenic zone is likely cooler and there­ vor the loss of the slab edge, rather than its aseisrnic fore less ductile than surrounding mantle rock, but extension. Flat-lying velocity anomalies in the tran­ brittle rheology may also be fostered by rapid min­ sition zone beneath the Bering Sea [Gorbatov et al., eral phase changes from metastable thermal condi­ 2000} and the southern Okhotsk Sea [van der Hilst tions [Kirby et al., 1996) and fluid-release cmbrittle­ et al., 1991] have been interpret.eel as residual slabs ment from overstepped dehydration metamorphism emplaced by past trench rollback. Fast seismic ve­ [Ague et al., 1998]. Thermal modelling [ Gorbatov locity anomalies in the north Kuriles and southern et al., 1997; Davaille and Lees, 2002] suggests that Kamchatka appear to extend into the lower mantle. conductive heating of ordinary Pacific lithosphere is however, suggesting slab penetration and a firmer an­ too slow to alter its brittle character. Go1·batov et chor for the subduction zone. al. (1997] suggest that the extreme edge of the Pa­ In a P-wave tomographic study restricted to sta­ cific plate is unusually thin when it enters the Kam­ tions and events in southern Kamchatka, Go1'batov chatka subduction zone, and therefore more prone et al. (1999] found larger (...... 6%) anomalies in seis­ to heating, due to thermal erosion associated with mic velocities than found in telescismic studies. The the Meiji of the Hawaii-Emperor hotspot slowest anomalies tended to cluster within the supra­ track. Davaille and Lees (2002] suggest that litho­ slab mantle wedge, directly beneath ("' 30 km depth) sphere beneath the Hawaii-Emperor chain is no thin­ the volcanic front of the subduction zone. Slow sub­ ner than surrounding Pacific plate, but that a thermal �foho mantle P velocities (7.4-8.0 km/sec) beneath anomaly inherited from the Hawaii hotspot induces the peninsula confirm estimates from earlier seismic delayed small-scale convection as the plate subducts. studies [Fedotov and Slavina, 1968; Kuzin, 1973}. The In Section 6 of this paper, we propose a third alter- fastest anomalies reported by Corbatov et al. (1999], 8 as expected, occur within the slab, whose geometry et al., 2002] and surface-wave dispersion [Shapiro et was fixed a priori from seismicity trends. al., 2000] have been used to constrain crustal prop­ Slab structure beneath Kamchatka can be probed erties. Crustal thickness throughout the peninsula with guided waves [Abe1·s, 2000] and teleseismic re­ lies between 3o-40 km in all estimates. Crustal P ceiver functions [Levin et al., 2002}. Using intermediate­ velocities are less than 6.7 km/s in most estimates, focus slab events, Abers [2000] finds unusual disper­ except one region within the Sredinny Range, near sion in P and S waves that can be fit with a thin the Klyuchevskoy volcano group, where lower-crustal (1-6 km) layer of low velocities (5-7% slower than sur­ P velocity (20-30 km) was estimated to be 7.0-7.2 rounding mantle) at 100-250-km depths at a number km/sec in refraction surveys [Utnasin et al., 1975]. of subduction zones in the Pacific. Dry /gabbro Evidence for maficlower crust, argued by Holbrook et and eclogite are too slow and too fast, respectively, al. [1999] to be a defining characteristic of thickened to fit the observations, so Abe1·s [2000] argues for "island-arc" crust, is therefore found only in a limited metastable hydrated oceanic crust (a blueschist as­ area. semblage with lawsonite, chlorite and amphibole) as Simultaneous estimates of crustal P and S veloc­ the main constituent of the low-velocity layer. ities are rare in past studies, but comparison of P Abers [2000] uses seismic station PET (Petropavlovsk­ velocities, e.g. from refraction, and S velocities, e.g. Kamchatsky) to probe the slab waveguide beneath from surface wave dispersion [Shapiro et al., 2000] the northern Kuriles and southern Kamchatka. The typically give Vp /Vs = 1.73 - 1.76, suggesting a fel­ dipping interfaces that bound this waveguide can sic composition. A felsic Vp /Vs ratio is also sup­ be detected beneath PET with teleseismic receiver ported by receiver functions, though the influence of functions (RFs), which reconstruct P-to-S converted anisotropy near the Moho complicates the interpre­ waves (also known as Ps waves) from the reverbera­ tation of Ps converted-wave amplitudes [Levin et al., tive P coda [Pa1·k and Levin, 2000; Levin et al., 2002] 2002]. (Figure 6). Single-station RF stacks from the SEKS portable seismic deployment detect a similar dipping feature at locations along the east coast of Kamchatka [Levin et al., 2002]. The expression of the slab inter­ 4. SEISMIC ANISOTROPY face is more prominent on the transverse horizontal AND MANTLE FLOW component than on the radial, suggesting that P-SH conversion is better developed than P-SV conversion. A major target of the "Side Edge of Kamchatka Such behavior appears common for Ps waves that Slab" (SEKS) portable seismological deployment was convert within the mantle lithosphere [Bostock, 1997; the interaction of slab descent and asthenospheric 1998; Savage, 1998; Levin and Pa1·k, 2000; Yu an et flow under the Kamchatka peninsula. Lattice-preferred al., 2002], and is taken as evidence for anisotropy. orientation (LPO) of olivine and orthopyroxene crys­ Olivine lattice-preferred orientation (LPO) can de­ tals in peridotite is thought to form by ductile flow velop in peridotite rock within a localized shear zone in the upper mantle [Christensen, 1984; !Ube, 1992; along the downgoing slab. In the subduction zone en­ Zhang and Karato, 1995; Zhang et al., 2000]. Olivine vironment, however, contributions to anisotropy from is highly anisotropic, and constitutes 40-60% of the hydrated serpentinites and blueschists may also be mantle above 420 km depth. Mantle flow, both ongo­ significant. ing and fossil, is the likely cause of shear-wave bire­ fringence in teleseismic body waves [ Vinnik et al., 3.3. Continental Crust in a Convergent Arc 1984; Silver and Chan, 1991] and Love-to-Rayleigh Setting? scattering in long-period surface waves [Yu and Park, Although it hosts a vigorous chain of arc volcanoes, 1994; Kobayashi and Nakanish·i, 1998; Levin ancl the crust of Kamchatka has "continental" bulk seis­ Park, 1998b]. In addition, Ps converted waves on the mic characteristics. Early studies included P trav­ transverse-horizontal component of motion can mark eltime studies from local earthquakes [Fedotov ancl sharp anisotropic gradients at depth in the crust and Slavina, 1968; Kuzin, 1973] and large-explosion pro­ mantle [Bostock, 1997; Savage, 1998; Levin and Park, files [ Utnasin et al., 1975; Balesta et al., 1977; Balesta 1998a]. None of these effects occur if the anisotropy and Gontovaya, 1985]. More recently, P-tomography has a near-vertical a.xis of symmetry, so they are use­ [ Gorbatov et al., 1999], Ps converted phases [Levin ful indicators of lateral mantle flow. 9

4.1. Regional Mantle Deformation beyond the Cretaceous "quiet" magnetic zone pre­ dicts that the paleospreading direction within the Shear-wave birefringence (splitting) parameters for slab under Kamchatka should be near-normal to the SK S phases in the SEKS deployment fall into two trench. This would imply trench-normal fast polar­ groups [Peyton et al., 2001). Stations located above ization above the slab, in conflict with observations. the Wadati-Benioff zone (APA ' PET KRO NITL ' ' ' At stations that lie just beyond the Wadati-Benioff TUM) show a trench-parallel fa st-polarization direc- zone, anisotropic fast-polarizations are trench-normal. tion. Stations away from the slab show other fast­ suggesting strain and/or mantle flow across the plate polarization orientations. Trench-normal directions boundary. Splitting at BNG is likely influenced by the at sites near the Aleutian-Kamchatka junction (ESS, distributed transcurrent deformation along the west­ KGB, BNG) rule out a strongly developed trench­ ern Aleutians [Cormier, 1975; Geist and Scholl, 1994]. parallel mantle fabric beyond the plate boundarv Splitting at KGB and ESS argues for flow around and comer, as might be expected in the case of stron g beneath the tattered slab edge. trench-parallel flow. Trench-normal splitting at ESS We analysed long-period surface waves recorded at in particular, argues that the slab does not extend PET for the presence of quasi-Love scattered waves. downdip beyond its seismogenic zone. This inter­ Events with shallow strike-slip source mechanisms pretation agrees with regional-scale P-wave tomog­ favor large-amplitude Love waves, and best facili­ raphy [ Go1·batov et al., 2000] which reports low seis­ tate the observation of mode-converted surfaces waves mic velocity in the uppermost mantle beneath central [Park and Yu, 1993; Yu and Park, 1994; Park, 1997]. Kamchatka. Only a handful of SK S phases exhibit Strong lateral gradients of anisotropy near a station splitting delay times 8t > 1 s, aud these are mostly ca se fundamental-mode Love-to-Rayleigh scattering, recorded at the northern stations. Typically, splitting � which can be observed as anomalous vertical motion delays of 0.4 < 8t < 0.8 s. with formal uncertainties in the Love-wave arrival window. 'Ve present data (lu) of 0.1-0.3 s, define the fa st-polarization trends from three events in Figure 9. Surface waves in the in Figure 7. The splitting delays 8t of Peyton et al. passband T > 100 sec are most sensitive to the prop­ [2001] are significantly smaller than those of previous erties of the mantle at depths e..xcceding 100 km, arc studies that used distant stations to make estimates of largely free of overtone surface waves, and should source-side anisotropy [Kaneshima and Silver, 1992; therefore reflect features of the anisotropic structure Fi scher and Ya ng, 1994]. The small number of ob­ below the lithosphere of the Pacific plate. servations may hide backazimuth and incident-angle variations in splitting values, behavior that could re­ Events 1 and 2 approach Kamchatka along direc­ solve depth variation in anisotropic parameters [Levin tions that are ...... , 45° from the strike of the trench, a ge­ et al., 1999]. ometry that would maximize Love-Rayleigh scatter­ ing in the presence of a strong trench-parallel astheno­ Two lines of evidence support the hypothesis that spheric flow gradient [Yu and Park, 1994]. Event 3 trench-parallel SKS splitting at APA, PET, KRO, approaches PET from the north, and would be sensi­ MIL, and TUM originates below the Wadati-Benioff tive to anisotropy gradients in the back-arc and slab­ zone: weak local-S splitting and the deformation of edge regions. Vertical waveforms from events arriv­ mantle xenoliths (see next section). S waves from ing from the NE and SW (events 1 and 2) contain earthquakes within the Kamchatka Wadati-Benioff little energy in the Love wave time window. This zone traverse the supra-slab mantle wedge and the suggests that neither Love wave encountered strong crust and do not sample anisotropy within and be­ gradients of trench-parallel seismic anisotropy along neath the slab. A group of such events exhibits only their paths. On the other hand, the data for event 3, weak splitting at stations PET and APA (Figure 8). with northerly backazimuth, contains a characteris­ Peyton et al. [2001J interpret the anisotropy implied tic quasi-Love phase on the vertical component in the by SKS splitting in terms of trench-parallel astheno­ Love wave time wi ndow. The exact location of the spheric flow beneath the Pacific plate. As for alterna­ anisotropic gradient responsible for this qLove phase tive interpretations of SK S splitting, we doubt that may only be determined through waveform modeling, the strain resides entirely in the slab itself, whether but the modest time separation between the parent due to along-trench extension or fossil fabric. Present­ Love and the "daughter" qLove phases implies that day slab extension would not occw· without similar an anisotropic gradient zone lies within 1000 km of flow in the adjoining asthenospbere. As for fossil the receiver. slab fabric, an extrapolation of magnetic anomalies 10

Weak or absent Love-to-Rayleigh scattering for developed corner flow in the mantle wedge is consis­ events 1 and 2 suggests that the mantle strain field tent with the tank experiments of Buttles and Olson offshore Kamchatka is less disturbed by subduction (1998]. than at other subduction zones. Quasi-Love waves ob­ Levin et al. (2002] analyze both radial- and transverse­ served by Yu and Park [1994] in :'.'lew Zealand had am­ component RFs in bin-averaged epicentral and back­ plitudes ,...,.253 relative to the incoming Love ampli­ azimuth sweeps, in order to detect Ps moveout and tudes, and could be modelled with a 100-km astheno­ polarity variations diagnostic of interface depth, in­ spheric layer in which 4% shear anisotropy rotates terface dip, and anisotropic fabric within the shal­ from trench-normal to trench-parallel ...... 500 km sea­ low mantle and crust. At some stations the radial ward of the I

Two-component mantle xenoliths have been iden­ basalts is indicated by the absence of a crustal magma tified in other arc-volcanism locales and have been re­ chamber beneath the volcano [Balesta et al., 1977; lated to infiltrating subduction-related and/or crustal Ozerov et al., 1997). Seismic sounding suggests that melts [Ertan and Leeman, 1996; Di1cea and Saleeby, basaltic Klyuchevskoy has a near-direct pipeline to 1998; Arai et al., 2000] . Mafic pyroxenite veins in the mantle, in contrast to the neighboring andesite­ upper mantle peridoties have been invoked to explain dacite Bezymianny volcano, beneath which a mid­ Os isotope variations in Hawaiian post-erosional vol­ crustal magma chamber is detected. If the Kelemen canism [Lassiter et al., 2000]. Such late-stage hotspot [1990] wallrock interaction model is a significant con­ volcanism taps a mantle source that has already un­ trol on calc-alkaline volcanics, one would expect the dergone the main-stage interval of tholeiitic volcan­ eruptive centers with extreme output volumes to fa­ ism [Ribe and Christensen 1999], and so plausibly vor the tholeiitic trend. CKD volcanism appears to had been infiltrated (i.e. "enriched" ) by basaltic melt fit this expectation. More comprehensive sampling of residuals. In northern Kamchatka, slab-melt metaso­ Kamchatka volcanics and updated estimates on vol­ matism could have occurred as a byproduct of sub­ canic productivity are needed, however, to verify the ducting the young, warm Komandorsky plate in the relationship properly. early Neogene (rvl0-25Ma), and become the source of "adakite" chemistry long after subduction had ceased. 6. DISCUSSION: The discordant dunite model of Kelemen [1990] re­ DYNAMICS OF A SLAB EDGE turns this section's exposition to its starting point. Kelemen [1990) proposes that an ascending tholei­ Plate-boundary dynamics in Kamchatka are com­ itic basalticmelt reacts chemically with the peridotite plex, and any explanation must fit data from both that bounds cracks and pore spaces, exchanging or­ present-day and past seismotectonics, plate motions thopyroxene (Opx) for olivine (Ol) in the wall rock. and volcanism. In this discussion, we distinguish The mantle develops "discordant" dunite veins in its three competing hypotheses for Quaternary tectonic harzburgite/lherzolite country rock as this interaction processes that govern the present geodynamics of the proceeds, and the basaltic melt composition evolves Aleutian-Kamchatka corner. We also consider how along a calc-alkaline trend, enriching more in silica the Cenozoic tectonic evolution of Kamchatka, since and AhOa than Fe as it evolves toward andesite and the Olyutorsky collision in the mid-Eocene, influenced dacite. Kelemen [1990] argues that "slow" ascent the transfer of active volcanism from the Sredinny of a tholeiitic melt through a hot mantle wedge will Range to the Eastern Volcanic Front. generate ca.le-alkaline magmatism at the surface. A Constraints on the timing of events in Cenozoic definitive distinction between "slow" and "fast" as­ Kamchatka volcanism and tectonics are farfrom com­ cent cannot be made without knowing the kinetics of plete, so our comparison of scenarios can only be pro­ 01-0px replacement reactions; Kelemen [1990] bases visional. Holocene volcanism has been dated with his model predictions on equilibrium thennodynam­ modern l4C methods [Braitseva et al., 1995], but the ics. If we add a third possibility that some ascend­ application of other modern radiometric methods to ing melts solidify as veins in the wedge to enrich the older igneous rocks is thusfar limited. Such studies are mantle, then Kelemen's conceptual model might be only beginning [e.g. Honthaas et al., 1995; Garver et extendable to the adakite problem as well. al., 2000; Hou1'igan et al., 2002]. Older syntheses of The dunite-replacement model forcalc-alkaline vol­ Kamchatka volcanism and tectonics [e.g., Erlich and canism can e>.."J>lain variations in eruptive style on Gorshkov, 1979] rely largely on stratigraphic relation­ a small spatial scale with correlated variations in ships, glaciation indicators and paleomagnetic data. magmatic fltL"< in either space or time. Small-scale eruptive variability is less easily explained by either 6.1. Quaternary Tectonic Scenarios wedge depletion [Miyashiro, 1974] or melt-column [Plank and Langmt1ir, 1988] mechanisms. Within We outline here three competing interpretations. the central Kamchatka depression, Hochstaedler et We do not claim that other scenarios, not discussed al. [1996] report the Klyuchevskoy volcanic group here, would not be possible. In addition, scenario as more tholeiitic than Sheveluch. The Klyuchevskoy details could be exchanged among the following three volcanic group also releases magma at a much faster hypotheses; they are formulated mainly to focus the rate. The vigor of the mantle source of Kluchevskoy interpretation of geologic data. The last hypothesis is the scenario favored by the authors. 14

(1) Steady-state geodynamics. The configuration of often initiate on the flanks of older calderas; Brnit­ slab and volcanic centers has undergone little if any seva et al. [1995] date most of these to be 30-40 ka. Quaternary change. Steady heating of its exposed For instance, ashes in the Bering Sea dated at 30 ka edge erodes the slab's thermal anomaly, erasing all a.re interpreted to be from the caldera eruption that symptoms of brittle and stiff rheology. Steady melt­ preceded the formation of Sheveluch. Radiocarbon ing of subducted crust at the slab edge contributes dating cannot be used to date volcanic events much adakite magma to Sheveluch volcano. older than 50 ka, but Erlich and Gor·shkov [1979) ar­ (2) Accelerated slab rollback. The Central Kam­ gue from other data (e.g. stratigraphic relationships) chatka Depression ( CKD) is an extensional graben as­ that the last "'50 kyr has been the most active time sociated with slab rollback stresses. The Kamchatka interval of Quaternary volcanism. arc is splitting into two arcs, Sredinny and Eastern Quaternary volcanism in the Sredinny Range and Volcanic Front (EVF), similar to past rift initiation farther west, aligned along the extension of the Aleu­ within the Phillipine plate [Karig et al., 1971; Hus­ tian island chain, poses a further problem for the song and Uyeda, 1982), and along the present-day steady-state model. Ta tsumi et al. [1994; 1995] in­ Tonga-Kermadec arc [W1'ight et al., 1996]. Rifting be­ terpret this volcanism in a steady-state context as a gan in response to an abrupt steepening of the Kam­ "third volcanic chain" of the Kamchatka arc, sup­ chatka slab, which has brought down a pulse of hy­ plied by a hydrous peridotite layer entrained down­ drous sediment to boost volcanism in the CKD (Fig­ ward with the slab - see also Churikova et al. (2001). ure 13). Quaternary volcanism represents a trans­ Seismic tomography and anisotropy argue that the fer of magma supply from the Sredinny Range to the slab edge is absent beyond the CKD [ Gorbatov et EVF as the slab steepens. The slab edge is lofted al., 1997; 2000; Peyton et al., 2001), so present-day by asthenospheric flow from the Pacific to the North mantle-wedge entrainment is unlikely. American plate back-arc region. Asthenospheric flow The hypothesis of accelerated slab rollback is mo­ induces frictional heating that erodes the slab edge. tivated largely by (1) an interpretation of the CKD (3) Slab-edge pinchofj. Slab rollback is gradual as an extensional graben, developing now while arc or absent. Local geodynamic processes have been magmatism shifts from the Sredinny Range to the pulsed by the recent pinchoff of the slab edge via EVF, (2) the torsion of the Kamchatka slab edge, per­ a necking instability [Davies and von Blankenburg, haps in response to asthenospheric flow around it as 1995]. The tattered slab edge has lofted in response it retreats [Ki ncaid and Olson, 1987; Buttles and Ol­ to the loss of load or to asthenospheric flow, and son, 1998, Hall et al., 2000; Peyton et al., 2001), and has induced uplift and pressure-release partial melt­ (3) a hypothetical geochemical cause for the increase ing in the overlying mantle (Figure 13). CKD mag­ in volcanic output since 50 ka. Several observations matism taps depleted mantle-wedge peridotite under are at variance with this model, however. Weak de­ the Kluchevskoy group, and adakite-metasomatized velopment of trench-parallel anisotropy beneath and mantle under Sheveluch. When the dense slab frag­ seaward of the I

The possible mechanisms for the third scenario, high velocity signature of a slab is seen both beneath slab-pinchoff, include (A) thermal instability leftover the northern and southern Apennines in the tomo­ from the Hawaii hotspot [Davaille and Lees, 2002]; graphic models, Wo1·tel and Sp akman (2000) conclude (B) ductile instability induced by a negatively buoy­ that slab detachment has occurred beneath central ant cold slab [Davies and von Blankenbmy, 1995], Italy i.e., under the Roman magmatic province. perhaps enhanced by the mass heterogeneity of the Trenchward of the slab window, a lofting of the hotspot track; (C) ductile instability induced by shear residual slab near the Aleutian junction would plau­ concentration at the slab edge associated with roll­ sibly lead to more intense volcanism. The volcanism back fl.ow. The relative merit of these causative mech­ would be more arc-like in trace elements, because the anisms is not clear, but the likely consequences of slab chemical composition of the upwelled mantle wedge pinchoff are suggested by several lines of evidence. A above the truncated slab would not differ much from negatively-buoyant slab fragment would fall rapidly a typical depleted arc peridotite. Uplift may generate 5 out of the upper mantle (10 -yr time scales, using melts from portions of the wedge that have accumu­ Stokes-flowas a model), and be replaced by upwelling lated the slab-derived hydrous component, similar to mantle. that identified by Kepezhinkskas et al. (1997], over a This upwelling flow might cause the observed tor­ significant time interval. Melt extraction after grad­ sion of the residual slab edge, but the loss of load after ual hydrous infiltration might explain the relatively slab pinchoff could also be a factor. Positive buoyancy long {>150 kyr) fluid residence times indicated by the in the slab edge may be present if, as Abers (2000) U-series disequilibrium study of Turner et al. (1998). suggests, the oceanic crust has resisted the eclogite Sisson and Bronto (1998] argue that a defining phase transformation to depths of 200 km or more. indicator of pressure-release melting in arc systems The time since pinchoff is limited by plate conver­ is a high equilibration temperature between basaltic gence rates and the present-day length of the trun­ melt and wedge peridotite, as estimated from mineral­ cated slab. Assuming a 80 mm/yr convergence rate, composition thermometry. The model of Ozerov et al. a 240-km segment of subducted slab edge would limit (1997] for the generation of primary high-magnesian the pinchoff to be within the last 3 Myr. Therefore, basalts under Klyuchevskoy suggests that partial melt rebound subsequent to slab pinchoff can plausibly be accumulates over 20-50 kbar pressure (correspond­ related to Quaternary volcanism near the Aleutian­ ing to 60-150 km depth) at 1350°-1400°C. Ozerov Kamchatka corner. [2000] models the fractionation of high-magnesian Above a new "slab window," weak pressure-release basalts to high-alumina basalts to occur over a P­ partial melting would be expected to produce widely T range of 1100° (7 kbar) to 1350°C (19 kbar). distributed alkalic basalt [Hole et al., 1991). The slab­ Kersting and Arcult£s [1994] report minimum crys­ window scenario is broadly consistent with Quater­ talization temperatures for Klyuchevskoy magmas of nary volcanism in the Sredinny Range and western 1100°-1200°C, depending on which rock sample and Kamchatka opposite the Aleutian corner, where flood mineral-system thermometer are applied. In these basalts are cited by Edich and Gorshkov [1979] and estimates, Klyuchevskoy magmatic processes occur Fedorov and Shapiro (1998). Both Vo lynets [1994) and at higher temperature than for typical arc-magma Fedorov and Shapiro (1998) argue for a rising man­ systems, though perhaps not as extreme (1320°C tle diapir as the source of inland Quaternary vol­ at 12 kbar) as the equilibrium P-T conditions Sis­ canics that are "i ntraplate" in their trace-element son and Bronto (1998) report for water-poor basalts signatures. Fedorov and Shapiro [1998] draw a par­ from Galunggung, . Finally, a geodynamic, allel between the trace-element signature of the Al­ rather than geochemical, impetus for the recent burst nei flood basalts of the Kamchatka isthmus and the of slab-edge volcanism can accommodate the petro­ leucite trachytes of the Roman magmatic province. logic variations reported between Klyuchevskoy and The Romananalog is suggestive, because central Italy Sheveluch. The upper mantle beneath Sheveluch may may present a related scenario. In P-wave tomogra­ have been metasomatized by slab melt leftover from phy models, the 50-250-km depth range of the upper the Miocene subduction of the young, warm Ko­ mantle beneath the central Apennine mountains and mandorsky plate, as suggested by mantle xenoliths the Roman magmatic province lacks a high-velocity from past volcanism from the Kan1chatka Isthmus anomaly, suggesting that subducted Adriatic litho­ [Kepezhinskas et al., 1996]. This secondary melting sphere is missing [Lucente et al., 1999]. Since the mechanism, and not the direct melting of the Pacific 16 lithosphere edge, may explain Sheveluch's adakite history especially difficult. The same can be said of chemical signature, and link its eruptive chemistry the Cenozoic tectonic history of the peninsula, as the with that of Neogene volcanoes of Northern Kam­ accretion of the Cape terranes is not dated secuTely. chatka. Many authors suggest a causal relation between the A major distinction between Quaternary hypothe­ accretion of the Cape terranes to southern Kamchatka ses (2) and (3) is predicted regional uplift. Abrupt and the eastward shift of volcanism from the southern slab rollback should induce subsidence in the back­ Sredinny Range to the Eastern Volcanic Front. Be­ arc region, at least until asthenospheric mantle man­ cause the constraining data are thusfar few in num­ ages to flow from the Pacific to the North American ber, proposed models differ significantly among dif­ side. The loss of a slab edge would lead to a grav­ ferent researchers. In this subsection, we suggest a ity low and asthenospheric upwelling to fill the gap. range of scenarios that have been proposed by, or can The tattered slab edge might be lofted by upwelling be extrapolated from, recent research in the region. flow, or else by loss of the downdip load if its crust As with the Quaternary tectonic scenarios, we do not is metastably buoyant. Both pinchoff scenarios pre­ claim our discussion to be conclusive or comprehen­ dict transient uplift in central Kamchatka. Though sive, but rather to highlight futme research opportu­ the detailed sequence of volcanism and uplift are as nities in the Kamchatka region. yet unknown, such uplift mightexplain why the Kam­ Two end-member scenarios for the formation of chatka river does not flow to the ocean along the en­ the Aleutian-Kamchatka junction, and the isolation tire length of the low-lying central Kamchatka depres­ of the Komandorsky Basin from the Pacific plate, sion to its coastline near Cape Ozernoy on the Bering can be recognized in recent research. The scenarios Sea, but rather turns toward the Aleutian-Kamchatka differ in how the Cape terranes accreted to south­ corner northeast of Klyuchevskoy. Rivers will change ern Kamchatka. In one case, the junction formed in course in response to vertical tectonic movements [e.g. the latest Cenozoic with the accretion of Cape Kam­ Mather, 2000]. Er-Zich and Gorshkov [1979] interpret chatka in its present position at 56°N. In the alternate the CKD as a shallow marine basin as late as the case, the junction formed somewhat earlier and far­ lower Pliocene ("-'3 6 Ma), so it is likely that the ther south, with the accretion of Cape Shipunsky, and Kamchatka river would have Bowed all the way to migrated northeast as a triple junction between the Cape Ozernoy unless the CKD uplifted unevenly. The North American, Pacific and Komandorsky plates. slab-pinchoffhypothesis predicts that transient uplift The accretion of Cape Shipunsky and Cape Kro­ of central I..istence of a Kronotsky island arc in the open Quaternary volcanic and continental deposits" on it. Cretaceous-Paleocene ocean (Figure 14), followed by The Kumroch Range is plausibly coeval with the translation of this arcfrom middle paleomagneticlati­ thrusting of the terrane collision that formed Cape tudes to its cuuent location on the Kamchatka coast­ Kamchatka, but this event is also not yet dated with line ("'55°N). Levashova et al. [2000] report paleo­ confidence. Hypothesis (3) therefore makes specific latitudes in the range 38°-45°N for the Kronotsky predictions regarding the sequence of volcanic and terrane. Note that the Kamcl1atka peninsula, as part tectonic events in central Kamchatka. These predic­ of the , is inferred to have mi­ tions should be testable with future field studies of grated southward a roughly equal interval of latitude the area. during this time. Although paleomagnetism of Cape Kronotsky and Cape Kamchatka has seen greater 6.2. Cenozoic Tectonics study, the island-arc affinity of the more southerly Since the Olyutorsky Collision Cape Shipunsky is also recognized, so that multi­ ple small island arcs are commonly hypothesized in The scarcity of radioisotope ages for Kamchatka plate reconstructions [e.g., Seliverstov, 1998]. The rocks makes reconstruction of its Cenozoic volcanic 17 demise of an ocean basin that formerly lay between volve continental lithosphere [ Wortel and Sp akman, Kamchatka and the accreted arcs has commonly been 2000), ocean-ocean subduction zones are more com­ cited forthe late-Cenozoic jump in volcanism from the monly 500-km or more in length. Also, each closure Sredinny Range to the EVF. of an ocean basin would have left a relict slab un­ Alternatively, in some early reconstructions [Jur·dy der the south Sredinny Range. No evidence for this and Gordon, 1984; Gor·don and Jurdy, 1986; Bazhenov has been reported, though a future interpretation of et al., 1991) the Pacific-North American plate bound­ late-Cenozoic volcanism in southern Kamchatka may ary was placed near the southern tip of Kamchatka in suggest slab rollback and/or detachment. the middle Eocene. However, the contemporary gap Triple-junction migrationpredicts an unusual mech­ in paleolatitude between Kamchatka and the Cape anism for the transfer of mid-Cenozoic island-arc vol­ terranes [Levashova et al., 2000) argues for a signifi­ canism from the Sredinny range to the EVF. The cant gap at the time of the Olyutorsky collision. Such relict 1vliocene spreading center in the present-day a gap between Kamchatka and the future Aleutian arc Komandorsky Basin suggests that the oceanic litho­ is a feature of more recent tectonic reconstructions sphere of the late-Cenozoic Komandorsky plate was [Seliverstov, 1998; Nokleberg et al., 2001). These new young and buoyant. After the accretion of Cape reconstructions, however, do not rule out the forma­ Shipunsky has isolated the Komandorsky from the tion of the Aleutian-Kamchatka junction at a south­ Pacific plate, the triple-junction migration scenario ern position at a later time. Rather than residing has the young, buoyant Komandorsky plate subduct on separate island-arc segments, the three Cape ter­ at a shallow angle and generate arc volcanism in the ranes could have formed as part of what we now rec­ inland Sredinny Range, forming a near-continuous ognize as the Aleutian arc. If so, they could have arc from near Cape Shipunsky to the Koryak high­ been thrust singly onto the east Kamchatka margin lands. Pacific plate subduction south of the Aleu­ as the triple junction moved north (Figure 15). Inthis tians would involve older denser lithosphere, subduct­ "caulking-gun" scenario, the former islands accreted ing at a steep dip. Therefore a slab tear would likely to Kamchatka as the Komandorsky triple junction develop at the Aleutian-Kamchatka triple junction migrated northeast from an initial location near the and propagate northeast with it in the late Ceno­ southern tip of Kamchatka, emplaced by transcurrent zoic. As the triple junction migrated up the Kam­ shear along the Pacific-North American plate bound­ chatka margin, the Eastern Volcanic Front would de­ ary. Such shear is now distributed across the western velop trenchward of the Sredinny Range, shifted both Aleutian arc [Ekstrom and Engdahl, 1989; McCafjrey, by the slab tear and any accreted Aleutian material. 1996]. Ave Lallement and Oldow [2000] argue that the Due to post-collisional igneous activity and/or the ef­ island chain experiences extensional thinning along fects of transient "slab windows," volcanism in the its length, consistent with the breakup of the Aleu­ Sredinny Range would persist after the triple junc­ tian arc into tectonic blocks of a few volcanic centers tion had passed, but with changes in style and chem­ each [Geist and Scholl, 1994; Myers and Frost, 1994]. ical signature. The successive accretion of individual islands along Ifthe Aleutian-Kamchatka triple junction migrated, the Kamchatka coast would be facilitated by along­ when did it start and stop moving? The emplace­ arc extension. ment of Cape Shipunsky likely bounds the starting A conclusive evaluation of the above models is not time, and the cessation of Komandorsky subduction possible at present. However, we favor the caulking­ bounds the stopping time. The collision time of Cape gun model, in which the Cape terranes accrete as a Shipunsky is currently unknown. Hochstaedler et al. byproduct of triple-junction migration, over the "mul­ [1994] and Honthaas et al. [1995] report "island­ tiple arc" model, in which separate accretion of the arc" magmatism in Northern Kamchatka as late as Shipunsky and Kronotsky capes involves the closure 15 Ma and 10 Ma, respectively, and "adakitic" mag­ of one or more small ocean basins. Our choice partly matism as early as 6-8 Ma. Adakiticmagmatism does stems from a bias toward a tectonic model with fewer not preclude the contnuedi subduction of the young moving parts. Multiple island arcs imply the forma­ Komandorsky lithosphere, but indicates that conver­ tion and maintenance of short (100-200 km) subduc­ gence has changed character. tion zones in the open ocean {Figure 14). Although An important timing constraint may be found in the length scale of subducting lithospheric fragments the revised history of Pacific absolute plate motions is observed to be short in convergence zones that in- (APM) by Wessel and Kroenke [1997; 2000], who of- 18 fer multiple lines of evidence for a significant change ter of unusually vigorous volcanoes, the Klyuchevskoy in Pacific plate motion during the 2-6 Ma interval in group and Sheveluch. the hotspot reference frame. Induced by the stalled The pattern of earthquake activity and estimates subduction of the Ontong Java Plateau, the inferred of seismic anisotropy from SK S splitting and Love­ change in local Pacific plate velocity is largest in the Rayleigh scattering suggest that mantle fabric tran­ southern hemisphere. Nevertheless, the Pacific plate sitions from a weakly developed trench-parallel fast­ APM near the Aleutian-Kamchatka junction is pre­ axis orientation beneath the seismogenic slab to trench­ dkted by Wessel and Kroenke [2000] to increase from normal near its northeast edge. Comparison with rv70 to 130 mm/yr, and to deflect "" 15° counter­ other arcs, particularly Tonga-Kermadec, suggests clockwise. The Pacific APM change does not de­ that slab rollback beneath Kamchatka is weak. Inthe termine changes in relative plate motions uniquely, supra-slab mantle wedge, path-integrated anisotropy but the magnitude and sense of the change is instruc­ from local S splitting is weak, but anisotropic gradi­ tive. If we assume that the present-day Pacific mo­ ents in the upper 50 km of the lithosphere induce tion of Wessel and Kroenke [1997] is parallel to the strong conversions of teleseismic P- to $-polarized western Aleutians, the Miocene-Pliocene shift in their scattered waves. Shallow near-Moho anisotropy ap­ APM model predicts rvl8 mm/yr convergence prior pears to have a trench-normal symmetry axis under to the shift. Because evidence for mid-Cenozoic sub­ the EVF, but stations in or adjacent to the CKD have duction under the western Aleutians is scarce [Yo­ more variable symmetry-axis orientations. This sug­ godzinski et al., 1993], this "convergence" can be in­ gests that any shear-coupled corner Bow in the Kam­ terpreted as northeast migration of lhe Komandorsky chatka wedge is limited to the immediate vicinity of triple junction, in agreement with the caulking-gun the trench-volcano arc. model. If 18 mm/yr is interpreted as a northeast mi­ Kamchatka plate dynamics have been transient on gration velocity, with plate convergence transferred to several time scales. Upper-mantle tomography and the Komandorsky-North America convergent margin, the ratio of seismic-moment release to inferred plate the 400 km between Cape Shipunsky and Cape Kam­ motion suggest that the downgoing Pacific lithosphere chatka implies -22 Myr of plate motion. The true is anchored to the lower mantle via slab penetration Pacific-Komandorsky motion is surely somewhat dif­ and strongly coupled to the overlying North Amer­ ferent, but the rough velocity estimate supports the ican plate, while the adjoining Kurile and Aleutiau feasibility of the triple-junction migration hypothesis. have suffered rollback at intervals over the Ifthe Wessel and Krnenke [2000] APM shift caused past 100 My. Rifting of the Komandorsky Basin the cessation of Kommandorsky subduction, then and its subduction under northern Kamchatka ceased subduction persisted at least until 6 Ma, and perhaps sometime since 10 l\la, perhaps in response to a as recently as 2 Ma. The latter date raises the possi­ change in absolute Pacific plate motion starting at bility that the cessation of Komandorsky subduction 6 Ma. Several lines of evidence suggest that the slab and the loss of the Pacific slab edge manifest a single edge has suffered the loss of downdip material since 2 regional tectonic event. This possibility deserves at­ Ma, either from thermo-convective erosion or a neck­ tention, even though it is an extreme interpretation ing instability. Evidence for Quaternary uplift and of the scant geological data now available. enhanced volcanism near the Aleutian-Kamchatka junction are consistent with the recent catastrophic loss of a slab fragment. The vigorous volcanoes of the 7. SUMMARY CKD are quite young ( < 50 ka). The cow-se of the Kamchatka River may have been deflected towards Present-day Kamchatka plate dynamics involve the Pacific coast away from the CKD by vertical the subduction of a finite slab, terminated at the tectonic movements. Alkalic Quaternary volcanism Aleutian-Kamchatka junction. The edge of the sub­ in the Kamchatka in terior may be related to broad ducting slab is tattered and torsioned into a dip shal­ "slab-window" asthenospheric upwelling induced by lower than that of the main I

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Jeffrey Park, Vadim Levin, Mark T. Brandon, Dept. of Geology and Geophysics, Box 208109, Yale University, New Haven, CT 06520 (email: [email protected]) Jonathan M. Lees, Department of Geological Sci- ences, Campus Box 3315, Mitchell Hall, Univ. of North Carolina, Chapel Hill, NC 27599-3315 Valerie Peyton, U.S.G.S., Seismo Lab, Bldg 10002, Kirtland AFB-East, Albuquerque, NM 87115 Evgenii Gordeev and Alexei Ozerov, Russian Academy of Sciences, Far Eastern Branch, Petropavlovsk-Karnchatsky Book chapter· in press with "Plate Boundary Zones, " edited by Seth Stein and Jeff1·ey Ff·eymuller

1Yale University, New Haven, CT, USA 2University of North Carolina, Chapel Hill, NC, USA This preprint was prepared with AGU's lJ\'fEX macros 3U.S. Geological Survey, Albuquerque, NM, USA v5.0l. File pbz'cam formatted Marcll 15, 2002. 26

135° 150° 165° 180° 195°

65° 65°

60° 60°

55° 55°

50° 50°

45° ) (Pacific Plate) 45°

135° 150° 165° 180° 195°

Figure 1. The North Pacific Region. The active boundary between the Pacificand North American plates (dashed grey line) extends westward from the Alaskan mainland along the Aleutians to Cape Kamchatka, and continues SW along the coast of southern Kamchatka to Northern Japan via the Kurile Island chain. The inferred boundary between the Emasian and North American plates is indicated with a dashed grey line, based on DeMets (1992ab). Arrows indicate the sense of present-day Pacific-plate motion, relative to North America. 27 156° 160° 164° 168° 172°

• Shevelu�h ° 64° Ii. 64

r--�- r-.... � � Ushkovs�_,.,- � •: Klyuchev oy .8 Bezymian y ... -� Tolbachik 0 Q) Q. {J 60° 60° .

· \s. '"s K'/ Kar a9 P Ko mando rsky Basin

� Q) > ° 56° a: � � 56 m ... � �Q .::it:. � � t:I - � m � .c � � ' $� �$?. (,) �� �� $ E � m ro$ C\� � 1;. f$)� o � � �ro$ o� 52° 52° ��· "'� �� ;;t Petropavlovsk- ""' · Kamchatsky �

156° 160° 164° 168° 172°

Figure 2. Map of Kamchatka. Solid triangles denote the location of volcanoes classified as "Holocene" in recent global compilations. However, the dates of past activity for inland volcanoes is poorly determined at present due to scarce radiometric dates. Er-Zich and Gorschov (1979] identify most inland activity as "Quaternary" based on stratigraphic constraints. The inset presents the major volcanoes of the Central Kamchatka Depression (CKD). 28

Figure 3. Topographic relief and major terranes of the southern Kamchatka peninsula. Solid triangles denote the location of volcanoes classified as "Holocene" in recent global compilations. However, the dates of past activity for inactive volcanoes in the Sredinny Range is poorly determined at present due to scarce radiometric dates. 29

• CapeKamchatka terrane, mid- Cretaceous tolate Eocene oceanic arc

1'V:71 Vetlovsky subduction complex, formed lL..!.J in ?early Tertiary (covered: lighter grey) Olyutorsky terrane: Cret oceanic arc (covered: light gray), and associated metamorphic rocks(o) � Continental arc, mid-Cretaceous to IL!!J early Tertiary volcanics and plutons 1.-:-1 Ukelayetflysch: mid-Cretaceous to late L....:...J Eocene forearc basin

l7M1 Accreted terranes: assembled before � Late Cretaceous

Continental margin units, mainly ._���� ....� --����������---t D Pa�ozoic

Figure 4. Tectonic map of the Kamchatka region, adapted from Gar'Ver et al. [2000]. The Russian Far East was formed by accretion of a succession of island-arc, ophiolitic and flysch terranes, starting in the Mesozoic. The last large terrane to arrive was the Olyutorsky terrane, which now straddles the Aleutian-Kamchatka junction. 30

Kamchatka-Kurile Seismicity

148° 152° 156° 160° 164° 168°

54°

48°

0 Mb = 4.5

Figure 5. Intermediate {75-200km) and deep (> 200km) earthquake epicenters for the Kamchatka region, located for the period 1964-1999 by Engdahl et al. (1998]. All events with M > 4.5 in the catalog are plotted. Note the scarcity of deep earthquake hypocenters under the Kamchatka peninsula itself, suggesting the loss of the slab edge. 31

PET Receiver Function Sweeps, 2 Hz cutoff frequency Predicted polarity of slab Ps phase: triangle - Ps negative; circle - Ps positive Radial RF Transverse RF

360

330

300

270

240

210

180

150

120

90

60

30

0

0 s 10 15 20 25 0 s 10 15 20 25 Ps Delay Time (sec) Ps Delay Time (sec)

Figure 6. Back-azimuth dependence of receiver functions for station PET (Petropavlovsk-Kamchatsky, Russia). P-coda from 442 earthquakes at teleseismic distances were bin-stacked using the multiple-taper receiver function estimator of Park and Lev-in [2000]. Transverse RF amplitude is boosted by a factor of two for visual comparison. Note the strong transverse pulses at 7- to 10-sec time delay at 150°-200° back-azimuth. This signal appears to be a complex P-to-S conversion from the steeply-dipping slab beneath PET. The curved line indicates the predicted delay and polarity for a Ps converted wave at the interface between a 55°-dipping slab and overlying mantle wedge. The polarity of the double-pulse conversion on the transverse RF is consistent with Ps conversions from the top and bottom of a low-velocity oceanic crust. 32

I' TKl-,h. 60N 250 km ., '\" �,/_, OSO?Y -t { _,//TIG 1 sec fUHRTUM ��GB 56N ESS� �\ . & z : �<=> h Mil./ <§5 /!I � KRO ti"'� I • PET .,. 52N '°",§ I/ � I I I � ...._� 156E 160E 164E 168E

Figure 7. Shear-wave splitting observations at permanent seismological station PET and a portable broadband seismic network in Kamchatka region. Arrows represent single-record birefringence observations. The contours of the Wadati-Benioffzone under Kamchatka are adapted from Gorbatov et al. (1997). The thick gray arrow shows the direction of Pacific plate motion (subduction along the Kamchatka trench and transcurrent motion along Bering Fault). The transcurrent boundary, distributed across the overriding North American plate [Geist and Scholl, 1994), is indicated by two thin grey lines. Two volcanoes are marked on the map: K - Klyuchevskoy; S- Sheveluch. 33

55°N splitting of S waves / from local earthquakes I

I 54°N

53°N

52°N 0.5 sec

100 km

157°E 158°E 159°E 160°E

Figure 8. Shear-wave splitting observations from local S waves at stations APA and PET. Arrows represent single­ record birefringence observations, placed at the epicenter of earthquakes in the Kamchatka subduction zone. Lo­ cations are from the reviewed catalog of the Kamchatka Experimental-Methodical Seismic Department (KEMSD). Note the scale difference for splitting delay &t relative to Figure 7. 34

60°N

0� 1000 2000 gh

R Love @ T 56°N

v 0 1000 eg .��-v---v�A"""'v-v f\-AA J��� i h Lovevv- iv•vvv- 520N T 250 km

48°N 0 1000 2000 156°E 160°E 164°E 168°E time, sec

Figure 9. Quasi-Love waves observed (and not observed) at GSN station PET (Petropavlovsk-Kamchatsky). Horizontal motion is plotted for the radial (R) and transverse (T) directions. The expected time windows for quasi-Love waves on the vertical (V) components are indicated by arrows beneath the Love waves. A quasi-Love scattered wave is most evident for event (3), at northerly back-azimuth. The QL wave for event (3) is distinguished by an early pulse on the vertical component that separates from the main Rayleigh wave. In contrast, the Rayleigh wave for event (2) extends into the Love-wave window, but waveform appears to be spread by dispersion, not a precursory phase. Events (1) and (2) approach at roughly 45° and 135° to the Kamchatka trench, optimal back­ azimuth for Love-Rayleigh scattering by a local concentration of anisotropy with a trench-parallel fast-polarization. 35

!CRUSTAL I ITHICKNESS

Figure 10. Crustalthickness values derived from individual models. Depth to the bottom of the lowest layer with Vs < 4.0 km/sis shown. Asterisks mark sites where we believe the transition is gradational. Values for ZUP and PET are derived on the basis of comparing their transverse RFs with that of KRO, and thus are shown with a question mark. Black arrow shows the direction of convergence along the Kamchatka trench. Thin black lines in the ocean denote the depth contour of 5500 m, providing an outline of the Kamchatka and Aleutian deep-water trenches. 36

MANTLE ANISOTROPY NEAR MOHO

a

500 km

Figure 11. Seismic anisotropy orientation in the upper mantle under Kamchatka. Orientations of anisotropic symmetry axes derived from individual models are plotted by black arrows centered on sites. Open arrows at ZUP and PET show directions inferred through comparative analysis of transverse RFs for PET, ZUP and KRO. Black arrow shows the direction of convergence along the Kamchatka trench. Thin black lines in the ocean denote the depth contour of 5500 m, providing an outline of the Kamchatka and Aleutian deep-water trenches. 37

CRUST AND UPPER MANTLE OF KAMCHATKA PENINSULA

near-surface mid-crustal low velocity interface

15km __ / t :���:J______.....--- 35±5km t ······· Q ·····�;��; · PACIFIC PLATE t

50 - 70 km 1111111111111111111111

______i ...... � ... I , , I ' I ,

I ' , I , I , anisotropic ' mantle I , I ' layering , , I I , ,

sub-slab flow

Figure 12. Summary cartoon of seismic anisotropy orientation in the upper mantle wedge beneath Kamchatka. Sub-slab anisotropy is inferred from SKS splitting. Mantle wedge and crustal fabric am inferred from receiver­ function analysis of data from the SEKS portable seismic network. P-to-SH converted phases from all mantle-wedge layer interfaces are not observed at all stations, but RFs from stations along the Eastern Volcanic Front exhibit qualitatively similar features. 38

Incipient Pressure-Release Rifting Volcanism -

s � s � 0 � N N

Pulse of Subducted Sediment

Asthenospheric Flow Sluggish Slab Rollback

Figure 13. Schematic representation of competing scenarios for Quaternary volcanism and tectonics near the Kamchatka slab edge. Left: accelerated slab rollback and volatile transport into the mantle wedge. Right: slab­ edge pinchoff and the upward rebound of its remnant. 39

65 Ma

+

20'

+

1000 km

Figure 14. for the north Pacific from Selive1·stov [1998}. Legend in the 65 Ma map: 1) active spreading center, 2) subduction zone, 3) , 4) trace of the Hawaiian hotspot, 5) oceanic volcanic arc, 6) Hawaiian hotspot {hs), (7) line marking material points that currently lie at the Aleutian-Kamchatka­ Kuril subduction zones, (8) magnetic lineations with major chrons labeled, (9) inferred magnetic lineations, (10) grabens, (11) minor faults, and (12) outline of oceanic plateaux. Abbreviations for oceanic plates: K, F, P = Kula, Farallon, and Pacific. Oceanic plateaux: Xe = Hess rise; WT = Shatsky rise. Active subduction zones: I=

Cape Kamchatka oceanic arc, II = Olyutorsky oceanic arc, III = continental subduction zone along northeastern Asia margin. Components of the Cape Kamchatka arc, from north to south in lower left panel: Shirshov ridge, Bowers ridge, Komandorsky Islands (western Aleutians), Cape Kamchatka terrane, Cape Kronotsky terrane, Cape Shipunsky terrane. Emperor chain indicated by dashed line that extends from Hawaiian hotspot (hs). 40

Middle Cenozoic

·"

· Plate Okhotsk Extension .,.

Komandorsky Ocean Basin

Pacific Plate Pacific Plate

Figure 15. Plate reconstruction for hypothesized late-Cenozoic tectonics of the Kamchatka region. Left: The first of the present-day "Cape" terranes (Shipunsky) collides with the Kamchatka peninsula, which has rotated CCW in response to extension in the Sea of Okhotsk. Collision timing is poorly constrained, but extrapolating the Pacific absolute plate motion proposed by Wessel and Kroenke (2000] suggests 25-30 Ma. Active subduction of the Komandorsky Basin occurs along the full eastern Kamchatka coastline. Subducted Komandorsky Basin is young and buoyant, fostering shallow slab dip and the formation of a volcanic arc (Sredinny Range) far inland. Pacific plate motion has a component normal to the Bering Fault, but rather than resume subduction under the western Aleutians, the Komandorsky plate drifts NE in response. Right: Cape terranes accrete sequentially onto the Kamchatka coast. Pacific plate changes direction during 2-6 Ma in response to collision of the Ontong Java Plateau with the Solomon subduction zone. As a result, the remnant of the Komandorsky Basin no longer subducts into the north Kamchatka subduction zones, and island-arc volcanism ceases.