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RUSSIAN JOURNAL OF EARTH SCIENCES, VOL. 12, ES1004, doi:10.2205/2011ES000504, 2011
Eocene arc-continent collision in northern Kamchatka, Russian Far East
Alexey V. Soloviev,1 John I. Garver,2 Mikhail N. Shapiro,3 Mark T. Brandon,4 and Jeremy K.
Hourigan5
Received 27 May 2011; accepted 8 June 2011; published 20 June 2011.
The collision of the Cretaceous Olutorsky Island Arc with eastern Eurasia margin in the northwest Pacific is a critical event in Cenozoic evolution of the Pacific Rim. The timing of synorogenic flysch deposition (Paleocene-Middle Eocene, as young as 45-50 Ma), cross-cutting Shamanka pluton (45 Ma), and an overlap sequence of the Kinkil volcanics (45 Ma) tightly bracket the timing of collision of the far-traveled Olutorsky terrane to the Middle Eocene. This collision may have driven change in plate movement that resulted in the establishment of the Aleutian Arc at this time, which trapped a fragment of the Kula plate (now in the Bering Sea). KEYWORDS: Kamchatka; Olutorsky Arc; Eocene collision;
Gssion-track and U/Pb dating; zircon; nanoplankton.
Citation: Soloviev, Alexey V., John I. Garver, Mikhail N. Shapiro, Mark T. Brandon, and Jeremy K. Hourigan (2011), Eocene arc-continent collision in northern Kamchatka, Russian Far East, Russ. J. Earth. Sci., 12, ES1004, doi:10.2205/2011ES000504.
Introduction
A major tectonic event in the northwest Pacific was the collision of the Upper Cretaceous Olutorsky terrane (island arc) with the NE Eurasian margin, which was an Andean-style convergent margin at that time [Bogdanov et al., 1990; Geist et al., 1994; Stavsky et al., 1990; Worrall, 1991].
The term “Olutorsky terrane” has precedence in the literature [Bogdanov et al., 1990]. This terrane represents mid-Cretaceous to early Tertiary volcanic rocks, oceanic sediments built on oceanic crust. This terrane includes similar rocks referred to as “Olyutorka-Kamchatka island arc ter-rane” [Nokleberg et al., 1998], “Iruneiskiy island arc terrane” [Nokleberg et al., 1998], “Achaivayamian-Valaginian volcanic arc” [Shapiro, 1995].
Remnants of the far-traveled Olutorsky terrane can be traced for almost the entire length of the Kamchatka Peninsula, and likely extend eastward into the Bering Sea as the Shirshov and Bowers Ridge [Seliverstov, 1998; Worrall, 1991] (Figure 1). Together, these onshore and offshore fragments
1Geological Institute, Russian Academy of Sciences, Moscow, Russia
2Geology Department, Union College, Schenectady, New York, USA
3Institute of the Physics of the Earth, Russian Academy of Sciences, Moscow, Russia
4Department of Geology and Geophysics, Yale University, New Haven, Connecticut, USA
5Department of Earth and Planetary Sciences, UC Santa Cruz, Santa Cruz, CA, USA
Copyright 2011 by the Geophysical Center RAS. http://elpub.wdcb.ru/journals/rjes/doi/2011ES000504.html
suggest that this Cretaceous arc may have been as much as 3000 km long but only the part on Kamchatka is exposed for direct observation. Understanding the timing of the collision of the Olutorsky arc with Eurasian continent is crucial to any Cenozoic tectonic reconstruction of northwest Pacific.
The highlight of this Cenozoic collisional suture zone is the spectacular Vatyna-Lesnovsk1 thrust, which is well exposed in northern Kamchatka (Figure 1). The Vatyna-Lesnovsk thrust is a part of the suture formed during collision of the Cretaceous Olutorsky island arc with Eurasian continental margin. Paleomagnetic data from Cretaceous volcanic rocks (Olutorsky terrane) indicate a paleo-latitude of 49.7°±5.6°N [Levashova et al., 1998], ~ 20° to the south of the expected latitudes for Eurasia and North America (~ 70°N). During the Late Cretaceous to early Tertiary, the Olutorsky arc was transported northward and eventually collided with Eurasian continental margin [Chekhovich et al., 1999; Geist et al., 1994; Konstantinovskaia, 2000; Shapiro, 1995; Soloviev et al., 1998; Worrall, 1991]. The total overthrusting along Vatyna-Lesnovsk suture zone based on simple structural reconstructions and by interpretations of gravity data suggests that there has been at least 50 km [Bogdanov et al., 1990] but may be as much as 100 km [Soloviev et al., 2001].
The timing of the Olutorsky collision has been controversial for years and published estimates for the timing to Olu-torsky collision range from the Cretaceous to the Miocene [Chekhovich et al., 1999; Geist et al., 1994; Konstantinovskaia, 2000; Stavsky et al., 1990; Worrall, 1991]. The
1 The northeast and central segments of this fault are referred to as the Vatyna thrust (Koryak Highlands), the Lesnovsk thrust (Lesnovsk Highlands in the Kamchatka Isthmus) respectively.
0 10 20 km
^Complexes of > >aleo-Asian Margiiy
Shamanka / Massif
| Cenozoic cover of Kamchatka
■ Olutorsky arc-terrane (K2-Pj)
□ Ukelayat - Lesnovsk basin (K^-P^
UF J Metamorphic complexes t J of Kamchatka
1^1 Vatyna - Lesnovsk thrust
/Modem Trench^^A Komandorsky r l^^strike-slip zone
X/ Fossil Trench ^
Spreding center
OCVB - Okhotsk - Chukotka Volcanic belt (K.-KJ WKVB - Western Kamchatka Volcanic belt (P,-Ps)
Samples
• FT
♦ U/Pb
■ nanoplankton
Kinkil Gr. volcanics Shamanka
(basal age 45 Ma) pluton (45 Ma)
Thrust plate of Olutorsky Arc
Post-orogenic volcanics N„
K2-P2* \
Deformed syn-orogenic Lesnovsk flysch (young as Middle Eocene)
0
Scale 5 10 km
Figure 1. [A] General geological setting Kamchatka (modified from [Shapiro, 1995; Til’man, Bogdanov, 1992; Worrall, 1991]). The Olutorsky arc-terrane lies outboard of Vatyna-Lesnovsk thrust and well exposed in northern Kamchatka. [B] Schematic geologic map showing the geological structure of the Kamchatka Isthmus (left picture) (modified from [Markovsky, 1989; Soloviev et al, 2001]). [C] Simplified geological profile cross Lesnovsk Highland.
004 SOLOVIEV ET AL.: EOCENE ARC-CONTINENT COLLISION ES1004
key to understanding the timing of collision is to constrain the age of rocks caught up in the collisional suture, which is marked by the Vatyna-Lesnovsk thrust. Here, we present new age data for the age of: a) deformed footwall strata below the Vatyna-Lesnovsk thrust; b) stitching intrusions that cut all rocks in the suture zone; and c) neoauthochtonous deposits that unconformably overlie deformed rocks in the collision zone. Taken together, these data tightly bracket the timing of Olutorsky collision. The timing of the collision bears on the evolution of the North Pacific including the establishment of the Aleutian Islands.
Geological Background
The geologic framework of the northwest Pacific includes oceanic and arc terranes of Paleozoic to Cenozoic age that have been accreted to northeast Eurasian margin from the Late Jurassic to Tertiary [Nokleberg et al., 1998; Stavsky et al., 1990]. The Cretaceous Okhotsk-Chukotka volcanic belt (OCVB) is a continental arc built on part of this collage of terranes assembled prior to the Albian. The OCVB is a laterally extensive Andean-style arc that persisted along the southern margin of the western and northern shore Sea of Okhotsk, Bering shelf, and western Alaska [Filatova, 1988; Nokleberg et al., 1998; Tikhomirov et al., 2006]. The duration of magmatic activity in OCVB is debated but generally thought to include the Middle Albian to Campanian time [Filatova, 1988; Zonenshain et al., 1990]. Arc-related mag-matism ceased at ~ 81 Ma within the Okhotsk sector of the OCVB [Hourigan, Akinin, 2004]. Following a possible brief hiatus, basaltic lavas with within-plate chemical affinities were erupted over a large part of the belt. 40Ar/39Ar data from these volcanic rocks indicate that this latter activity spanned the interval from 78-73 Ma [Hourigan, Akinin, 2004]. The OCVB was the source of a thick package of volcanic sediments deposited offshore into the forearc, and these strata are represented by turbidites of the Ukelayat Group in the Koryak Highlands [Garver et al., 2000] and the Lesnovsk Group in the Lesnovsk Highlands in Kamchatka Isthmus (Figure 1).
Fragments of the Upper Cretaceous part of the Olutorsky island arc formed about 2000 km to the south in the Pacific far from the Eurasian continental margin, and the arc activity was inferred to be largely intra-oceanic [Chekhovich et al., 1999; Levashova et al., 1998]. Today, this far-traveled island arc complex has been obducted on to the continental margin and the late thrust that appears to be the suture is the Vatyna-Lesnovsk thrust zone. In this spectacular overthrust, island arc rocks of the Olutorsky terrane sit on a footwall comprised of Cretaceous to Paleogene terrigenous deposits of the Ukelayat basin, which was likely deposited in the forearc region of the OCVB. The Vatyna-Lesnovsk thrust can be mapped as a flat or slightly tilted surface that can be mapped for hundreds of kilometers on northern Kamchtaka (Figure 1) [Soloviev et al., 2001]. In the two areas where we mapped extensively both footwall and hanging wall rocks are exposed but in many part of the Peninsula the structure
is buried by younger volcanic rocks of the Kamchatka arc, which was established after collision.
The footwall in the collision zone appears to be comprised almost exclusively of the Cretacoues to Paleocene forearc strata of the Ukelayat basin that contains strata inferred to have been largely derived from the OCVB [Garver et al., 2000]. Based on the age of the youngest strata in the Uke-layat basin, the Olutorsky arc was inferred to collide with Eurasian margin in the Middle Eocene (c. 45-50 Ma) [Garver et al., 2000; Soloviev et al., 1998].
Finally, the collision zone appears to have a significant syn- to early post-collisional suite of volcanic rocks and in-trusives that play a key role in bracketing deformation. The Kinkil volcanic belt is located on the northern part of the Western Kamchatka and has also been described as a part of the Western Kamchatka-Koryak volcanic belt (WKKV) (see (Figure 1) (for example, [Filatova, 1988]). The collision is inferred to have driven the locus of volcanism eastward to the Kinkil volcanic belt, which includes Upper Eocene to Lower Miocene neoautochthonous deposits that rest uncon-formably on deformed rocks of the Vatyna-Lesnovsk suture zone. Locally intrusives related to these volcanic rocks are present in the collision zone, and the most notable of these in the Shamanka granite.
Geology of Collision Zone
In the Kamchatka Isthmus area, the low-angle Lesnovsk thrust is well exposed and the underlying deformed rocks of Ukelayat basin are exposed in structural windows along the Vatapvayam dome and the Shamanka dome [Shantser et al., 1985; Soloviev et al., 2001] (Figure 1). Upper plate rocks of the Olutorsky island arc include the Upper Cretaceous MORB and arc-volcanic rocks, pelagic cherts, poorly dated mafic intrusions, and greenschists [Shantser et al., 1985]. The structural thickness of the upper plate is about 2-4 km [Soloviev et al., 2001] (Figure 1). The folds are not typical in the upper plate rocks partly because there is little stratal continuity in these units, but axes of map-scale folds trend northeast-southwest, parallel to the orogenic belt. West-vergent asymmetrical folds in greenschist facies mylonitic rocks occur along faults between the upper plate nappes [Soloviev et al., 2001].
The Ukelayat-Lesnovsk Group is a very thick, deformed turbidite assemblage that makes up the lower plate in the collision zone. A key issue is that the age range of unit has been poorly constrained. The unit is generally composed of poorly fossilliferous medium- to thin-bedded, fine- to medium-grained sandstone and interbedded shale and siltstone. Sandstones are compositionally uniform and are mainly quartzofeldspathic and feldspathic-quartzose graywackes. The composition of sandstones from rocks in northern Kamchatka indicates a source region dominated by a dissected continental arc and paleocurrents indicate largely axial S-to-N sedimentary transport [Garver et al., 2000]. These lower plate rocks are exposed in the center of the two structural domes which appear to have been exhumed
Figure 2. U/Pb isochron diagrams: (a) for the Shamanka granodiorite (sample Sh1/99); (b) for the rhyolites at the base of the Kinkil volcanics(sample Sh4/99).
in the Late Eocene [Soloviev et al., 2002] (Figure 1). These turbidites in the lower plate are dominated by westward-overturned isoclinal folds with north-northeast fold axes that are parallel to the thrust front, which indicate shortening in the direction of thrusting [Soloviev et al., 2001].
The Lesnovsk and Vatyna thrusts were originally interpreted as a stratigraphic contact and thus the turbidites below the upper plate were inferred to older [Markovsky, 1989]. As a result of this initial misunderstanding, the turbiditic strata were originally mapped as Cretaceous because the overlying upper plate rocks have a number of mid-to Upper Cretaceous fossils; this simple observation indicated to early workers that the deformed underlying unfos-siliferous strata of the Ukelayat were Cretaceous (or older) [Markovsky, 1989]. New stratigraphic age constraints presented here and elsewhere include new nanonfossil biostratigraphy and fission-track grain ages on detrital zircon [Garver et al., 2000; Soloviev et al., 2002]. These data indicate that along the thrust front here and to the north, these foot-wall strata range in age from Upper Cretaceous to Middle Eocene.
Below the frontal thrust zone, large blocks occur enclosed in a chaotic structure and this has been mapped as olis-tostromal deposits that are the sedimentary record of the collisional process. The matrix in these deformed rocks is composed of mudstones with thin interlayers of fine-grained sandstone that have been pulled apart and tectonically dismembered. Blocks are represented by several lithologies including a distinctive tuff with broken Inoceramus shells, as well as chert, aphyric pillow-basalt, and volcanic sandstone all correlative with upper plate lithology of Vatyna Group (i.e. rocks of the Irunei Group). The ages of chert blocks in the olistostromal deposits and from chert in the Irunei Group in the upper plate are Santonian to Maastrichtian as indi-
cated by radiolaria [Soloviev et al., 2001]. The olistostromal unit represents deformed basin strata with blocks derived from the upper plate that was likely deposited during thrusting, but the age of deposition is not directly constrained. Mass wasting deposits are common in settings where the blocks from upper plate rocks are deposited immediately in front of an advancing thrust sheet and then involved to post-depositional deformations.
The Lesnovsk thrust is well exposed and we evaluated its occurrence and small-scale structures in several places in northern Kamchatka [Soloviev et al., 2001]. The thrust zone is marked by a narrow zone of pervasive brittle deformation commonly less then 100 m thick. The fault zone varies in thickness, and consists of lens-like phacoids with black and green cataclasite with wavy fibered and flaser structure. The thickness of the cataclasite zone varies from 0.5 to 30 meters, but is normally from 5 to 10 meters. Riedel structures in this zone indicate top to the north-northeast transport for thrust in the Vatapvayam Dome area and top to the northwest transport for the thrust in the Shamanka Dome [Soloviev et al., 2001].
Post-orogenic neo-autochthonous strata are well exposed east of the thrust in the isthumus area. These rocks include gently deformed volcanic rocks of Kinkil Group that overlap folded lower plate deposits with an angular unconformity. Rhyolites and their tuffs dominate at the base of the Kinkil Group, but basaltic rocks occur upward in the stratigraphic sequence. Middle to Late Eocene flora from strata of the Kinkil Group have been reported in the literature [Gladenkov et al., 1991] and we determined radiometric ages from several locations (see below).
One of the most crucial relationships that constrain the timing of thrusting occurs in the Kamchatka Isthmus. In this area lower plate rocks are well exposed in a window,
Table 1. U-Pb Isotopic Data and Ages From Shamanka Granodiorite (Sample Sh1/99) and Kinkil Rhyolite (Sample Sh4/99)
Grain type Grain wt.(^g) _Q Pb (pg U (ppm) 206Pbm 206Pbc 204Pb 208Pb 206 Pb* 238u Apparent ages (Ma) 207Pb* 235u 207Pb* 206Pb*
5A 31 6 241 Sample Sh1/99 571 9.6 45.6 ± 1.3 45.7 ± 1.6 51 ± 49
5A 32 11 216 310 5.9 45.3±1.3 45.3 ± 2.2 46 ± 88
5A 29 8 303 502 7.7 45.3±1.1 44.9 ± 1.5 20 ± 55
1Br 11 9 471 283 5.9 45.2 ± 1.8 45.5 ± 2.6 63 ± 99
5Ar 29 9 386 556 3.6 45.4 ± 0.9 45.5 ± 1.1 52 ± 37
5Ar 31 6 1301 Sample Sh4/99 3200 21.9 47.1 ± 0.3 47.6 ± 0, 5 71 ± 15
5Ar 32 11 3151 4100 20.3 46.1 ± 0.3 46.4 ± 0.5 61 ± 20
5Ar 36 11 1915 2690 20.3 44.9 ± 0.3 46.1 ± 0.5 105 ± 16
5Ar 66 8 1809 9180 13.1 61.7 ± 0.4 78.9 ± 0.6 637 ± 7
5Ar 70 12 1462 5900 19.9 61.1 ± 0.4 80.1 ± 0.7 692 ± 10
* — radiogenic Pb, Grain type: A=~ 100 B=~ 200 ^, r = 5:1 rod-shaped grain (number of grains analyzed is also shown), 206Pb/204Pb is measured ratio, uncorrected for blank, spike, or fractionation, 206Pb/208Pb is corrected for blank, spike, and fractionation, Most concentrations have an uncertainty of 25% due to uncertainty in weight of grain, Constants used: 238u/235U = 137.88. Decay constant for 235U = 9.8485 X 10-10, Decay constant for 235U = 1.55125 X 10-10. All uncertainties are at the 95% confidence level, Pb blank ranged from 2 to 10 pg, U blank was < 1 pg, Interpreted ages for concordant grains are 206Pb * /238U ages if < 1.0 Ga and 207Pb*206Pb* ages if > 1.0 Ga. Interpreted ages for discordant grains are projected from 100 Ma. All analyses conducted using conventional isotope dilution and thermal ionization mass spectrometry, as described by [Gehrels, 2000].
both lower plate and upper plate rocks are intruded by the Shamanka granite (Figure 1). The pluton has a distinctive contact aureole and the intrusion is accompanied by numerous dikes of basic and felsic composition both of which are cross-cut upper and lower plate rocks. The spatial distribution of dikes has been used to infer that they are related to the effusive rocks of the nearby Kinkil Group [Shantsev et al., 1985]. The basal units of the Kinkil Group, which are up to 200-250 m contains pebbles and cobbles of granites, rocks similar to the dikes and hornfels. Flora in these conglomerates of the Kinkil Group is Upper Eocene [Shantsev et al., 1985].
New Age Data
We have worked on the overthrust zone in several places in northern Kamchatka [Soloviev et al., 2001]. One area that provides crucial relationships that constrain the timing of thrusting and collision is in the Kamchatka Isthmus. Here we provide the results of new detrital zircon fission track ages (from lower plate sandstones), new determinations of nanoplankton (from mudstones in the lower plate), and previously unreported U/Pb, K/Ar and Rb/Sr ages (Figure 2, Figure 3) on cross-cutting plutonic rocks and their extrusive equivalents (Table 1, Table 2, Table 3).
Table 2. Rb-Sr Isotopic Data and Ages for Shamanka Granodiorite (Sample Sh1/99)
Mineral Rb, ppm Sr, ppm 87Rb/87Sr 87Sr/86Sr Isochrone age, Ma Pair minerals Pair age, Ma (87Sr/86Sr)
Plagioclase 6.314 759.0 0.02410± 0.70388± 44.4 ± 0.1 Plagioclase- 47.1 ± 1.1 0.70386±
0.00007 0.00002 (87Sr/86Sr)0 = hornblende 0.00002
Hornblende 9.566 15.86 1.7447± 0.70503± 0.70389± Plagioclase- 44.37 ± 0.04 0.70386±
0.0016 0.00002 0.00003 biotite 0.00002
Biotite 325.4 3.240 295.72± 0.89023± Biotite- 44.35 ± 0.04 0.70393±
0.25 0.00003 hornblende 0.00002
All analyses conducted using isotope dilution with mixed tracer 85Rb—84Sr and mass spectrometry Sector54 (Micromass). The international standard for Sr—SRM987 was used for control. The isotope composition of Sr normalized by 86Sr/88Sr = 0.1194. Sample was dated by V. N. Golubev (Institute of ore deposit, petrography, mineralogy and geochemistry RAS, Moscow, Russia).
Figure 3. Rb-Sr isochron diagram for the Shamanka granodiorite (sample Sh1/99). Pl - plagioclase, Hb - hornblende, Bi - biotite.
Zircon fission-track data. We collected thirteen new sandstone samples (Table 4) from lower plate deposits for the detrital fission track grain-age analysis (Figure 1), and these data augment our previous data set that has 27 samples [Garver et al., 2000]. Zircon concentrates were prepared using standard methods of separation, and zircons were analyzed using the external detector method [Bernet, Garver, 2005; Garver et al., 2000; Tagami, O’Sullivan, 2005]. For each sample 45 to 90 zircon grains were counted giving a total of about 1000 single-grain determinations. A binomial peak-fitting method was used to establish principal grain-age components for each sample [Brandon, 1996, 2002; Galbraith, Green, 1990] (Table 5). This approach requires that maximum temperatures should be < 200° C to ensure that zircon fission-track ages are primary cooling ages rather than reset during thrusting [Brandon et al., 1998]. Several observations indicate that maximum temperatures of the Ukelayat strata were generally well below ~ 150 or 200°C. The sampled Lesnovsk Group shows only brittle deformation, with no evidence of pressure solution or cleavage formation. FT dating of apatite from the dated zircon samples indicates
that detrital apatites are also largely unreset or partially reset and retain old ages that reflect cooling events in the source region (Table 6). From these observations, we conclude that our samples remained well below ~ 150° C after deposition (probably ~ 120° C or less based on apatite resetting patterns) and therefore the zircon fission-track system has certainly remained closed.
In all samples several age components (or peaks) are present and the different age populations, P1, P2 and P3 (Table 5), represent cooling events in the source-region, either related to near surface magmatism, tectonic exhumation, or erosion [Bernet, Garver, 2005; Brandon, Vance, 1992; Garver et al., 1999, 2000]. Because we are primarily interested in constraining depositional age we focus on the P1, which defines the minimum grain-age and thus maximum de-positional age for each sample [Bernet, Garver, 2005; Brandon, Vance, 1992; Carter et al., 1995; Garver et al., 1999, 2000; Kowallis et al., 1986]. The youngest population (P1) is composed of from 5 to 50% of the dated grains (Table 5). The P1 component appears to have been derived from active part of a volcanic arc or rapidly exhumed rocks in source
Table 3. K-Ar Data and Ages for Shamanka Granodiorite (Samples Sh1/99) and Kinkil Rhyolite (Sample Sh4/99)
Sample No. Mineral K, %, і а 40 Arrad (ng/g), і а Age, Ma, і1.6 а
Sh1/99 Biotite 6.67 і 0.06 22.0 і 0.3 47.0 і 1.3
Sh1/99 Hornblende 0.54 і 0.01 1.65 і 0.06 44.0 і 2.5
Sh4/99 Biotite 6.75 і 0.06 21.8 і 0.3 46.0 і 1.3
The concentrations of radiogenic 40Ar conducted using isotope dilution with tracer 38Ar and mass spectrometry MI-1201 IG. The concentrations of K determined by flame spectrophotometry. Constants used: Afc = 0.581 X 10-10y-1, = 4.962 X 10-10y-1,
40 K = 0.01167 (at. %). Sample was dated by M. M. Arakelyants (Institute of ore deposit, petrography, mineralogy and geochemistry RAS, Moscow, Russia).
Table 4. Locations of the Samples Analyzed by the Geochronological Methods
Field Number Latitude Longitude Elevation, m Rock type Unit
Sh1/99 N 59°31'30" E 161°48,35" 1160 granodiorite Shamanka massive
Sh2/99 N 59°32'50" E 161°38,30" 1070 sandstone Lesnovsk Group
Sh3/99 N 59°31'22" E 161°39,00" 396 sandstone Lesnovsk Group
Sh4/99 N 59°33'42" E 161°35,55" 870 rhyolite Kinkil volcanics
Sh15/99 N 59°27'57" E 161°44,43" 1036 sandstone Lesnovsk Group
Sh21/99 N 59°29'17" E 161°29,50" 370 sandstone Lesnovsk Group
L1 N 59°01,46" E 161W35" 874 sandstone Lesnovsk Group
L2 N 59°02W E 161° 04^3" 1197 sandstone Lesnovsk Group
L4 N 59°00^4" E 161°02,24,, 612 sandstone Lesnovsk Group
L6 N 59W25" E 160°22,55,, 897 monzogabbro Atvenaivayam M.
L7 N 59°00^5" E 160°29,50,, 691 monzogabbro Atvenaivayam M.
L9 N 59°11,09" E 161°11,40,, 451 sandstone Lesnovsk Group
L10 N 59° 1^40" e 412 sandstone Lesnovsk Group
L11 N 59°09^7" E 161°13,00,, 936 sandstone Lesnovsk Group
L12 N 59° 10^9" E 161°13,50,, 927 sandstone Lesnovsk Group
L13 N 59°11W E 570 sandstone Lesnovsk Group
L17 N 59° 1^05" E 161°03,10,, 180 sandstone Lesnovsk Group
area. This inference is supported by the fact that volcanic depositional age for sandstones derived from active volcanic
grains are common in the fraction of lithic fragments in the sources or continental arc terrains [Bernet, Garver, 2005].
sandstones and many appear to be first-cycle [Garver et al., The young population of grain ages (P1 ages) from this new
2000; Shapiro et al., 2001]. A number of other studies have data set from bedded sandstones span the interval between
shown that the FT minimum age can be used as a proxy for 58 to 44 Ma, and sample Sh15/99 which was a block, gives
Table 5. Zircon Fission-Track Peak Ages for Lesnovsk Flysch (Northern Kamchatka)
Nsamp. Nt Age of the zircon population
P1 (Ma) P2 (Ma) P3 (Ma)
Lesnovsk Highland (Shamanka Dome)
Sh3/99 60 51.6 ± 5.0 (27%) 86.7 ± 8.9 (55%) 131.4 ± 29.2 (18%)
Sh2/99 75 54.1 ± 8.9 (16%) 73.9 ± 13.9 (26%) 132.6 ± 9.2 (58%)
Sh21/99 60 56.1 ± 3.8 (37%) 106.0 ± 11.5 (47%) 150.3 ± 34.2 (16%)
Sh15/99 59 86.1 ± 6.1 (44%) 155.3 ± 11.0 (56%) -
Lesnovsk Highland (Vatapvayam Dome)
L12 67 43.7 ± 3.4 (17%) 70.6 ± 4.4 (67%) 107.0 ± 12.2 (16%)
L1 45 46.0 ± 2.7 (49%) - 107.3 ± 7.0 (51%)
L9 90 47.0 ± 3.8 (19%) 70.8 ± 5.7 (56%) 104.0 ± 11.9 (25%)
L2 90 48.1 ± 5.0 (7%) 78.1 ± 5.8 (53%) 116.0 ± 8.6 (40%)
L11 90 50.4 ± 5.6 (20%) 70.6 ± 6.6 (65%) 109.7 ± 25.0 (15%)
L10 90 53.9 ± 3.4 (40%) 87.5 ± 6.2 (50%) 176.5 ± 23.8 (10%)
L17 90 54.5 ± 10.4 (5%) 84.6 ± 6.5 (65%) 134.6 ± 18.9 (30%)
L13 89 55.5 ± 3.5 (34%) 93.0 ± 4.8 (66%) -
L4 90 58.1 ± 4.2 (36%) 83.3 ± 6.3 (51%) 130.5 ± 14.9 (13%)
Nt — number of grains; percentage of grains calculated in a specific peak; Age for each population is in Ma, uncertainties cited at ±1a. Zircons were dated using standard methods for FT dating using an external detector. Mounts were etched in a NaOH—KOH at 228°C for 15 and 30 hr and then irradiated at Oregon State with a fluence of 2 X 1015 n/cm2, along with zircon standards and dosimeter CN-5. Tracks were counted on an Olympus BH-P at 1256x, and a Z-factor of 305.01 ±6.91 was used. Fission-track ages were computed using the program Zetaage 4.7 [Brandon, 1996]. To discriminate the populations by age, we used the program Binomfit 1.8 [Brandon, 2002]. Sample treatment is described in [Bernet, Garver, 2005; Garver et al., 1999].
8 of 13
Table 6. Apatite Fission-Track Data From Lesnovsk Highland
No. Sample Unit Elev (m) ps Ns pi Ni pd n x2 Age -/ + 1 O' U±2se Track length(n)
Shi Shamanka M. 1160 6.37 187 2.25 660 Shamanka Dome 2.89 15 100.0 42.3 -4.8 +5.4 31.0 + 2.7
Sh2 Lesnaya Gr. 1070 3.45 195 0.98 556 2.88 25 0.4 44.6* -6.7 +7.8 13.6 + 1.3
Sh4 Kinkil Gr. 870 4.76 185 1.57 612 2.84 14 99.7 44.3 -5.0 +5.7 22.1 ± 2.0
L17 Lesnovsk Gr. 180 5.07 384 1.88 Vatapvayam 1422 3.22 Dome 42 1.8 13.0* -6.6 +13.4 23.2 + 1.4 11.72 + 1.59 (20)
L10 Lesnovsk Gr. 412 3.76 96 1.72 439 3.13 20 38.8 38.4 -4.7 +5.4 21.8 + 2.2 12.02 ± 1.49 (5)
L13 Lesnovsk Gr. 570 3.44 150 1.32 574 3.20 28 1.4 40.9* -4.9 +5.6 16.4+1.5 12.52 + 2.00 (14)
L4 Lesnovsk Gr. 612 6.84 247 2.07 747 3.04 22 69.2 56.3 -5.4 +5.9 27.1 ± 2.2 11.70+ 1.49 (19)
LI Lesnovsk Gr. 874 5.57 340 1.61 982 3.00 45 5.1 58.2 -5.2 +5.7 21.3+1.5 11.65 + 1.17 (21)
L12 Lesnovsk Gr. 927 5.66 246 1.53 667 3.18 28 62.8 65.6 -6.3 +7.0 19.2 + 1.6 11.70+ 1.32 (21)
Lll Lesnovsk Gr. 936 2.48 554 0.98 2189 3.15 89 0.0 40.6* -3.6 +3.9 12.4 + 0.6 11.81 ± 1.61 (60)
L2 Lesnovsk Gr. 1197 3.90 178 1.82 831 3.02 17 22.4 36.3 -3.7 +4.1 24.0+1.8 12.35 + 1.46 (11)
L6 Atvenaivayam M. 897 6.80 406 Gabbro 1.75 in ultramafic massif (upper plate 1043 3.06 30 76.2 rock) 66.6 -5.7 +6.3 22.7+1.6 13.32 + 1.35 (55)
L7 Atvenaivayam M. 691 6.77 475 1.48 1038 3.09 30 100.0 79.0 -6.6 +7.2 19.1 ± 1.3 13.12 + 1.34 (52)
In this table ps is the density (cm-2) of spontaneous tracks (xlO5) and Ns is the number of spontaneous tracks counted; pi is the density (cm-2) of induced tracks (xlO6); and pd is the density (cm-2) of tracks on the fluence monitor (xlO6); n is the number of grains counted; and x2 is the Chi squared probability in percent. Apatite fission track ages (d=l<7 were calculated using the Zeta method, and ages were calculated using the computer program and equations in Brandon (1992). The Zeta factor (for LI—L17 samples) of 112.49 d= 7.53 (d=l se) is based on forth Zeta determinations from both the Fish Canyon Tulf and the Durango apatite (for Shi—Sh21 — 103.73 ±8.77 is based on 3 Zeta determination). All ages that pass x2 (> 5%) are reported as pooled ages, otherwise mean ages are shown. Glass (CN-1) monitors, placed at the top and bottom of all irradiation packages (for Zeta Calculations) were used to determine the fluence gradient in each package. All samples were counted and track lengths were measured at 1562.5x using a dry lOOx objective (12.5 oculars and 1.25x multiplication factor) on an Olympus BH-2 microscope (for LI—L17 samples) and 1600x using a dry lOOx objective (10 oculars and 1.6x multiplication factor) on Olympus BX60 microscope (for Shi—Sh21 samples) fitted with an automated stage and a Calcomp digitizing tablet. Calibration for track length measurements (given as mean track length, standard deviation, and number measured) was accomplished by repeatedly measuring 20 /im increments on a graticule prior to actual measurements on these samples.
* - BinomFit ages [Brandon, 2002] (x2 < 5%); x2 (> 5%) - pooled ages.
ES1004 SOLOVIEV ET AL.: EOCENE ARC-CONTINENT COLLISION ES1004
Figure 4. Isotopic and biostratigraphic data from lower plate of Olutorsky collision zone and from neoautochthonous complex in the northern Kamchatka: implication for collision of Olutorsky Arc with Eurasian continental margin. The stratigraphic chart is by [Ogg et al., 2008].
a P1 age of 86.1 ± 6.1 Ma (Table 5, Figure 4). These compare to the age range of 88 to 44 Ma from our other data set (see [Garver et al., 2000]). Together, these ages are remarkable given the fact that the sampled units of bedded sandstone and shale were all previously mapped as Cretaceous, as discussed above: our results show that most are Paleocene. The new samples that we report here come entirely from the footwall of the Vatyna-Lesnovsk suture. We infer that the P1 ages are maximum ages for the deposition of the units, and that overthrusting must be younger
than the age of these sandstones. However, we suspect that there may be very short time separating deposition of the sandstones and overthrusting. P1 was probably derived, at least in part, from a contemporaneous volcanic source, because the young population is made of colorless euhedral zircon grains. In addition, U/Pb ages of grains from Ukelayat sandstones show that some grains are the same age as P1 [Hourigan et al., 2009; Reiners et al., 2005]. As discussed, the Ukelayat-Lesnovsk flysch contains olistostomes inferred to be mass wasting deposits that are evidence of syncolli-
10 of 13
Table 7. Nanoplankton from the Lesnovsk Group (Lesnovsk Highland, Northern Kamchatka)
Nannoplankton species Number of Sample
902- 1 9902- 5 9902- 7 9902- 11 9902- 20 9903- 4 9903- 5 9903- 6 9903- 11 9903- 15 9903- 16 9903- 18 9904- 7 9906- 7 9911- 7 9911- 8 9911- 12 9911- 17
Cyclicargolithus floridanus Coccolithus pelagicus Sphenolithus primus/moriformis Dictyococcites bisectus Reticulofenestra umbilicus R. haqii R. dictyoda Helicosphaera compacta Chiasmolithus cf. Nitidus Zyghablithus bijugatus Micula decussata Neochiastozygus sp. Thoracosphaera sp. Watznaueria barnesae Reinchardtites anthiphorus Eiffellithus turriseffeli 1 1 - - 1 1 1 1 1 1 III 1 ■ - 1 1
Mudstones were collected in Shamanka Dome area. The nanoplankton were identified by E. A. Shcherbinina (Geological Institute of RAS, Moscow).
ES1004 SOLOVIEV ET AL.: EOCENE ARC-CONTINENT COLLISION ES1004
0 20 40 60 80 100
Apatite fission-track age, Ma
Figure 5. Age-elevation relationship of fission-track data from the Lesnovsk Highland. FT depositional ages (P1) of zircon shown in red. These ages define a depositional interval (pink) of Middle Eocene and older. Apatite ages indicate slow post-orogenic exhumation and possibly post-orogenic heating from the top by the Kinkil volcanics.
sional deposition. Our conclusion is that the youngest of the P1 ages, which are Middle Eocene, are nearly synchronous with collision: the other P1 ages are precollisional and likely represent a long interval of deposition in the forearc basin that was outboard of the OCVB.
Nanoplankton. We collected mudstones from the same sequence that we took sandstones for ZFT analysis for bios-tratigraphic analysis. Nanoplankton from 18 samples (Table 7) from the upper part of the Lesnovsk Group yield Pa-leocene through Middle Eocene assemblages and these new ages closely correspond to the age of P1 zircons from sandstones [Soloviev et al., 2002]. Together these ages, obtained independently from a number of sites using different methods, now indicate with certainty that the upper part of the Ukelayat-Lesnovsk flysch is largely if not entirely Eocene in age (Figure 4).
U/Pb age on zircon. To further refine our age control, we collected one sample from the granitic rocks of the Shamanka massif (Table 4, Figure 1) and one from the Kinkil volcanics and dated them by several different methods (Table 1, Table 2, Table 3, Table 4, Figure 1, Figure 2, Figure 3). A new U/Pb age on zircon from the Shamanka Massif is 45.3 ± 1.0 Ma (95% CI), sample Sh1/99 (Table 1, Figure 2). The Kinkil volcanics, which are inferred to be co-magmatic with the Shamanka, give a U/Pb age on zircon of 45.5 ± 2.9 Ma (95% CI) (Table 1, Figure 2). from a sample from the basal member.
The overthrust zone is intruded by the Shamanka granitic massif, and the deformed Lesnovsk Flysch is unconformably overlain by the volcanics of Kinkil Group (Figure 1). The end of collision must be marked by intrusion of the Shamanka granites into the overthrust zone because the intrusion cuts both upper plate and lower plate rocks. Previous studies have demonstrated that sandstones and siltstones bearing Upper Eocene to Lower Oligocene mollusks are stratigraph-ically correlative to the Kinkil Group. Neoautochthonous strata contain basal conglomerates with pebbles derived from the Shamanka pluton. An Upper Eocene fossil flora [Shantser et al., 1985] occurs within the conglomeratic strata.
Post-Orogenic Cooling
The Middle-Upper Eocene Kinkil Volcanics rest uncon-formably on Lesnovsk suture zone and they are slightly tilted. The Neogene volcanics are horizontal. The fission-track ages from detrital apatite indicate partial resetting (Table 6), so collisional burial of lower plate flysch was less than ~ 4 km. The age-elevation relationship (Figure 5) indicates very slow post orogenic exhumation, less than 50 m/Myr. The partial resetting of apatite at high elevations (see Table 6, Figure 5) may reflect heating by the Kinkil volcanics which unconformably blanketed the structural dome immediately after collision at ~ 45 Ma.
Conclusions
Detrital zircon FT grain-age data in concert with nanoplankton biostratigraphic data suggest that part of the Ukelayat-Lesnovsk Group comprises principally Paleocene to Middle Eocene strata. The end of deposition of the lower plate sediments, deformation, and the subsequent onset of magmatism recorded in Kinkil Group occurred at c. 45 Ma (Middle Eocene). These data allow us to propose that thrusting driven by collision of the Olutorsky island-arc with northeastern Asia took place at about 45 Ma, perhaps over a short period of time (~ 1 Myr).
D. M. Worrall [1991] proposed that the Olutorsky arc collided with Eurasia at about 56 Ma and this collision, with its attendant changes in slab-pull stress on Kula plate, may have been the underlying cause of change in plate motion. Our data suggest that the collision of the Olutorsky arc with Eurasia happened at 45 Ma and we speculate that this collision may have driven Pacific plate reconstruction at 43 Ma [Engebretson et al., 1985]. As such, if the Olutorsky terrane continues offshore into the Bowers and Shirshov ridges in the Bering Sea [Worrall, 1991], then it is likely that the development of the Aleutian Ridge post-dated collision at 45 Ma. In this case, older ages reported from the Aleutians may not represent early onset of the Aleutian Arc, but rather older rocks that were somehow brought into the vicinity of the modern arc.
Acknowledgments. This study was supported by the grant of the President of the Russian Federation MD-1053.2010.5 (to Soloviev), by the Federal Targeted Research Program “Leading Scientific Schools” (SS-7091.2010.5), by the Program for Basic Research of the Division of Earth Sciences, Russian Academy of Sciences, no. 4 and no. 6. This work also was funded by the National Science Foundation (9418990 to Brandon, 9418989 to Garver), the Union College Faculty Development Fund (Garver/Soloviev). We are grateful to G. E. Gehrels for U/Pb dating, M. M. Arakelyants for K/Ar dating, V. N. Golubev for Rb/Sr dating, E. A. Shcherbinina for nanoplankton determinations. Thanks are due to all the workers of the Kamchatka Geological Survey Expedition whose logistical support enabled us to carry out this work. Irradiation of the samples was assisted by the U.S. DOE Reactor Use Sharing program awarded to the Oregon State nuclear reactor (USA).
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Mark T. Brandon, Department of Geology and Geophysics, Yale University, New Haven, Connecticut, USA.
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Alexey V. Soloviev, Geological Institute, Russian Academy of Sciences, Moscow, Russia.
