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Deep structure and margins of the Kurai Basin (Gorny Altai), from controlled-source resistivity data
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ScienceDirect Russian Geology and Geophysics 55 (2014) 98–107 www.elsevier.com/locate/rgg
Deep structure and margins of the Kurai Basin (Gorny Altai), from controlled-source resistivity data N.N. Nevedrova a,b,*, E.V. Deev a,b, A.M. Sanchaa a a
A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia b Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia Received 4 December 2012; accepted 10 April 2013
Abstract The deep structure of the Kurai basin and its junction with the flanking mountains has been studied by controlled-source resistivity surveys (vertical electric sounding and transient electromagnetic methods). According to the data processing results, the basin is the deepest along its northern, southern, and eastern margins. The sedimentary fill comprises two resistivity units corresponding to two sequences deposited at different stages of the basin history. The lower, less resistive unit consists of Paleogene–Neogene lacustrine clay and the higher-resistivity upper unit represents coarser Quaternary deposits. In Paleogene–Neogene time, the basin formed by the left-lateral pull-apart mechanism. The earliest Quaternary strike-slip faulting in the setting of overall compression produced the Central Kurai basin within the northern Kurai basin, while the flanking ranges and fault blocks thrust upon the basin transforming it into a ramp. Thus, piedmont steps rose along the basin margins, and the marginal grabens became ramps and half-ramps. © 2014, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: neotectonics; resistivity surveys; TEM; VES; Kurai basin; Gorny Altai
Introduction The Kurai intermontane basin is the second largest basin in Gorny Altai, which together with the Chuya basin makes up a Cenozoic system in the southeastern part of the area. The two basins are separated by the Sukor plateau and share a similar history of neotectonics and deposition. The Kurai basin has a complex tectonic relation with the flanking mountains and a heterogeneous structure with several subbasins in its limits inferred from geological and geophysical data (Bondarenko et al., 1968; Delvaux et al., 1995; Nevedrova et al., 2011). A large amount of vertical electric sounding (VES) profiles were collected in the Kurai basin in the 1960–1980s (Fig. 1), but some have been unsuitable to estimate the sediment thickness, for different reasons. Lately, in 2007 through 2011, more knowledge of the basin structure have been obtained due to integration of controlled-source VES and TEM (transient electromagnetic) surveys (Fig. 1). Inversion of TEM responses acquired in different years allows estimating the resistivity and
* Corresponding author. E-mail address: [email protected] (N.N. Nevedrova)
thickness of the basin sediments over the whole territory of the Kurai basin. In this study we interpret VES and TEM resistivity data to model the deep structure of the Kurai basin and its junction with the flanking mountains, with implications for its history and formation mechanism.
Geological and neotectonic background The Kurai basin has a rhomb-shaped geometry, with a ~35 km (W–E) and ~20 km (N–S) long and short axes, respectively (Fig. 1). The basin is surrounded by mountains. In the north, it is the Kubadru neotectonic uplift, with its highest summit (Verkhovie Ortolyka) at 3446 m asl. In the northeast there are the en-echelon Kurai and Tongulak Ranges, both being the highest (up to 3200 m) parts of the strongly asymmetric Kubadru–Bashkaus block with its long low-angle slope facing the Bashkaus valley; the slope facing the Kurai basin has a stepped profile with scarps at 2850–2950, 2400–2500, and 2000–2100 m. The same distinct scarps delineate the basin in the southwest, from the North Chuya Range rising 4000 m asl. The
1068-7971/$ - see front matter D 201 4, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2013.12.008 +
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Fig.1. Simplified neotectonics of the Kurai basin, with locations of VES and TEM profiles. 1–6, uplifted blocks, fully or partly buried under sediments, with different elevations above sealevel (m): higher than 3500 (1), from 3500 to 3000 (2), from 3000 to 2500 (3), from 2500 to 2000 (4), from 2000 to 1500 (5), and from 2500 to 2000 (6); 7, sedimentary sequences lying over blocks lower than 2000 m; 8, observed faults; 9, inferred faults; 10, VES stations: archive data (a); archive data used for modeling reported in the text (b); VES stations of 2010 (c); 11, TEM stations of 2007–2010 (a) and profile collected in 2011 (b). Number 1 in circle marks Central Kurai basin.
lower part of the slope comprises several subsided blocks with elevations 2000–2500 m, which are partly or fully covered by glacial till and related deposits. The Oroi, Belken, and Aktash blocks with elevations rarely exceeding 2200 m delineate the basin in the west, and the scarps of the Sukor (Chagan-Uzun) plateau (2500–2900 m asl) make up its southeastern and eastern borders. The Cenozoic basin sediments overlie the deformed Riphean–Paleozoic basement composed of sedimentary, volcanic-sedimentary, igneous, and metamorphic rocks and discontinuous Late Cretaceous–Early Paleogene bedrock eluvium (Turkin and Fedak, 2008). The basal redeposited eluvium (Karachum Formation) appears in outcrops and in drill sections in the basin north (Aleshko et al., 1962; Bondarenko et al., 1968). It occurs as whitish kaolinitic loamy sand, greenish-gray or canary-yellow loam with debris of local rocks, greenish-gray or reddishbrown clay, and canary sand, of a total thickness no more than 12.5 m. The formation belongs to the Paleocene according to the current Cenozoic stratigraphy of Gorny Altai (Shokalsky, 1999).
The sedimentary section proper consists of several sequences: (1) the Oligocene–Early Miocene Koshagach Formation of clay and marl with brown coal intercalations; (2) the Middle Miocene–Middle Pliocene Tueryk Formation rocks of different grain sizes: calcareous sandy clay in the basin center and pebble, gravel, sand, and clay along the margin; (3) Early Pleistocene Beken Formation of pebble, gravel, sand, and clay; (4) Late Pleistocene Bashkaus Formation of boulders and debris cemented with straw-yellow loam, found along the southern slope of the Kurai Range; (5) Late Pleistocene glacial, fluvial-glacial, and limnic facies, with embedded or overlying Holocene alluvium, proluvium, and colluvium (Aleshko et al., 1962; Bondarenko et al., 1968; Luzgin and Rusanov, 1992; Shokalsky, 1999).
Resistivity surveys: data acquisition and processing VES data (see Fig. 1 for profile locations) were acquired with a symmetrical quadrupole Schlumberger array at every 400 to 600 m on profiles spaced at 1.5 to 4 km, at the
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Fig. 2. Typical VES curves from different parts of the Kurai basin: field (1) and theoretical (2) curves, and resulting resistivity models.
maximum half-cable length AB/2 from 500 to 1000 m depending on the presumed local sediment thickness. At present, all available VES data collected over the basin territory have been processed using software for forward modeling and inversion with a layered 1D reference model (Epov et al., 1990). The field VES curves from different basin parts (detailed in the following section), synthetic curves, and the resulting models (Fig. 2) often fail to resolve the entire sedimentary section, for several reasons. The rugged topography of an intermontane basin may hamper laying large loops, and grounding may be problematic because of highly resistive rocks on the surface, which often deteriorates the data quality. However, the matter is rather in the very physics of the method, as continuous resistive surface layers (such as permafrost common to mountain valleys in Siberia) can screen the dc responses. The choice of the layered reference model has been discussed in our other publications (Deev et al., 2012; Nevedrova et al., 2011), and here we only note that it based upon all available geological evidence, from well logs and geological cross sections. The selected reference model applies throughout the Kurai basin, though some VES data cannot be interpreted in terms of a layered earth. The latter curves were processed in 3D to estimate approximately the typical distor-
tions. All processing was with advanced software for 2D and 3D forward modeling and inversion. TEM data were acquired with central-loop or offset-loop configurations, the transmitter loop being ungrounded. The central-loop configuration was used in profile surveys (Fig. 1), with the transmitter and receiver loop sizes 200 × 200 m2 and 100 × 100 m2, respectively, and the 50 A transmitter current. This system has high lateral and depth resolution, and the transient responses are stable against distortion from shallow structures. The TEM points were spaced at 400 m, except the station TEM 299 located 1500 m far from TEM 300. The data processing was with ERA and EMS software (Epov et al., 1990; Khabinov et al., 2009). The resistivity patterns obtained by inversion of transients from the eastern basin part (Fig. 3), compared to computed curves, show that the layered-earth model is not always applicable. The misfit between the measured and computed apparent resistivity (ρτ) curves for TEM 300 exceeds 5% at early times. At TEM 306, errors appear near the minimum and at late times. The errors may be due to basement heterogeneity, as in the case of TEM 306 (Epov et al., 2006), or to resistive shallow structures, which is a more likely explanation for TEM 300.
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Fig. 3. Field (1) and theoretical (2) TEM curves from the eastern Kurai basin.
The TEM data have been used to estimate the resistivity of sediments and their thickness over 1500 m. The obtained resistivity patterns (Fig. 3) demonstrate a good depth resolution of the method, which is suitable for studying the deep structure of intermontane basins.
Structure of the Kurai basin from geological and geophysical data The Kurai basin comprises three independent subsided blocks within its structure (Fig. 1): the Central Kurai basin and the Aktash and Eshtykel grabens (Nevedrova et al., 2011). The Aktash graben occupies the northernmost part of the Kurai basin. Its northern border follows a Quaternary thrust of Paleozoic rocks over Cenozoic sediments, which belongs to a system of thrusts of different ages within the Kurai transcrustal fault zone and is expressed geomorphically as a scarp along the foot of the Kurai and Kubadru mountains (Bondarenko et al., 1968). The graben presumably extends westward along the Kubadru foothills, then follows the tectonic gorge of the Chibitka River, and finally opens into the Sorulukel basin. This inference is based on drilling evidence: wells 328, 326 and 323 tap Cenozoic soft sediments (215 m thick in well 328) filling a narrow buried graben beneath the hanging wall of the thrust near the Yarlyamry River (Aktash ore field), and wells 396 and 1/6 also strip the graben sediments but do not penetrate beyond the Cenozoic section, with their bottoms at 264 and 342 m below the surface, respectively (Bondarenko et al., 1968). Being overlain by Late Pleistocene glacial deposits, the thrust must be no younger than Late Pleistocene; the amount of displacement may reach 200 m (Bondarenko et al., 1968; Mukhin and Kuznetsov, 1939). The Aktash graben is separated from the remainder Kurai basin by a narrow S-shaped low horst composed of Devonian and Carboniferous rocks, which makes a piedmont step (Fig. 1), an example of a neotectonic inversion uplift. Its fault boundaries are active, judging by deformation induced by the Chuya (Altai) earthquake of 2003 which has partly rejuve-
nated 1.5–2 m high older fault scarps (Rogozhin and Platonova, 2002; Rogozhin et al., 2008). Trenching in the scarp along the horst northern border reveals reverse faults dipping toward the horst axis, with the amount of displacement from a few centimeters to 2 m. Radiocarbon ages of paleosols from colluvial wedges show that the fault scarp was repeatedly rejuvenated by multiple events at 7820 ± 140 years BP (GIN9085, calendar age 8589–8602 yr), 2850 ± 110 yr (GIN-9081, calendar age 2847–3083 yr BP) and 1040 ± 80 years BP (GIN-9082, calendar age 906–1057 yr). Reverse faults with displacement from 5 to 15 cm dipping northeastward have been found also in trenches along the southern fault scarp. The 14C ages of the deformed sediments, 345 ± 30 (IGAN1702) and 324 ± 30 (IGAN-1692) years BP, indicate that the causative event occurred about 1700 AD. There are three VES profiles available from the Aktash subsided block. The cross section in Fig. 4A was obtained by recent processing of archive VES data of profile 8 traversing the graben. See Fig. 2 for an example processed VES curve (station 193) from the middle of the profile. The soundings with a maximum half-cable length of no more than 750 m, in the presence of surface resistive rocks, failed to provide the required depth resolution as far as the basement. Some old VES data had errors more than 5% (most likely, instrumental error). Nevertheless, the depths to the basement were resolvable at VES points 187–189 (southwest) and 203–205 (northeast). The suggested geological model of the basement surface, as imaged by the resistivity section along profile 8, suggests a ramp origin of the Aktash graben: the Kurai Range and the piedmont step, which border it in the north and south, respectively, are uplifted in steps on a system of thrusts. The archive VES data allow estimating the sediment thickness only approximately, as exceeding 600 m. The resistivity pattern represents a complex structure of the sedimentary section, with a strongly heterogeneous 50 m thick top layer and three relatively low-resistive layers below (60–250, 130–330, and 50–220 Ohm⋅m). The upper one may be assigned to the lower Quaternary deposits and early Pleistocene gravelly loam, sand, and gravlestone, proceeding from parametric VES data (Deev et al., 2012) and well logs from the western part of the graben
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Fig. 4. Resistivity cross sections along VES profiles that traverse the Aktash graben. A: Cross section along profile 8, from archive VES data; B: cross section along profile of 2010; C: field data of 2010 (1) and theoretical curves (2). Red dash lines are inferred faults. Colors show layers with different resistivities.
(Bondarenko et al., 1968). The two other layers may represent Neogene clay and silt, locally with intercalations of sand and coarser deposits, which may have resistivities as high as 400–600 Ohm⋅m. Note that the thicknesses of all layers change markedly from one TEM station to another, this being evidence of sediment deformation at the final ramp stage. VES data collected in 2010 along a short profile in the center of the Aktash graben (Fig. 4B) resolved the basementsediment boundary at two stations (Ku_3, Ku_4), where the maximum half-cable length (AB/2) reached 1 km, and the sediment thickness was found to be 600 m. However, the depth resolution was low at some points in the western profile flank (Ku_1, Ku_2) as AB/2 longer than 600 m was impossible
because of the topography, and only resistivities were estimated. Field VES curves at all stations collected in 2010 (Fig. 4C) had a quality sufficient for interpretation with reference to a layered earth model. Both the instrumental and inversion errors were within 5 to 7%, depending on shallow effects. Inversion of the field VES data gave resistivity patterns consisting of four layers, with sediments and a highly resistive basement. Thus, recent VES data have allowed updating the resistivity model of the Aktash graben and estimating its sediment thickness. The TEM and VES data from the Central Kurai basin and its eastern border likewise have provided more complete deep
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Fig. 5. Resistivity cross sections along profiles that traverse the central and southeastern Kurai basin. A: N–S VES profile 9; B: TEM data. Other symbols as in Fig. 4.
structure evidence. Most of VES profiles across the Central Kurai basin ran northeastward from the Chuya River as far as the basin boundary with the horst (piedmont) along the Aktash graben. According to inversion of TEM responses (Nevedrova et al., 2011), sediments are 300–500 m thick within the basin and are faulted, as well as the basement, at the southern border of the piedmont step (Fig. 5A). The resistivity cross section in Fig. 5A follows N–S profile 9 (Fig. 1) across the Central Kurai basin, with its southern end reaching the piedmont step before the Sukor plateau where the uplift involves the basin sediments (Fig. 6). The profile was collected in the 1980s, at every 500 m, with the maximum half-cable length from 500 to 1000 m, often 500 m. The field (VES 18) and computed resistivity curves,
and the respective model for the Central Kurai basin, are shown in Fig. 2. The acquired VES data turned out to be of insufficient depth resolution. Inversion of responses from the northern profile end, with equivalence and to a large error, gave only tentative sediment thickness estimates, of the order of 400– 500 m. The sediment thicknesses in the southern and central parts of the profile were beyond the resolution of the method. The resulting pattern is divided into two distinct resistivity units, the sediments being less resistive (within 9–80, 110– 150, and 230–450 Ohm⋅m) below and more resistive above, with the thicknesses 300–400 m and 50–300 m, respectively. The lower unit may correspond to the Neogene Koshagach and Tueryk Formations of clay and sand with clay, while the
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Fig. 6. Piedmont step along the western boundary of the Sukor block, view from M-52 roadway (Novosibirsk–Tashanta). White dash lines are inferred faults.
upper one may represent Quaternary deposits, judging by the available parametric data from the neighbor Chuya basin (Deev et al., 2012) and well logs from the central and southern Kurai basin (Luzgin and Rusanov, 1992). More rigorous constraints on the deep structure in the eastern Kurai basin were obtained from TEM data due to their depth resolution much better than the dc VES method. The TEM profile ran in the NW–SE direction, crossed the N–S VES profile, and had the last point TEM 299 at the top of the piedmont step before the Sukor plateau (Fig. 1). Example measured and computed transient responses, as well as the respective models, are shown in Fig. 3 for two TEM stations. The two resistivity patterns are both layered and consist of alternating low- and high-resistivity zones, but differ notably, this being evidence of a heterogeneous structure of the eastern Kurai basin. It was clear already from a preliminary qualitative analysis that the apparent resistivity (ρτ) curves had ascending righthand branch and were suitable for estimating the sediment thickness and the basement resistivity. Quantitative processing of the collected data yielded the resistivity pattern of the whole sedimentary section along the TEM profile (Fig. 5B). The model images the basement descending in steps on a system of normal faults from northwest to southeast and making a deep half-ramp delineated by a reverse fault from
Fig. 7. Eshtykel graben, view from roadway at the saddle before Lake Dzhangyskol. White dash lines are inferred faults.
the side of the piedmont. The thickest sediments reach almost 1600 m at TEM 300. Low-resistivity layers (26–43 and 70– 90 Ohm⋅m) compensating the basement surface subsidence are correlated with rather argillic sediments of the Tueryk and Koshagach Formations according to well logs (Luzgin and Rusanov, 1992), as well as another low-resistivity layer (11–16 Ohm⋅m), possibly part of the Tueryk Formation. The highly resistive layers upsection may represent coarse deposits of the Beken Formation (boulders, pebbles, and gravel) and Quaternary sediments above it; more conductive layers may be sand-clay facies. The patterns derived from VES and TEM data (Fig. 5) differ in the number of layers and resistivity values, which may result from different physics and technologies of the two methods and, on the other hand, from structural heterogeneity of the respective basin areas. Nevertheless, both models image the key features of the eastern Kurai basin: (1) deep subsidence in the southern profile ends; (2) two units, with lower resistivity below and higher resistivity above; (3) greater sediment thicknesses within the subsided block, in both units. The lower sequence becomes thicker as it fills the lower part of the graben expressed in the basement topography, while the upper sequence compensates the subsidence before the front of the growing piedmont. The Eshtykel graben strikes in the NW direction along the foot of the North Chuya Range. Its southern fault boundary with the North Chuya Range and its northern piedmont are partly buried under Late Pleistocene glacial deposits. The Northern fault which delineates the graben from Lake Karakol to the Akturu River shows up on the surface as a scarp, locally reaching tens of meters high, with bedrock exposed in its wall (Fig. 7). The fault borders of the Eshtykel graben were revealed in the hypocenter pattern of aftershocks that followed the Chuya (Altai) earthquake of 2003. Their projections onto a plane across the graben highlighted two faults spaced about 4 km and dipping beneath the North Chuya Range at 70° (Novikov et al., 2008). The earthquake of 2003 produced surface rupture (systems of fractures 6 to 40 m long but with minor displacement) and landsliding (Vysotsky et al., 2004). The alluvium of the Akturu high floodplain terrace and the overlying slope wash became cut by a system of reverse faults with their planes dipping toward the North Chuya Range (Rogozhin et al., 2008).
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Fig. 8. Resistivity cross sections along VES profiles 2 and 3.
Several N–S oriented VES profiles, with the 1000 m maximum half-cable length, collected earlier in the southwestern Eshtykel graben have been reprocessed recently using SONET software. See Fig. 8 for resistivity cross sections along profiles 2 and 3. High-resistivity rocks crop out in the northern part of the profiles, at stations 42, 41, 40, 35, 36 (profile 2) and 105–111 (profile 3), which correspond to the Salgandui basement block. The latter, together with the Akkoba and Karakol basement blocks, separates the Central Kurai basin and the Eshtykel graben. The top surfaces of the blocks lie at the elevations 1500–2000 m (Fig. 1). The northern Salgandui block is covered with 100–150 m thick soft sediments (VES stations 30–34, 38, 39 of profile 2 and 97–104 of profile 3). The VES data resolve the whole sedimentary section upon the northern Salgandui block, and the basement surface. Sharp changes in sediment thicknesses, down to zero, mark fault bounds of the basin superposed on the Salgandui block. According to the VES data, the section in this part of the Kurai basin consists of a highly resistive upper unit (layers 1 and 2) corresponding to Quaternary deposits and two more layers (3 and 4) in the lower unit. The latter may correspond to Neogene fine clastics with variable resistivities; the basement has a resistivity of 1000 Ohm⋅m.
South of the Salgandui block, both sections image the Eshtykel graben filled with more than 600 m of sediments and thrust over by the North Chuya Range. About 150–200 m of sediments are Quaternary rocks of diverse lithologies and different resistivities. Two layers below, with the resistivities 150–180 and 50–60 Ohm⋅m, well defined along profile 3, may correspond to the Tueryk and Koshagach Formations, respectively (Luzgin and Rusanov, 1992), as evidenced by log data (wells 13, 14, 15). However, the boundary between the two layers is poorly traceable along profile 2, and the sediments mostly look uniform, more than 400 m thick, with resistivities from 70 to 120 Ohm⋅m. Two layers, 29–100 and 140– 400 Ohm⋅m, appear only at VES 43–45. The ~1000– 3000 Ohm⋅m zone below the sediments is interpreted as the Paleozoic basement with reference to geological data. The sediment thickness in the southwestern part of the graben is, however, unresolved in the VES data and is only inferred to exceed 600–650 m.
The structural history of the Kurai basin The Kurai basin, with its structure consisting of several fault blocks marked by abrupt changes in sediment thickness,
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Fig. 9. Model of a basin formed in a transtension setting, after (Burtman et al., 1963). 1 and 2 are structures of shear and extension, respectively.
Fig. 10. Model of extrusion thrusts in Kurai Range (Chikov et al., 2008).
has had a multistage history. The mountains flanking the Kurai and Chuya basin underwent the first event of slow uplift in the Early Paleogene, when the Late Cretaceous period of tectonic stability with surface planation, intense denudation and chemical weathering was followed by redeposition of eluvium (Karachum Formation) composed of slope-wash and talus deposits with coarse intercalations of debris, pebble, and gravel (Devyatkin, 1965; Delvaux et al., 1995; Deev et al., 2012). The pattern of sediment thicknesses in the Kurai basin imaged by the resistivity surveys, with the deepest subsidence along the northern (Aktash graben), southern (Eshtykel graben), and eastern borders, prompts that it originated as a pull-apart basin. Judging by the basin geometry, it was apparently a setting of transtension with left-lateral strike-slip faulting, when a large undeformed block between fault walls could subside as a single whole (Burtman et al., 1963). Subsidence of large blocks may occur between converging and diverging shear zones and give rise to flat-bottom basins with heavily faulted sides. Thus, the basins become the deepest along the sides and acquire a particular geometry of their transversal profiles (Fig. 9). This inference is consistent with the fault pattern in the western flank of the Quaternary thrust, near the Krasnye Vorota locality, where Delvaux et al. (1995) reconstructed tentatively a Tertiary stress tensor corresponding to an oblique WNW reverse motion with a sinistral strike-slip component, on an NE dipping plane. The predominant strike-
slip faulting explains the absence of Paleogene vertical offset inferred from fission-track thermochronology (De Grave et al., 2007; Glorie et al., 2012). During the Paleogene stage, the Kurai basin underwent limnic deposition of fine clay-size sediments, with coarser grain sizes (pebble, gravel, and sand) along the basin sides (Devyatkin, 1965; Delvaux et al., 1995; Luzgin and Rusanov, 1992), which agrees with mostly low resistivities of the respective geoelectric layers. In Quaternary time, the Central Kurai basin formed within the northern Kurai basin, in a setting of right-lateral (?) strike-slip faulting; several grabens appeared also in the Chuya valley between the Sukor block and the Kurai Range, as well as between the Belkenek, Karakol, and Akkoba blocks on one side and the Aktash block, with other blocks separating the Aktash graben, on the other side. The tectonic setting changed to compression (along the NE–SW principal axis). The flanking mountains grew rapidly to the present-day elevations (De Grave et al., 2007; Glorie et al., 2012) and, at the same time, were thrusting upon the basin along the border faults; the latter experienced vertical motions by the extrusion mechanism (Fig. 10) and the basin itself became a ramp (Delvaux et al., 1995). The same process gave rise to uplifts along the basin margins, and the marginal grabens turned into ramps and half-ramps, partly being controlled by counterclockwise rotation of the Sukor block in the eastern part the basin (Buslov et al., 1999). The Quaternary neotectonic stage was the time of coarse sand and molasse deposition (brownish rocks of the Beken and Bashkaus Formations), while the area of limnic deposition reduced considerably. Later in the Late Pleistocene, the area experienced glaciations intermittent with warmer periods, in the changed climate and geomorphic settings with rather high elevations (Zolnikov, 2009). Deposition of that stage formed a complex gray till sequence consisting of moraine diamictite, fluvioglacial boulders and pebbles, glacial-lacustrine and galcier-damlake sand, silt, and clay, and deposits of cataclysmic superstreams. Besides the glacial facies, there exist multifacies Holocene deposits on the basin surface, which are either superposed upon or embedded into older sedimentary sequences. Generally, the Quaternary deposits are of coarser grain sizes than the Paleogene–Neogene rocks and, correspondingly, have much higher resistivities.
Conclusions The controlled-source resistivity (VES and TEM) surveys have provided more constraints on the structure of the Kurai basin and its junction with the flanking mountains. The basin is the deepest along its northern (Aktash graben), southern (Eshtykel graben), and eastern margins, where sediments reach thicknesses of 600–1600 m (against 300–500 m in the Central Kurai basin). The sedimentary fill comprises two resistivity units corresponding to two sequences deposited at different stages of the basin history. The lower less resistive unit consists of Paleogene–Neogene lacustrine clay and the higherresistivity upper unit represents coarser Quaternary deposits.
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In Paleogene–Neogene time, the basin formed by the left-lateral pull-apart mechanism, when its weakly deformed central part subsided as a single whole between fault walls. The earliest Quaternary strike-slip faulting in a setting of overall compression produced the Central Kurai subbasin within the northern Kurai basin, while the flanking ranges and fault blocks thrust upon the basin to transform it into a ramp. Thereby piedmont steps rose along the basin margins and the marginal grabens became ramps and half-ramps. The study was supported by grant 12-05-33048- mol_a_ved from the Russian Foundation for Basic Research.
References Aleshko, Yu.B., Landa, M.N., Stolbina, I.V., Rakovets, O.A., 1962. Geological Map of the USSR, Scale 1:200,000. Ser. Altai. Sheet M-45-XVI. Explanatory Note [in Russian]. Gosgeoltekhizdat, Moscow. Bondarenko, P.M., Devyatkin, E.V., Liskun, I.G., 1968. Cenozoic neotectonics and stratigraphy of the Aktash region in the Kurai neotectonic zone of Gorny Altai, in: Problems of Geomorphology and Neotectonics of Mountain Provinces in Siberia and Russian Far East. Proc. USSR Workshop on Geomorphology and Neotectonics of Siberia and Russian Far East [in Russian]. Nauka, Novosibirsk, Book II, pp. 65–73. Burtman, V.S., Lukiyanov, A.V., Peive, A.V., Ruzhentsev, S.V., 1963. Strike-slip faulting and some methods of its studies, in: Faults and Horizontal Crustal Motions [in Russian]. Izd. AN SSSR, Moscow, pp. 5–34 (Transactions, GIN AN SSSR, Issue 80). Buslov, M.M., Zykin, V.S., Novikov, I.S., Delvaux, D., 1999. The Cenozoic structure and geodynamics of the Chuya intermontane basin in Gorny Altai. Geologiya i Geofizika (Russian Geology and Geophysics) 40 (12), 1720–1736 (1687–1701). Chikov, B.M., Zinoviev, S.V., Deev, E.V., 2008. Mesozoic and Cenozoic collisional structures of the southern Great Altai. Russian Geology and Geophysics (Geologiya i Geofizika) 49 (5), 323–331 (426–438). De Grave, J., Buslov, M.M., Van den Haute, P., Dehandschutter, B., Delvaux, D., 2007. Meso-Cenozoic evolution of mountain range—intramontane basin systems in the Southern Siberian Altai mountains by apatite fission-track thermochronology, in: Lacombe, O., Lavé, J., Roure, F., Vergés, J. (Eds.), Thrust Belts and Foreland Basins: From Fold Kinematics to Hydrocarbon. Frontiers in Earth Sciences, Berlin–Heidelberg, pp. 457– 470. Deev, E.V., Nevedrova, N.N., Zolnikov, I.D., Rusanov, G.G., Ponomarev, P.V., 2012. Geoelectrical studies of the Chuya basin sedimentary fill (Gorny Altai). Russian Geology and Geophysics (Geologiya i Geofizika) 53 (1), 92–107 (120–139).
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Delvaux, D., Theunissen, K., Van-der-Meer, R., Berzin, N.A., 1995. Dynamics and paleostress of the Cenozoic Kurai–Chuya depression of Gorny Altai (South Siberia): tectonic and climatic control. Geologiya i Geofizika (Russian Geology and Geophysics) 36 (10), 31–51 (26–45). Devyatkin, E.V., 1965. Cenozoic Deposition and Neotectonics in the Southeastern Altai [in Russian]. Nauka, Moscow (GIN Transactions, Issue 126). Epov, M.I., Dashevsky, Yu.A., Eltsov, I.N., 1990. Machine-Aided Interpretation of EM Data [in Russian]. IGiG SO AN SSSR, Novosibirsk (Preprint No. 3). Epov, M.I., Nevedrova, N.N., Antonov, E.Yu., 2006. A method for TEM data processing with regard to typical distortions of transient responses from active seismic areas. Geofizicheskii Vestnik, No. 6, 8–14. Glorie, S., De Grave, J., Buslov, M.M., Zhimulev, F.I., Elburg, M.A., Van den Haute, P., 2012. Structural control on Meso-Cenozoic tectonic reactivation and denudation in the Siberian Altai: Insights from multimethod thermochronometry. Tectonophysics 544–545, 75–92. Khabinov, O.G., Chalov, I.A., Vlasov, A.A., Antonov, E.Yu., 2009. The EMS system for TEM data processing, in: Proc. GEO–Sibir’-2009. A Collection of Papers [in Russian]. Izd. SGGA, Novosibirsk, pp. 108–113. Luzgin, B.N., Rusanov, G.G. 1992. Characteristics of formation of Neogene deposits in the southeastern Gorny Altai. Geologiya i Geofizika (Russian Geology and Geophysics) 33 (4), 23–29 (18–23). Mukhin, A.S., Kuznetsov, V.A., 1939. Quaternary thrusts in the Southeastern Altai. Vest. ZSGU, No. 1, 49–52. Nevedrova, N.N., Sanchaa, A.M., Deev, E.V., 2011. The geoelectrical and tectonic structure of the Kurai basin in Gorny Altai. Geofizika, No. 6, 56–64. Novikov, I.S., Emanov, A.A., Leskova, E.V., Batalev, V.Yu., Pybin, A.K., Bataleva, E.A., 2008. The system of neotectonic faults in southeastern Altai: orientations and geometry of motion. Russian Geology and Geophysics (Geologiya i Geofizika) 49 (11), 859–867 (1139–1149). Rogozhin, E.A., Platonova, S.G., 2002. Source Areas of Holocene Large Earthquakes in Gorny Altai [in Russian]. OIFZ RAN, Moscow. Rogozhin, E.A., Ovsyuchenko, A.N., Marakhanov, A.V., 2008. Holocene great earthquakes in southern Gorny Altai. Izv. RAN, Fizika Zemli, No. 6, 31–51. Shokalsky, S.P. (Ed.), 1999. Geological Map of the Russian Federation. Scale 1:200,000. Legend: Altai Series, 2nd ed. Explanatory Note [in Russian]. Novokuznetsk. Turkin, Yu.A., Fedak, S.I., 2008. Geology and Sedimentary Complexes of Gorny Altai [in Russian]. STT, Tomsk. Vysotsky, E.M., Novikov, I.S., Agatova, A.R., Deev, E.V., Skobelitsin, G.A., Makarova, D.D., 2004. The structure of ground failure zone after the Chuya earthquake of 2003 (Gorny Altai), in: Geomorphic Processes: Theory, Practice, and Methods. Proc. XXVIII Plenum of the Geomorphological commission of the Russian Academy of Sciences [in Russian]. IG SO RAN, Novosibirsk, pp. 65–67. Zolnikov, I.D., 2009. Late Pleistocene super-floods in Gorny Altai: links with the deposition and geomorphic history of the West Siberian Plain. Bull. Commission on the Quaternary, No. 69, 58–70.
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ScienceDirect Russian Geology and Geophysics 55 (2014) 98–107 www.elsevier.com/locate/rgg
Deep structure and margins of the Kurai Basin (Gorny Altai), from controlled-source resistivity data N.N. Nevedrova a,b,*, E.V. Deev a,b, A.M. Sanchaa a a
A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia b Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia Received 4 December 2012; accepted 10 April 2013
Abstract The deep structure of the Kurai basin and its junction with the flanking mountains has been studied by controlled-source resistivity surveys (vertical electric sounding and transient electromagnetic methods). According to the data processing results, the basin is the deepest along its northern, southern, and eastern margins. The sedimentary fill comprises two resistivity units corresponding to two sequences deposited at different stages of the basin history. The lower, less resistive unit consists of Paleogene–Neogene lacustrine clay and the higher-resistivity upper unit represents coarser Quaternary deposits. In Paleogene–Neogene time, the basin formed by the left-lateral pull-apart mechanism. The earliest Quaternary strike-slip faulting in the setting of overall compression produced the Central Kurai basin within the northern Kurai basin, while the flanking ranges and fault blocks thrust upon the basin transforming it into a ramp. Thus, piedmont steps rose along the basin margins, and the marginal grabens became ramps and half-ramps. © 2014, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: neotectonics; resistivity surveys; TEM; VES; Kurai basin; Gorny Altai
Introduction The Kurai intermontane basin is the second largest basin in Gorny Altai, which together with the Chuya basin makes up a Cenozoic system in the southeastern part of the area. The two basins are separated by the Sukor plateau and share a similar history of neotectonics and deposition. The Kurai basin has a complex tectonic relation with the flanking mountains and a heterogeneous structure with several subbasins in its limits inferred from geological and geophysical data (Bondarenko et al., 1968; Delvaux et al., 1995; Nevedrova et al., 2011). A large amount of vertical electric sounding (VES) profiles were collected in the Kurai basin in the 1960–1980s (Fig. 1), but some have been unsuitable to estimate the sediment thickness, for different reasons. Lately, in 2007 through 2011, more knowledge of the basin structure have been obtained due to integration of controlled-source VES and TEM (transient electromagnetic) surveys (Fig. 1). Inversion of TEM responses acquired in different years allows estimating the resistivity and
* Corresponding author. E-mail address: [email protected] (N.N. Nevedrova)
thickness of the basin sediments over the whole territory of the Kurai basin. In this study we interpret VES and TEM resistivity data to model the deep structure of the Kurai basin and its junction with the flanking mountains, with implications for its history and formation mechanism.
Geological and neotectonic background The Kurai basin has a rhomb-shaped geometry, with a ~35 km (W–E) and ~20 km (N–S) long and short axes, respectively (Fig. 1). The basin is surrounded by mountains. In the north, it is the Kubadru neotectonic uplift, with its highest summit (Verkhovie Ortolyka) at 3446 m asl. In the northeast there are the en-echelon Kurai and Tongulak Ranges, both being the highest (up to 3200 m) parts of the strongly asymmetric Kubadru–Bashkaus block with its long low-angle slope facing the Bashkaus valley; the slope facing the Kurai basin has a stepped profile with scarps at 2850–2950, 2400–2500, and 2000–2100 m. The same distinct scarps delineate the basin in the southwest, from the North Chuya Range rising 4000 m asl. The
1068-7971/$ - see front matter D 201 4, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2013.12.008 +
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Fig.1. Simplified neotectonics of the Kurai basin, with locations of VES and TEM profiles. 1–6, uplifted blocks, fully or partly buried under sediments, with different elevations above sealevel (m): higher than 3500 (1), from 3500 to 3000 (2), from 3000 to 2500 (3), from 2500 to 2000 (4), from 2000 to 1500 (5), and from 2500 to 2000 (6); 7, sedimentary sequences lying over blocks lower than 2000 m; 8, observed faults; 9, inferred faults; 10, VES stations: archive data (a); archive data used for modeling reported in the text (b); VES stations of 2010 (c); 11, TEM stations of 2007–2010 (a) and profile collected in 2011 (b). Number 1 in circle marks Central Kurai basin.
lower part of the slope comprises several subsided blocks with elevations 2000–2500 m, which are partly or fully covered by glacial till and related deposits. The Oroi, Belken, and Aktash blocks with elevations rarely exceeding 2200 m delineate the basin in the west, and the scarps of the Sukor (Chagan-Uzun) plateau (2500–2900 m asl) make up its southeastern and eastern borders. The Cenozoic basin sediments overlie the deformed Riphean–Paleozoic basement composed of sedimentary, volcanic-sedimentary, igneous, and metamorphic rocks and discontinuous Late Cretaceous–Early Paleogene bedrock eluvium (Turkin and Fedak, 2008). The basal redeposited eluvium (Karachum Formation) appears in outcrops and in drill sections in the basin north (Aleshko et al., 1962; Bondarenko et al., 1968). It occurs as whitish kaolinitic loamy sand, greenish-gray or canary-yellow loam with debris of local rocks, greenish-gray or reddishbrown clay, and canary sand, of a total thickness no more than 12.5 m. The formation belongs to the Paleocene according to the current Cenozoic stratigraphy of Gorny Altai (Shokalsky, 1999).
The sedimentary section proper consists of several sequences: (1) the Oligocene–Early Miocene Koshagach Formation of clay and marl with brown coal intercalations; (2) the Middle Miocene–Middle Pliocene Tueryk Formation rocks of different grain sizes: calcareous sandy clay in the basin center and pebble, gravel, sand, and clay along the margin; (3) Early Pleistocene Beken Formation of pebble, gravel, sand, and clay; (4) Late Pleistocene Bashkaus Formation of boulders and debris cemented with straw-yellow loam, found along the southern slope of the Kurai Range; (5) Late Pleistocene glacial, fluvial-glacial, and limnic facies, with embedded or overlying Holocene alluvium, proluvium, and colluvium (Aleshko et al., 1962; Bondarenko et al., 1968; Luzgin and Rusanov, 1992; Shokalsky, 1999).
Resistivity surveys: data acquisition and processing VES data (see Fig. 1 for profile locations) were acquired with a symmetrical quadrupole Schlumberger array at every 400 to 600 m on profiles spaced at 1.5 to 4 km, at the
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Fig. 2. Typical VES curves from different parts of the Kurai basin: field (1) and theoretical (2) curves, and resulting resistivity models.
maximum half-cable length AB/2 from 500 to 1000 m depending on the presumed local sediment thickness. At present, all available VES data collected over the basin territory have been processed using software for forward modeling and inversion with a layered 1D reference model (Epov et al., 1990). The field VES curves from different basin parts (detailed in the following section), synthetic curves, and the resulting models (Fig. 2) often fail to resolve the entire sedimentary section, for several reasons. The rugged topography of an intermontane basin may hamper laying large loops, and grounding may be problematic because of highly resistive rocks on the surface, which often deteriorates the data quality. However, the matter is rather in the very physics of the method, as continuous resistive surface layers (such as permafrost common to mountain valleys in Siberia) can screen the dc responses. The choice of the layered reference model has been discussed in our other publications (Deev et al., 2012; Nevedrova et al., 2011), and here we only note that it based upon all available geological evidence, from well logs and geological cross sections. The selected reference model applies throughout the Kurai basin, though some VES data cannot be interpreted in terms of a layered earth. The latter curves were processed in 3D to estimate approximately the typical distor-
tions. All processing was with advanced software for 2D and 3D forward modeling and inversion. TEM data were acquired with central-loop or offset-loop configurations, the transmitter loop being ungrounded. The central-loop configuration was used in profile surveys (Fig. 1), with the transmitter and receiver loop sizes 200 × 200 m2 and 100 × 100 m2, respectively, and the 50 A transmitter current. This system has high lateral and depth resolution, and the transient responses are stable against distortion from shallow structures. The TEM points were spaced at 400 m, except the station TEM 299 located 1500 m far from TEM 300. The data processing was with ERA and EMS software (Epov et al., 1990; Khabinov et al., 2009). The resistivity patterns obtained by inversion of transients from the eastern basin part (Fig. 3), compared to computed curves, show that the layered-earth model is not always applicable. The misfit between the measured and computed apparent resistivity (ρτ) curves for TEM 300 exceeds 5% at early times. At TEM 306, errors appear near the minimum and at late times. The errors may be due to basement heterogeneity, as in the case of TEM 306 (Epov et al., 2006), or to resistive shallow structures, which is a more likely explanation for TEM 300.
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Fig. 3. Field (1) and theoretical (2) TEM curves from the eastern Kurai basin.
The TEM data have been used to estimate the resistivity of sediments and their thickness over 1500 m. The obtained resistivity patterns (Fig. 3) demonstrate a good depth resolution of the method, which is suitable for studying the deep structure of intermontane basins.
Structure of the Kurai basin from geological and geophysical data The Kurai basin comprises three independent subsided blocks within its structure (Fig. 1): the Central Kurai basin and the Aktash and Eshtykel grabens (Nevedrova et al., 2011). The Aktash graben occupies the northernmost part of the Kurai basin. Its northern border follows a Quaternary thrust of Paleozoic rocks over Cenozoic sediments, which belongs to a system of thrusts of different ages within the Kurai transcrustal fault zone and is expressed geomorphically as a scarp along the foot of the Kurai and Kubadru mountains (Bondarenko et al., 1968). The graben presumably extends westward along the Kubadru foothills, then follows the tectonic gorge of the Chibitka River, and finally opens into the Sorulukel basin. This inference is based on drilling evidence: wells 328, 326 and 323 tap Cenozoic soft sediments (215 m thick in well 328) filling a narrow buried graben beneath the hanging wall of the thrust near the Yarlyamry River (Aktash ore field), and wells 396 and 1/6 also strip the graben sediments but do not penetrate beyond the Cenozoic section, with their bottoms at 264 and 342 m below the surface, respectively (Bondarenko et al., 1968). Being overlain by Late Pleistocene glacial deposits, the thrust must be no younger than Late Pleistocene; the amount of displacement may reach 200 m (Bondarenko et al., 1968; Mukhin and Kuznetsov, 1939). The Aktash graben is separated from the remainder Kurai basin by a narrow S-shaped low horst composed of Devonian and Carboniferous rocks, which makes a piedmont step (Fig. 1), an example of a neotectonic inversion uplift. Its fault boundaries are active, judging by deformation induced by the Chuya (Altai) earthquake of 2003 which has partly rejuve-
nated 1.5–2 m high older fault scarps (Rogozhin and Platonova, 2002; Rogozhin et al., 2008). Trenching in the scarp along the horst northern border reveals reverse faults dipping toward the horst axis, with the amount of displacement from a few centimeters to 2 m. Radiocarbon ages of paleosols from colluvial wedges show that the fault scarp was repeatedly rejuvenated by multiple events at 7820 ± 140 years BP (GIN9085, calendar age 8589–8602 yr), 2850 ± 110 yr (GIN-9081, calendar age 2847–3083 yr BP) and 1040 ± 80 years BP (GIN-9082, calendar age 906–1057 yr). Reverse faults with displacement from 5 to 15 cm dipping northeastward have been found also in trenches along the southern fault scarp. The 14C ages of the deformed sediments, 345 ± 30 (IGAN1702) and 324 ± 30 (IGAN-1692) years BP, indicate that the causative event occurred about 1700 AD. There are three VES profiles available from the Aktash subsided block. The cross section in Fig. 4A was obtained by recent processing of archive VES data of profile 8 traversing the graben. See Fig. 2 for an example processed VES curve (station 193) from the middle of the profile. The soundings with a maximum half-cable length of no more than 750 m, in the presence of surface resistive rocks, failed to provide the required depth resolution as far as the basement. Some old VES data had errors more than 5% (most likely, instrumental error). Nevertheless, the depths to the basement were resolvable at VES points 187–189 (southwest) and 203–205 (northeast). The suggested geological model of the basement surface, as imaged by the resistivity section along profile 8, suggests a ramp origin of the Aktash graben: the Kurai Range and the piedmont step, which border it in the north and south, respectively, are uplifted in steps on a system of thrusts. The archive VES data allow estimating the sediment thickness only approximately, as exceeding 600 m. The resistivity pattern represents a complex structure of the sedimentary section, with a strongly heterogeneous 50 m thick top layer and three relatively low-resistive layers below (60–250, 130–330, and 50–220 Ohm⋅m). The upper one may be assigned to the lower Quaternary deposits and early Pleistocene gravelly loam, sand, and gravlestone, proceeding from parametric VES data (Deev et al., 2012) and well logs from the western part of the graben
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Fig. 4. Resistivity cross sections along VES profiles that traverse the Aktash graben. A: Cross section along profile 8, from archive VES data; B: cross section along profile of 2010; C: field data of 2010 (1) and theoretical curves (2). Red dash lines are inferred faults. Colors show layers with different resistivities.
(Bondarenko et al., 1968). The two other layers may represent Neogene clay and silt, locally with intercalations of sand and coarser deposits, which may have resistivities as high as 400–600 Ohm⋅m. Note that the thicknesses of all layers change markedly from one TEM station to another, this being evidence of sediment deformation at the final ramp stage. VES data collected in 2010 along a short profile in the center of the Aktash graben (Fig. 4B) resolved the basementsediment boundary at two stations (Ku_3, Ku_4), where the maximum half-cable length (AB/2) reached 1 km, and the sediment thickness was found to be 600 m. However, the depth resolution was low at some points in the western profile flank (Ku_1, Ku_2) as AB/2 longer than 600 m was impossible
because of the topography, and only resistivities were estimated. Field VES curves at all stations collected in 2010 (Fig. 4C) had a quality sufficient for interpretation with reference to a layered earth model. Both the instrumental and inversion errors were within 5 to 7%, depending on shallow effects. Inversion of the field VES data gave resistivity patterns consisting of four layers, with sediments and a highly resistive basement. Thus, recent VES data have allowed updating the resistivity model of the Aktash graben and estimating its sediment thickness. The TEM and VES data from the Central Kurai basin and its eastern border likewise have provided more complete deep
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Fig. 5. Resistivity cross sections along profiles that traverse the central and southeastern Kurai basin. A: N–S VES profile 9; B: TEM data. Other symbols as in Fig. 4.
structure evidence. Most of VES profiles across the Central Kurai basin ran northeastward from the Chuya River as far as the basin boundary with the horst (piedmont) along the Aktash graben. According to inversion of TEM responses (Nevedrova et al., 2011), sediments are 300–500 m thick within the basin and are faulted, as well as the basement, at the southern border of the piedmont step (Fig. 5A). The resistivity cross section in Fig. 5A follows N–S profile 9 (Fig. 1) across the Central Kurai basin, with its southern end reaching the piedmont step before the Sukor plateau where the uplift involves the basin sediments (Fig. 6). The profile was collected in the 1980s, at every 500 m, with the maximum half-cable length from 500 to 1000 m, often 500 m. The field (VES 18) and computed resistivity curves,
and the respective model for the Central Kurai basin, are shown in Fig. 2. The acquired VES data turned out to be of insufficient depth resolution. Inversion of responses from the northern profile end, with equivalence and to a large error, gave only tentative sediment thickness estimates, of the order of 400– 500 m. The sediment thicknesses in the southern and central parts of the profile were beyond the resolution of the method. The resulting pattern is divided into two distinct resistivity units, the sediments being less resistive (within 9–80, 110– 150, and 230–450 Ohm⋅m) below and more resistive above, with the thicknesses 300–400 m and 50–300 m, respectively. The lower unit may correspond to the Neogene Koshagach and Tueryk Formations of clay and sand with clay, while the
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Fig. 6. Piedmont step along the western boundary of the Sukor block, view from M-52 roadway (Novosibirsk–Tashanta). White dash lines are inferred faults.
upper one may represent Quaternary deposits, judging by the available parametric data from the neighbor Chuya basin (Deev et al., 2012) and well logs from the central and southern Kurai basin (Luzgin and Rusanov, 1992). More rigorous constraints on the deep structure in the eastern Kurai basin were obtained from TEM data due to their depth resolution much better than the dc VES method. The TEM profile ran in the NW–SE direction, crossed the N–S VES profile, and had the last point TEM 299 at the top of the piedmont step before the Sukor plateau (Fig. 1). Example measured and computed transient responses, as well as the respective models, are shown in Fig. 3 for two TEM stations. The two resistivity patterns are both layered and consist of alternating low- and high-resistivity zones, but differ notably, this being evidence of a heterogeneous structure of the eastern Kurai basin. It was clear already from a preliminary qualitative analysis that the apparent resistivity (ρτ) curves had ascending righthand branch and were suitable for estimating the sediment thickness and the basement resistivity. Quantitative processing of the collected data yielded the resistivity pattern of the whole sedimentary section along the TEM profile (Fig. 5B). The model images the basement descending in steps on a system of normal faults from northwest to southeast and making a deep half-ramp delineated by a reverse fault from
Fig. 7. Eshtykel graben, view from roadway at the saddle before Lake Dzhangyskol. White dash lines are inferred faults.
the side of the piedmont. The thickest sediments reach almost 1600 m at TEM 300. Low-resistivity layers (26–43 and 70– 90 Ohm⋅m) compensating the basement surface subsidence are correlated with rather argillic sediments of the Tueryk and Koshagach Formations according to well logs (Luzgin and Rusanov, 1992), as well as another low-resistivity layer (11–16 Ohm⋅m), possibly part of the Tueryk Formation. The highly resistive layers upsection may represent coarse deposits of the Beken Formation (boulders, pebbles, and gravel) and Quaternary sediments above it; more conductive layers may be sand-clay facies. The patterns derived from VES and TEM data (Fig. 5) differ in the number of layers and resistivity values, which may result from different physics and technologies of the two methods and, on the other hand, from structural heterogeneity of the respective basin areas. Nevertheless, both models image the key features of the eastern Kurai basin: (1) deep subsidence in the southern profile ends; (2) two units, with lower resistivity below and higher resistivity above; (3) greater sediment thicknesses within the subsided block, in both units. The lower sequence becomes thicker as it fills the lower part of the graben expressed in the basement topography, while the upper sequence compensates the subsidence before the front of the growing piedmont. The Eshtykel graben strikes in the NW direction along the foot of the North Chuya Range. Its southern fault boundary with the North Chuya Range and its northern piedmont are partly buried under Late Pleistocene glacial deposits. The Northern fault which delineates the graben from Lake Karakol to the Akturu River shows up on the surface as a scarp, locally reaching tens of meters high, with bedrock exposed in its wall (Fig. 7). The fault borders of the Eshtykel graben were revealed in the hypocenter pattern of aftershocks that followed the Chuya (Altai) earthquake of 2003. Their projections onto a plane across the graben highlighted two faults spaced about 4 km and dipping beneath the North Chuya Range at 70° (Novikov et al., 2008). The earthquake of 2003 produced surface rupture (systems of fractures 6 to 40 m long but with minor displacement) and landsliding (Vysotsky et al., 2004). The alluvium of the Akturu high floodplain terrace and the overlying slope wash became cut by a system of reverse faults with their planes dipping toward the North Chuya Range (Rogozhin et al., 2008).
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Fig. 8. Resistivity cross sections along VES profiles 2 and 3.
Several N–S oriented VES profiles, with the 1000 m maximum half-cable length, collected earlier in the southwestern Eshtykel graben have been reprocessed recently using SONET software. See Fig. 8 for resistivity cross sections along profiles 2 and 3. High-resistivity rocks crop out in the northern part of the profiles, at stations 42, 41, 40, 35, 36 (profile 2) and 105–111 (profile 3), which correspond to the Salgandui basement block. The latter, together with the Akkoba and Karakol basement blocks, separates the Central Kurai basin and the Eshtykel graben. The top surfaces of the blocks lie at the elevations 1500–2000 m (Fig. 1). The northern Salgandui block is covered with 100–150 m thick soft sediments (VES stations 30–34, 38, 39 of profile 2 and 97–104 of profile 3). The VES data resolve the whole sedimentary section upon the northern Salgandui block, and the basement surface. Sharp changes in sediment thicknesses, down to zero, mark fault bounds of the basin superposed on the Salgandui block. According to the VES data, the section in this part of the Kurai basin consists of a highly resistive upper unit (layers 1 and 2) corresponding to Quaternary deposits and two more layers (3 and 4) in the lower unit. The latter may correspond to Neogene fine clastics with variable resistivities; the basement has a resistivity of 1000 Ohm⋅m.
South of the Salgandui block, both sections image the Eshtykel graben filled with more than 600 m of sediments and thrust over by the North Chuya Range. About 150–200 m of sediments are Quaternary rocks of diverse lithologies and different resistivities. Two layers below, with the resistivities 150–180 and 50–60 Ohm⋅m, well defined along profile 3, may correspond to the Tueryk and Koshagach Formations, respectively (Luzgin and Rusanov, 1992), as evidenced by log data (wells 13, 14, 15). However, the boundary between the two layers is poorly traceable along profile 2, and the sediments mostly look uniform, more than 400 m thick, with resistivities from 70 to 120 Ohm⋅m. Two layers, 29–100 and 140– 400 Ohm⋅m, appear only at VES 43–45. The ~1000– 3000 Ohm⋅m zone below the sediments is interpreted as the Paleozoic basement with reference to geological data. The sediment thickness in the southwestern part of the graben is, however, unresolved in the VES data and is only inferred to exceed 600–650 m.
The structural history of the Kurai basin The Kurai basin, with its structure consisting of several fault blocks marked by abrupt changes in sediment thickness,
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Fig. 9. Model of a basin formed in a transtension setting, after (Burtman et al., 1963). 1 and 2 are structures of shear and extension, respectively.
Fig. 10. Model of extrusion thrusts in Kurai Range (Chikov et al., 2008).
has had a multistage history. The mountains flanking the Kurai and Chuya basin underwent the first event of slow uplift in the Early Paleogene, when the Late Cretaceous period of tectonic stability with surface planation, intense denudation and chemical weathering was followed by redeposition of eluvium (Karachum Formation) composed of slope-wash and talus deposits with coarse intercalations of debris, pebble, and gravel (Devyatkin, 1965; Delvaux et al., 1995; Deev et al., 2012). The pattern of sediment thicknesses in the Kurai basin imaged by the resistivity surveys, with the deepest subsidence along the northern (Aktash graben), southern (Eshtykel graben), and eastern borders, prompts that it originated as a pull-apart basin. Judging by the basin geometry, it was apparently a setting of transtension with left-lateral strike-slip faulting, when a large undeformed block between fault walls could subside as a single whole (Burtman et al., 1963). Subsidence of large blocks may occur between converging and diverging shear zones and give rise to flat-bottom basins with heavily faulted sides. Thus, the basins become the deepest along the sides and acquire a particular geometry of their transversal profiles (Fig. 9). This inference is consistent with the fault pattern in the western flank of the Quaternary thrust, near the Krasnye Vorota locality, where Delvaux et al. (1995) reconstructed tentatively a Tertiary stress tensor corresponding to an oblique WNW reverse motion with a sinistral strike-slip component, on an NE dipping plane. The predominant strike-
slip faulting explains the absence of Paleogene vertical offset inferred from fission-track thermochronology (De Grave et al., 2007; Glorie et al., 2012). During the Paleogene stage, the Kurai basin underwent limnic deposition of fine clay-size sediments, with coarser grain sizes (pebble, gravel, and sand) along the basin sides (Devyatkin, 1965; Delvaux et al., 1995; Luzgin and Rusanov, 1992), which agrees with mostly low resistivities of the respective geoelectric layers. In Quaternary time, the Central Kurai basin formed within the northern Kurai basin, in a setting of right-lateral (?) strike-slip faulting; several grabens appeared also in the Chuya valley between the Sukor block and the Kurai Range, as well as between the Belkenek, Karakol, and Akkoba blocks on one side and the Aktash block, with other blocks separating the Aktash graben, on the other side. The tectonic setting changed to compression (along the NE–SW principal axis). The flanking mountains grew rapidly to the present-day elevations (De Grave et al., 2007; Glorie et al., 2012) and, at the same time, were thrusting upon the basin along the border faults; the latter experienced vertical motions by the extrusion mechanism (Fig. 10) and the basin itself became a ramp (Delvaux et al., 1995). The same process gave rise to uplifts along the basin margins, and the marginal grabens turned into ramps and half-ramps, partly being controlled by counterclockwise rotation of the Sukor block in the eastern part the basin (Buslov et al., 1999). The Quaternary neotectonic stage was the time of coarse sand and molasse deposition (brownish rocks of the Beken and Bashkaus Formations), while the area of limnic deposition reduced considerably. Later in the Late Pleistocene, the area experienced glaciations intermittent with warmer periods, in the changed climate and geomorphic settings with rather high elevations (Zolnikov, 2009). Deposition of that stage formed a complex gray till sequence consisting of moraine diamictite, fluvioglacial boulders and pebbles, glacial-lacustrine and galcier-damlake sand, silt, and clay, and deposits of cataclysmic superstreams. Besides the glacial facies, there exist multifacies Holocene deposits on the basin surface, which are either superposed upon or embedded into older sedimentary sequences. Generally, the Quaternary deposits are of coarser grain sizes than the Paleogene–Neogene rocks and, correspondingly, have much higher resistivities.
Conclusions The controlled-source resistivity (VES and TEM) surveys have provided more constraints on the structure of the Kurai basin and its junction with the flanking mountains. The basin is the deepest along its northern (Aktash graben), southern (Eshtykel graben), and eastern margins, where sediments reach thicknesses of 600–1600 m (against 300–500 m in the Central Kurai basin). The sedimentary fill comprises two resistivity units corresponding to two sequences deposited at different stages of the basin history. The lower less resistive unit consists of Paleogene–Neogene lacustrine clay and the higherresistivity upper unit represents coarser Quaternary deposits.
N.N. Nevedrova et al. / Russian Geology and Geophysics 55 (2014) 98–107
In Paleogene–Neogene time, the basin formed by the left-lateral pull-apart mechanism, when its weakly deformed central part subsided as a single whole between fault walls. The earliest Quaternary strike-slip faulting in a setting of overall compression produced the Central Kurai subbasin within the northern Kurai basin, while the flanking ranges and fault blocks thrust upon the basin to transform it into a ramp. Thereby piedmont steps rose along the basin margins and the marginal grabens became ramps and half-ramps. The study was supported by grant 12-05-33048- mol_a_ved from the Russian Foundation for Basic Research.
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Editorial responsibility: M.I. Epov