Inherited strike-slip faults as an origin for basement-cored uplifts: Example of the Kungey and Zailiskey ranges, northern Tian Shan
Abstract
[1] Basement-cored uplifts bounded by steeply dipping reverse faults are mechanically difficult to explain. Reactivation of strike-slip faults that aid the formation of new, high-angle reverse faults in the surrounding crust may provide one origin for these structures. This hypothesis is explored by examining the late Miocene to Quaternary evolution of the Kungey and Zailiskey ranges in the northern Tian Shan. These ranges are cored by the Kemin-Chilik fault (KCF), an inherited Paleozoic structure with sinistral separation of basement terranes. Range growth in response to northward propagation of the Tian Shan has taken place along a network of steeply dipping reverse and oblique-slip faults surrounding the KCF. Deformation of a low relief unconformity separating Neogene strata from Paleozoic basement records structural growth in response to fault slip. Deformed river terraces surrounding the ranges are correlated to a well preserved chronosequence in the southern Kungey Range. Cosmogenic10Be dating of this chronosequence combined with offset measurements yields slip rates ranging from 0.07 to 0.37 mm/yr for dip-slip faults, and 1.1 to 1.5 mm/yr for strike-slip faults Late Quaternary activity in the Kungey-Zailiskey ranges is consistent with the longer-term, outward stepping pattern of range growth. Based on cross sections constrained from the folded unconformity surface, deformed Neogene strata and Quaternary terraces, faults building the Kungey Range are inferred to steepen at depth and emanate from a shear zone co-located with the reactivated KCF. This geometry is consistent with a slip partitioned system developed by an obliquely slipping reactivated fault at depth.
Key Points
- Inherited strike-slip faults can initiate high-angle reverse faults
- The Kungey and Zailiskey ranges grew surrounding an inherited structure
- Observed fault geometry is consistent with a slip-partitioned system
1. Introduction
[2] Basement involved thrust belts (BITB), here defined as belts of shortening characterized by high-angle (dip > 45°) reverse fault-bounded uplifts, are common on the margins of many orogenic belts [Tapponnier and Molnar, 1979; Jordan and Allmendinger, 1986; Rogers, 1987; Bump, 2003; Hilley et al., 2005]. Individual thick-skinned structures within these deformation belts typically absorb only minor amounts of shortening [Erslev, 1991; Narr and Suppe, 1994], thus significant shortening accommodated in BITB must involve slip on multiple structures. This is in contrast with thin-skinned thrust belts, where thrust sheets or wedges are detached from basement that is subducted beneath a gently dipping, long-lived, large-offset thrust fault [e.g.,Hubbert and Rubey, 1959; Chapple, 1978; Davis et al., 1983; Burchfiel et al., 1999; Scharer et al., 2004]. Though the amount of shortening across BITB is typically low (<200 km) the rates of deformation in presently active BITB can be a significant portion of the total shortening rate across an orogen [Jordan and Allmendinger, 1986; Abdrakhmatov et al., 1996; Molnar and Ghose, 2000; Meade and Hager, 2001].
[3] Development of BITB requires repeated throughgoing failure of the basement to initiate new or reactivate pre-existing structures. However, initiation of slip on high-angle reverse faults in BITB poses two problems: development of new high-angle reverse faults violates the prediction from Mohr-Coulomb mechanics [Anderson, 1951], and motion on high-angle structures is kinematically unfavorable to absorb shortening [e.g.,Sibson, 1985]. One explanation for the development of BITB is the re-activation of an older structural grain. Many studies have shown that inherited faults exert a strong influence on the loci of deformation in intracontinental orogens [e.g.,Holm and Cloud, 1990; Allen and Vincent, 1997; D'Lemos et al., 1997; Holdsworth et al., 1997; Darby and Ritts, 2002; Bump, 2003; Mouthereau and Lacombe, 2006; Butler et al., 2006; Webb and Johnson, 2006]. Inheritance of older structures may be favored where reactivation of pre-existing faults requires significantly less work than initiation of new, optimally oriented faults [Scholz, 2002]. Pre-existing high-angle faults may also become mechanically favorable when pore fluid pressure within the crust surrounding such faults sufficiently decreases friction along the fault surface [Sibson et al., 1988; Hilley et al., 2005]. However, both of these conditions cannot explain the generation of new high-angle faults in BITB. Thus, either all faults in these settings are reactivated, mechanically favorable high-angle faults or there exists a poorly understood process that initiates new high-angle reverse faults, despite their mechanical shortcomings.
[4] In this study, we explore the possible role of oblique deformation in the origin of high-angle reverse faults in BITB. Commonly, reverse faults present in BITB are oriented oblique, rather than orthogonal, to overall compression [Tapponnier and Molnar, 1979; Jordan and Allmendinger, 1986; Ding et al., 2004]. In regions of complex structural history, a pre-existing structural grain is unlikely to be oriented optimally to absorb compression; nonetheless these weaker areas of crust may be preferentially reactivated [Webb and Johnson, 2006]. Reactivation of high-angle structures at depth in a compressional regime can cause deformation to propagate upwards as a partitioned system of reverse and obliquely slipping faults, potentially representing a lowest energy condition [Michael, 1990; Molnar, 1992; Bowman et al., 2003]. These high-angle reactivated faults may evolve into oblique-reverse faults with continued deformation [Sibson, 1985] and core a slip-partitioned system, providing a possible origin for high-angle reverse faults in BITB.
[5] To explore the role of inherited structures in the formation of high-angle reverse faults, we study the Tian Shan of central Asia (Figure 1). The Tian Shan are a BITB formed as a result of Indo-Eurasian convergence [Burtman, 1975; Molnar and Tapponnier, 1975; Tapponnier and Molnar, 1979; Avouac et al., 1993; Abdrakhmatov et al., 2001; Thompson et al., 2002], and are dominated by north-south convergence along east-west striking high-angle reverse faults [Abdrakhmatov et al., 1996; Burbank et al., 1999; Korjenkov, 2000; Mikolaichuk, 2000; Molnar and Ghose, 2000; Abdrakhmatov et al., 2001; Bullen et al., 2001; Meade and Hager, 2001; Thompson et al., 2002; Bullen et al., 2003; Mikolaichuk et al., 2003; Bowman et al., 2004; Sobel et al., 2006; De Grave et al., 2007; Korjenkov et al., 2007]. Strike-slip faulting is also prevalent in the Tian Shan (Figure 1b), ranging in scale from the ∼600-km long Talas Fergana fault that bisects the entire orogen [Burtman et al., 1996] to somewhat shorter strike-slip faults that are embedded within the orogen [Delvaux et al., 1999; Mikolaichuk, 2000; Buslov et al., 2003]. The objective of this study is to determine the role of these embedded strike-slip faults in accommodating north-south shortening and northward propagation of the Tian Shan.
[6] We focus in detail on the late Miocene to recent growth of the Kungey and Zailiskey Ranges in the northern Tian Shan. These ranges are separated by the Kemin-Chilik fault (KCF), a 240 km-long inherited, active sinistral fault [Delvaux et al., 1999] (Figure 1b). Analysis of synorogenic Neogene stratigraphy shows that the Kungey and Zailiskey Ranges formed in response to northward propagation of the Tian Shan. Preservation of a pre-orogenic unconformity as a low-relief upland surface (Figure 2) provides an unusually extensive structural marker from which we can measure integrated deformation. The temporal evolution of uplift and reactivation of structures in building these ranges is inferred from comparison of this deformed marker surface to rates and styles of late Quaternary fault activity. The geometry of faulting at depth is constrained from folding and faulting of the unconformity surface, and from folding of Neogene strata and Quaternary fluvial terraces. Though the KCF is inherited, we argue that range growth took place along newly generated, steeply dipping reverse faults propagating upward from the strike-slip fault at the core of the ranges. The distribution of active structures shows that range growth occurs via integrated shortening and strike-slip faulting, with gradients in deformation rates that result from trade-off between strike-slip and reverse-slip fault systems. These observations are consistent with the development of new high-angle reverse faults in a BITB through interaction of regional compression with a re-activated, obliquely oriented strike-slip fault.
2. Setting
[7] The Tian Shan is an ∼2500 km long, 300–400 km wide orogenic belt composed of a series of east-west trending basement cored uplifts. The central and western Tian Shan are bounded on the north and south by the Kazakh Platform and Tarim Basin, respectively (Figure 1a). Although located 1000–1500 km north of the collisional plate boundary between India and Eurasia, the Tian Shan have been built as a direct result of this collision [Burtman, 1975; Molnar and Tapponnier, 1975; Tapponnier and Molnar, 1979; Avouac et al., 1993] and have absorbed ∼200 ± 50 km of crustal shortening [Avouac et al., 1993; Yin et al., 1998]. Current shortening across the Western Tian Shan is ∼20 mm/yr [Abdrakhmatov et al., 1996; England and Molnar, 1997a, 1997b; Larson et al., 1999; Meade and Hager, 2001; Reigber et al., 2001; Zubovich et al., 2010], accommodated largely near the northern and southern margins of the Tian Shan.
[8] The basement rocks of the Tian Shan are a part of the Central Asian Orogenic Belt, a series of microcontinents, accretionary wedges, and volcanic arcs accreted onto the edge of the Siberian craton to form the Kazakh Platform from ∼540–250 Ma [Sengor et al., 1993; Mossakovsky et al., 1994; Carroll et al., 2001; Windley et al., 2007]. Evidence of minimal Mesozoic tectonism in the western Tian Shan is preserved as deposition of Jurassic shales in a few minor normal fault-bounded basins [Cobbold et al., 1994]. The Cretaceous and early Cenozoic were dominated by erosion, beveling down the Paleozoic rocks to form a quasi-planar erosional surface [Makarov, 1977; Sadybakasov, 1990]. This low-relief unconformity surface separates basement from overlying late Cenozoic pre- and synorogenic deposits [Chediya, 1986] and is widely preserved as a relict landform throughout uplifted basement ranges in the Tian Shan (Figure 2), thus providing an excellent structural and geomorphic marker [e.g., Burbank et al., 1999; Abdrakhmatov et al., 2001; Thompson et al., 2002; Mikolaichuk et al., 2003; Oskin and Burbank, 2005; Sobel et al., 2006; Oskin and Burbank, 2007].
[9] Orogenic growth initiated in the eastern and southern Tian Shan circa 25–20 Ma, based on thermochronologic and magnetostratigraphic studies [Hendrix et al., 1994; Sobel and Dumitru, 1997; Yin et al., 1998; Dumitru et al., 2001; Heermance et al., 2007]. Thick-skinned deformation appears to have propagated northward as a series of east-west striking basement uplifts, and thin-skinned deformation propagated south into the northern Tarim Basin [e.g.,Scharer et al., 2004]. Examination of pre-and syn-orogenic stratigraphy and thermochronology from the Kyrgyz Range in the northwestern Tian Shan demonstrate a history of Miocene burial prior to exhumation starting at ∼11 Ma [Bullen et al., 2001, 2003; Sobel et al., 2006], consistent with northward propagation into a former foreland basin.
[10] Ranges internal to the Tian Shan are separated by basins filled with Cenozoic syn-orogenic deposits [Burtman, 1975; Burbank et al., 1999; Abdrakhmatov et al., 2001; Thompson et al., 2002]. Quaternary deformation is concentrated within these basins where faults and folds deform the Cenozoic sedimentary deposits. Active shortening structures are often located kilometers from the basement-bounding structures [Thompson et al., 2002]. Surface expressions of faults and folded strata define a shallow, ramp-style geometry at depth which transfers deformation on to steep crustal-scale faults at the range fronts [Molnar et al., 1994; Burbank et al., 1999; Burchfiel et al., 1999; Thompson et al., 2002; Goode et al., 2011].
[11] The majority of range fronts and river networks in the Tian Shan contain a regionally correlative suite of alluvial fans and fluvial terraces. Previous studies correlate and group these terraces on the basis of morphology and geomorphic position into four main divisions, Q1, Q2, Q3 and Q4 [Grigorienko, 1970; Makarov, 1977] (as cited by Thompson et al. [2002]). These terraces and fans provide planar, ideal markers with which to measure modern deformation amount and style across active structures [e.g., Weldon and Sieh, 1985; Avouac et al., 1993; Molnar et al., 1994; Lave and Avouac, 2000; Abdrakhmatov et al., 2001; Thompson et al., 2002].
2.1. The Issyk-Kul Basin
[12] The Issyk-Kul basin in northeastern Kyrgyzstan is the largest intramontane basin in the Tian Shan (∼230 km east-west and ∼80 km north-south) (Figure 1b). The basin is deepest along its southern margin, where the south-dipping pre-Cenozoic unconformity surface is overlain by ∼4 km of sedimentary fill [Mikolaichuk, 2000; Buslov et al., 2003]. The south side of the basin is bounded by the Terskey Range, where over 7 km of structural relief has developed, evidenced by preservation of the unconformity surface at high elevations (∼4 km) within the Terskey Range. The 700 m-deep Issyk-Kul (Kul = lake) with a surface elevation of ∼1600 m currently occupies the majority of the basin [Buslov et al., 2003].
[13] The Kungey and Zailiskey Ranges are the northernmost ranges of the Tian Shan and bound the northern margin of the Issyk-Kul basin. These ranges are juxtaposed by the historically active north-northeast striking sinistral KCF [Delvaux et al., 1999, 2001; Crosby et al., 2007]. Fission track studies of the Kungey Range indicate an absence of reset ages [De Grave et al., 2007], along with the widespread preservation of the pre-Cenozoic unconformity surface at high elevations supports relatively recent uplift with little incision when compared to the adjacent Kyrgyz Range [Sobel et al., 2006]. The margins of the Kungey and Zailiskey ranges are bounded by active fold and thrust belts where deformation is accommodated by slip along north- and south-vergent thrust and reverse faults as well as by slip on obliquely oriented strike-slip faults.
2.2. Neogene-Quaternary Stratigraphy
[14] Pre- and syn-orogenic Neogene and Quaternary strata of the northern Tian Shan can be regionally correlated despite facies changes along strike of the range fronts (Figure 3). Syn-orogenic strata consist of a wedge of fluvial sediments derived from southern portions of the Tian Shan during its northward propagation. This wedge is overlain by a coarsening-upward series of sandstones and conglomerates that records erosion of the growing Kungey and Zailiskey Ranges.
[15] The basal Neogene unit is the Kokturpak (Suluterek) formation which unconformably overlies the Paleozoic basement, varies from 10 to 80 m in thickness, and likely predates the onset of foreland basin sedimentation at any given location [Korjenkov, 2000; Mikolaichuk, 2000; Abdrakhmatov et al., 2001]. The base of the Kokturpak Fm. consists of thin paleosols with carbonate nodules, minor gypsiferous sandstones, and thin local lacustrine carbonate deposits. Locally in the western Issyk-Kul basin basal layers of the Kokturpak Fm. are overlain by interbedded red siltstones and sandstones with abundant gypsum beds. Basalts flows present in the Kokturpak Fm. near Toru-Aygir have been dated at 53 ± 1 Ma [Sobel and Arnaud, 2000].
[16] A conformable, gradational contact separates the Kokturpak Fm. from the overlying Shamsi Fm. Oldest of the syn-orogenic stratigraphy, the Shamsi Fm. consists of massive red sandy clay units interbedded with paleosols and minor gravel conglomerates. Basal layers contain gypsiferous horizons which become absent higher in the column. Along the southern margin of the Kungey Range, the Shamsi Fm. is absent in the east, thickening to ∼500 m in the west. In adjacent intermontane basins thickness of correlative Shamsi Fm. has been measured at several hundred meters [Abdrakhmatov et al., 2001]. Paleomagnetic data from the exposures fringing the nearby Kyrgyz Range places a depositional time of ∼12–8 Ma [Abdrakhmatov et al., 2001; Bullen et al., 2001].
[17] The widespread basin-filling unit is referred to as the Chu Fm. [Korjenkov, 2000; Mikolaichuk, 2000; Abdrakhmatov et al., 2001] and locally noted as the Issyk-Kul Fm. [Bowman et al., 2004; Korjenkov et al., 2007]. We divide the Chu Fm. into two members based on clast lithology indicative of source area. The Terskey member (Tc(t)) consists of sandstones, gravels and gravel conglomerates interbedded with massive silt and sand layers. Individual clasts include metavolcanic rocks, metasedimentary rocks, gneiss, low-grade schists, and felsic plutonic rocks. Paleo-transport measurements (n = 52) from abundant low-angle planar cross beds indicates south-to-north transport (Figure 3). On the basis of paleocurrent directions and diversity of clast lithology, the Terskey member is inferred to be syn-tectonic with the growth of the Terskey Range south of the Issyk-Kul basin. The Kungey member of the Chu Fm. (Tc(k)) is composed of locally derived granitic material forming poorly indurated massive sand layers with interbedded lithified sandstones and gravel conglomerates. Planar cross-bedding in sandstone and conglomerate layers (n = 68) indicates a north-to-south transport direction (Figure 3). Flanking the western Kungey Range, the >600 m-thick Terskey member of the Chu Fm. conformably overlies the Shamsi Fm. The Kungey member is preserved only locally at the top of the section. In outcrops along the central Kungey Range, as little as 10 m to as much as 200 m of the Terskey member conformably overlies the Kokturpak Fm. and is in turn conformably overlain by the Kungey member.
[18] The Sharpyldak Fm. conglomerate overlies the Kungey member of the Chu Fm. with a gradational coarsening-upward contact. This conglomerate is composed of locally derived, dominantly granitic clasts, are >500 m thick, and record erosion of the Kungey Range and consequent progradation of coarse material into the Issyk-Kul Basin. Where present proximal to the Kungey Range, its constituent clasts are purely granitic in composition with clasts up to 2m in diameter supported by a sand matrix. Correlation with massive, locally derived conglomerates in the adjacent Chu Basin places the onset of deposition of the Sharpyldak Fm. at mid-Pliocene or ∼4 Ma [Abdrakhmatov et al., 2001]. However, because the coarse-grained facies of the Sharpyldak Fm. are linked to the uplift and erosion of local source areas, its age varies between 6 and 3 Ma within the Tian Shan [Abdrakhmatov et al., 2001].
[19] Reconnaissance investigations indicate that the Neogene stratigraphy of the Zailiskey Range consists of similar Kokturpak and Shamsi Fm. equivalents that are overlain by coarser Chu and Sharpyldak Fm. equivalents that record range uplift and erosion. An exposure of a tilted, north-dipping section lying above the unconformity surface east of Chilik shows that the Kokturpak equivalent is ∼80 m thick, Shamsi equivalent ∼400 m thick, and Chu equivalent >500 m thick.
[20] Late Quaternary deposition in the northern Tian Shan is recorded by a regionally correlative, well-preserved suite of fluvial terraces and alluvial fans. Composition of clasts composing these deposits in the northern Issyk-Kul basin is predominantly granitic, reflecting the composition of the Kungey Range source area. These surfaces can be classified using morphology and geomorphic position [Grigorienko, 1970; Makarov, 1977] (as cited by Thompson et al. [2002]) and additional criteria we define in this study (Table 1). Subdivisions of major terrace groups are defined by surface morphology, pavement development, soil development, and clast characteristics to 50 cm depth. We divide Q2 into Q21 and Q22, Q3 into Q31, Q32 and Q33, and define recent (likely Holocene) surfaces as Q4. Because Q32 is the most widely preserved terrace and fan deposit, the majority of measurements were made where faults displace its surface. These same criteria were used to define a chronosequence of terraces on the northern flank of the Zailiskey Range, using Q4, Q3, and Q2 nomenclature for deposits analogous to those flanking the Kungey Range.
| Surface | Surface Morphology | Clast Weathering | Pavement Development | Soil Development |
|---|---|---|---|---|
| Q4 | Subdued bar and swale topography (<1 m local relief) | Subangular to subrounded clasts, poor light brown varnish development | Immature, isolated pavement with large gaps of dust in between clasts | <5 cm thick AV horizon with minimal stage 1 carbonate development |
| Q32 | Extensive flat, well-defined surfaces with slight concavity. No preservation of remnant topography, minor frost ridges present | Angular to subrounded clasts, thin varnish development (<0.25 mm) | Immature, patchy pavement with little dust in between clasts | 10 cm thick AV horizon with minor stage 1 carbonate development on clasts to 50 cm depth |
| Q22 | Flat surfaces with no preservation of remnant topography. Frost ridges are present and oriented sub-parallel to contours | Angular to subrounded clasts, varnish development on non-granitic clasts of 0.5 mm. Significant break-up of larger clasts | Patchy areas of interlocking clasts | 10–15 cm thick AV horizon with stage 1 to stage 2 carbonate development to 50 cm depth |
| Q21 | Ridge and ravine topography with areas of flat, preserved surfaces. Low-amplitude frost ridges present. | Angular to subrounded clasts, granitic clasts have advanced granular attrition. 0.75–1.5 mm thick varnish development on non-granitic clasts | Patchy pavement development in areas of small, angular clasts | 15–17 cm thick AV horizon with stage 2 to minor stage 3 carbonate development to 50 cm depth |
3. Structures of the Kungey and Zailiskey Ranges
[21] Neogene and Quaternary geology of the Kungey and Zailiskey Ranges was mapped on aerial photography, LANDSAT imagery, and 1:25,000 topographic maps. Structural measurements were taken in deformed Neogene strata to determine fold and fault geometries. Deformed terrace surfaces were identified; vertical and lateral offsets were measured with a Trimble GeoXT differential GPS to determine displacement.
3.1. Western Kungey Range
[22] The western Kungey Range consists of a basement-cored uplift bounded by sinistral-oblique faults with reverse faults at its western end [Mikolaichuk, 2000; Delvaux et al., 1999, 2001] (Figure 4). The unconformity surface has been folded and faulted over the range crest with >3 km of structural relief. The northwest side of the range is bounded by the sinistral Kemin-Chilik fault, and the southeast side of the range by the sinistral Toguz-Bulak fault. These faults are linked at the western end of the Kungey Range by a set of reverse faults that bound the intensively deformed Kok-Moynok basin. This small basin lies between the Kungey Range and the Kyrgyz Range to the west. A series of east-west striking, south-vergent reverse faults step southward from the eastern end of the Toguz-Bulak fault, near the Toru-Aygir River valley. The range front also steps south from here to form the central Kungey Range. In this area, at high elevations within the range, additional east- to southeast-striking reverse faults are inferred from offset of the low-relief relict unconformity surface.
[23] Structures exposed along the Toru-Aygir River reveal a pattern of deformation stepping southward from the Kungey Range [Bowman et al., 2004] (Figure 4b). In the Ak-Teke hills, along the lower Toru-Aygir River the Chu (Terskey) is deformed into an anticline in the hanging wall of the north-dipping Ak-Teke fault [Korjenkov, 2000; Mikolaichuk, 2000; Bowman et al., 2004; Korjenkov et al., 2007]. The Ak-Teke anticline is ∼3 km in width and extends at least 30 km east-west. The anticline is asymmetric with a backlimb dip of 30° north, a broad flat crest, and overturned beds in the southern limb where the Ak-Teke thrust fault outcrops at the surface. Morphology and geometry of the fold lead to the interpretation that the Ak-Teke anticline is a fault-propagation fold formed synchronously with slip on the Ak-Teke fault [Suppe, 1983; Suppe and Medwedeff, 1990]. North of the Ak-Teke anticline lies the Kyzyl-Kultor basement uplift and the Jai-Lo syncline. The Kyzyl-Kultor hills consist of Paleozoic rocks folded and uplifted along the south-vergent Kyzyl-Kultor fault. The Jai-Lo syncline deforms both the Paleozoic bedrock and overlying Neogene sediments. Faults rooting into Paleozoic rocks are responsible for local shortening near the syncline axis.
[24] Most of the range-bounding structures of the western Kungey Range show abundant evidence of late Quaternary activity. Two sites along the Toguz-Bulak fault sinistrally offset Q32 alluvial fans (Figure 5). These sites are separated by the Balikche fault, a northwest-striking reverse fault that branches southward from the Toguz-Bulak fault into the Issyk-Kul basin. West of the Balikche fault, the Toguz-Bulak fault sinistrally displaces a Q32 alluvial fan and several inset channels by 125 ± 8 m (Figures 5b and 5c). East of the Balikche fault, the fault displaces the margins of a channel inset into Q32 by 95 ± 8 m (Figure 5d). The Balikche fault itself shows evidence of Holocene activity, with a northeast-facing scarp and displacement of a Q4 fan by 7 ± 2 m (Figure 5a). Strike-slip on the Toguz-Bulak fault feeds northeast into active, east-west striking thrust faults and folds along the Toru-Aygir River valley [Korjenkov, 2000; Bowman et al., 2004; Korjenkov et al., 2007]. The Ak-Teke fault is the most active and largest spatially of these and displaces and warps preserved Q21, Q22, Q31, Q32, Q33 and Q4 surfaces (Figure 6a). Displacement along the Ak-Teke fault was measured at 48 ± 4 m in Q22 and 32 ± 3 m in Q32. A scarp in Q4 shows a vertical offset of ∼2 m [Korjenkov, 2000]. East of Toru-Aygir slip on the Ak-Teke fault decreases to 21 ± 2 m in Q32 (Figure 6b). South of the Ak-Teke fault, the Tekren anticline deforms Q22, Q32 and Q4 surfaces with only a minor amount of shortening, 10 ± 4 m, measured from tilting of Q22gravels in the backlimb of the fold. Thrust faulting in the core of the Jai-Lo syncline also deforms Q32, Q33 and Q4 surfaces with an offset of 30 ± 6 m in Q32 (Figure 6b).
3.2. Central Kungey Range
[25] Basement uplift across the central Kungey Range is accomplished by a series of north- and south-vergent reverse faults (Figure 4c). North of the town of Cholpon-Ata, these faults displace the unconformity surface. Where preserved at the range front, this surface dips uniformly south ∼25°. South of the range front, the south-vergent Cholpon-Bosteri fault bounds an open, east-west trending syncline developed in Neogene strata. Paleozoic rocks are locally exposed on the south limb of the syncline, thus the basement is presumably folded along with the Neogene strata. Conformable Neogene strata on the north limb of the syncline are locally overturned on the south limb in the Svalka anticline. East of Cholpon-Ata the unconformity surface and overlying Neogene strata uniformly dip ∼25–30° south near the range front. South-vergent faulting with minor displacements disrupts the overlying Neogene and Quaternary deposits, but likely roots into basement at shallow depths. Evidence of faulting disappears east of the Chon-Aksu River and exposure of the Neogene strata defines a south-dipping homocline overlying the unconformity surface.
[26] Active deformation in the central Kungey Range is expressed by slip along the Cholpon-Bosteri fault system. Unlike the western Kungey Range, all late Quaternary deformation along the central Kungey Range front occurs via thrust or reverse faulting. Where bounding the southern limb of the Neogene syncline, the Cholpon-Bosteri fault dips ∼55° north as calculated by local topographic expression of the fault trace. Near the town of Cholpon-Ata, this fault displaces Q22, Q32 and Q4 surfaces. Slip in Q32 measured at 16 ± 2 m (Figures 6c and 6d). East of Cholpon-Ata, north-facing scarps in deformed terraces indicate a change of fault geometry and decrease of deformation magnitude. Deformation appears to transfer from the Cholpon-Bosteri fault to north-vergent back-thrusts near the town of Korumdu. Older, inactive back-thrusts deform Q22surfaces, younger back-thrusts are expressed via small scarps and hanging wall anticlines in Q32. Slip measured across the scarp and back limb of an anticline on one of these back-thrusts is 6 ± 2 m in Q32 (Figure 6d). Further east, fault scarps in Q22 and Q32surfaces indicate a return to south-vergent, shallow structures. These scarps diminish eastward and disappear entirely near the Chon-Aksu River.
3.3. Chon-Kemin and Chon-Aksu Valleys
[27] The east-northeast striking KCF runs longitudinally through the Chon-Kemin river valley, separating the Kungey Range from the Zailiskey Range (Figure 1b). The KCF is a re-activated Paleozoic-Mesozoic sinistral fault zone with offset basement terranes [Delvaux et al., 1999, 2001]. Late Quaternary sinistral motion is shown by sinistral offset in undifferentiated fluvial terraces along strands of the fault, while historical ruptures along the fault zone show both sinistral and reverse motion [Tapponnier and Molnar, 1979; Molnar and Ghose, 2000; Delvaux et al., 2001; Crosby et al., 2005, 2007].
[28] The Chon-Aksu valley cuts east-west through the Kungey Range northeast of Cholpon-Ata. Running through this valley is the 45°–60° northeast-dipping Chon-Aksu fault [Delvaux et al., 2001; Crosby et al., 2005]. Trenching along the Chon-Aksu fault shows ∼4 major events within the past 12 kyr, the most recent being the M 8.2 1911 Kemin earthquake [Delvaux et al., 1999; Delvaux et al., 2001; Arrowsmith et al., 2004; Crosby et al., 2005]. The 1911 event also ruptured segments of the KCF with sinistral-oblique motion expressed at the surface [Delvaux et al., 1999]. Ruptures along the Chon-Aksu fault associated with this event display both reverse and normal motion with vertical displacements of 2–8 m [Arrowsmith et al., 2004].
3.4. Northern Zailiskey Range
[29] The basement uplifts of the Zailiskey Range are bounded on the north by a system of north-vergent thrust and reverse faults (Figure 4d). Where preserved, the Cenozoic unconformity surface is north-dipping and generally overlain by a conformable Neogene sedimentary section. Along the eastern range front near Chilik, basement is thrust above a series of south-vergent reverse faults that step progressively northward [Tibaldi et al., 1997].
[30] Near Almaty, the Neogene section is buried by loess of unknown thickness. Based upon topographic expression and limited exposures of Neogene strata, the range front east of Almaty appears to have been warped into a series of east-west-trending anticlines (Figure 4d). Further east, the loess cover diminishes and outcrops of Neogene sedimentary units are common. South of Chilik, these sediments have been uplifted and warped into east-west-trending, east-plunging anticlines. East of Chilik the Neogene section is exposed in a 6°–17° north-dipping conformable section on top of the Cenozoic unconformity surface.
[31] Quaternary deformation along the Zailiskey Range front is expressed by fault scarps and folding of a regional terrace sequence that appears correlative to the terrace sequence in the Issyk-Kul basin. Unnamed faults that displace the terraces have reverse and minor oblique-slip geometries. Analogous Q4 surfaces along the range front that cross reverse faults have scarps ∼3–5 m in height, Q3 and Q2 surfaces have progressively more deformation. South of Chilik the range front shows evidence for Quaternary activity along two north-vergent thrust faults. Displacement of Q2 surfaces was measured at 40 ± 20 m along the southern fault proximal to the range, and 72 ± 30 m on the northern fault (Figure 7).
4. Geochronology and Late Quaternary Deformation Rates
[32] We selected a chronosequence of alluvial terraces along the Toru-Aygir River for cosmogenic10Be exposure age dating (Figures 4c and 8). This location in the western Issyk-Kul Basin was chosen for its high degree of surface preservation, paucity of loess deposits, and previous terrace-age studies [Bowman et al., 2004]. Samples for analysis were collected from depth profiles to simultaneously determine terrace age and account for 10Be inherited during erosion, transport, and deposition of sediment [Anderson et al., 1996; Repka et al., 1997; Perg et al., 2001].
[35] Samples were taken from flat terrace surfaces well away from convex, actively eroding edges (Figure 8). Pits were dug into Q21, Q22 and Q32terraces to a depth of 170 cm. Quartz-rich clasts ranging in size from 1 to 10 cm were taken by hand from depths of 170, 100, 50, 25 and 10 cm and from the surface of the terrace proximal to each pit. Samples were crushed and sieved to isolate a 250–500μm fraction. Standard quartz separation techniques were followed, and targets of 10Be were prepared for analysis at the University of Minnesota Cosmogenic Lab. Analysis was performed at the Lawrence Livermore National Laboratory accelerator mass spectrometer.
[36] Production rates (Po) for 10Be in the western Kungey Range were calculated using methods described by Pigati and Lifton [2004]. Time-averaged production rates vary with altitude, and to a lesser extent, with terrace age. We find Po values of 26.0, 24.8 and 24.3 atoms 10Be/g/yr for Q21, Q22 and Q32, respectively and use equation (3) to solve for the age of each surface. Values calculated for inheritance (Ci) are similar to measured values of 10Be concentration from modern stream sediments (Table 2). The depth profiles are exceptionally well behaved (Figure 9). Our model ages, reported with a 95% confidence interval, are 139.9 ± 6.5 ka for Q21, 126.4 ± 10.8 ka for Q22 and 85.6 ± 7.6 ka for Q32 (Figure 9 and Table 2). In all of the profiles, the surface samples are consistent with linear regression of the profile. This would not be the case if erosion had preferentially removed fine materials from the surface. Independent age constraints further support minimal surface lowering. The Q32 age is only slightly younger than an OSL age of 96.0 ± 8.6 ka determined from sand collected from the same excavation [Bowman et al., 2004], and the ages for Q2 match those determined by Thompson et al. [2002] in other areas of the Tian Shan.
| Terrace | Sample ID | Depth (cm) | Sample Weight (g) | 10Be (atoms ×107) | 10Be Errorb (atoms ×107) | 10Be (atoms/g ×106) | 10Be Error (atoms/g ×106) |
|---|---|---|---|---|---|---|---|
| Q32 | TA-05-01 | 0 | 55.6 | 13.6 | 0.209 | 2.45 | 0.0376 |
| TA-05-03 | −170 | 42.9 | 2.83 | 0.0405 | 0.659 | 0.0105 | |
| TA-05-04 | −100 | 31.6 | 3.29 | 0.163 | 1.04 | 0.0514 | |
| TA-05-05 | −50 | 29.4 | 4.05 | 0.0621 | 1.38 | 0.0211 | |
| TA-05-06 | −25 | 39.2 | 6.68 | 0.129 | 1.71 | 0.0329 | |
| TA-05-07 | −10 | 27.5 | 6.45 | 0.0936 | 2.35 | 0.035 | |
| Q22 | TA-05-02 | 0 | 52.3 | 2.08 | 0.357 | 3.97 | 0.0683 |
| TA-05-09 | −170 | 52.5 | 5.45 | 0.114 | 1.04 | 0.0217 | |
| TA-05-10 | −100 | 47.1 | 7.77 | 0.109 | 1.65 | 0.0231 | |
| TA-05-11 | −50 | 36 | 7.63 | 0.251 | 2.12 | 0.0698 | |
| TA-05-12 | −25 | 27 | 7.82 | 0.181 | 2.9 | 0.0672 | |
| TA-05-13 | −10 | 32.6 | 10.3 | 0.36 | 3.15 | 0.1103 | |
| Q21 | TA-05-08 | 0 | 50.2 | 20.5 | 0.255 | 4.09 | 0.0508 |
| TA-05-14 | −170 | 30 | 3.32 | 0.08 | 1.11 | 0.0266 | |
| TA-05-15 | −100 | 46.2 | 6.55 | 0.14 | 1.42 | 0.0302 | |
| TA-05-16 | −50 | 45.9 | 11.2 | 0.166 | 2.45 | 0.0361 | |
| TA-05-17 | −25 | 41 | 12.9 | 0.202 | 3.13 | 0.0492 | |
| TA-05-18 | −10 | 46 | 17.5 | 0.285 | 3.8 | 0.062 | |
| TA-05-19 | modern stream | 52.7 | 3.33 | 0.0761 | 0.631 | 0.0144 | |
| Terrace | Sample Locations | Production Rates (Following Pigati and Lifton [2004]) | |||||
| Q32 | 42°34′36.46″ N, 76°23′59.39″E, 1906 m | 24.3 at/g/yr | |||||
| Q22 | 42°34′59.14″N, 76°23′37.82″E, 1937 m | 24.8 at/g/yr | |||||
| Q21 | 42°35′24.03″N, 76°23′26.75″E, 2003 m | 26 at/g/yr | |||||
[37] Late Quaternary deformation expressed as folded and faulted fluvial terraces was studied to determine styles and rates of deformation (Table 3 and Figure 10). Fault scarps in preserved terrace surfaces were field mapped on aerial photography and surveyed with a differential GPS to determine offset and fault geometry. Lateral offsets of terrace edges and ephemeral stream channels were measured to determine strike-slip displacement, where present. Amount of slip along thrust faults was calculated from scarp profiles using the methodology ofThompson et al. [2002]taking into account uncertainty in the dip of hanging wall and footwall terrace surfaces and backlimb shortening, where applicable. Fault dip was measured from topographic exposure of scarp faces, estimated from similarly deforming areas with known fault dip, or based on cross-section interpretations. Slip rates were calculated using10Be ages for offset surfaces. Monte Carlo simulations with 10,000 calculations give slip rates with a 95% confidence interval (Table 3).
| Fault | Location | Fault Dip (deg) | Surfacea | Offset (m) | Slip Rateb (mm/yr) |
|---|---|---|---|---|---|
| Toguz-Bulakc | West of Balikche | n/a | Q32 | 125 +/− 8c | 1.5 +/− 0.2c |
| North of Balikche | n/a | Q32 | 95 +/− 8c | 1.1 +/− 0.2c | |
| Ak-Teke | Toru-Aygir river valley | 30 +/− 2 | Q32 | 32 +/− 3 | 0.37 +/− 0.04 |
| 30 +/− 2 | Q22 | 48 +/− 4 | 0.38 +/− 0.04 | ||
| East of Toru-Aygir | 30 +/− 5 | Q32 | 21 +/− 3 | 0.24 +/− 0.04 | |
| Un-named | Jai-lo syncline | 11 +/1 2 | Q32 | 30 +/− 6 | 0.35 +/− 0.07 |
| Cholpon-Bosteri | North of Cholpon-Ata | 55 +/− 10 | Q32 | 16 +/− 2 | 0.19 +/− 0.03 |
| North of Korumdu | 30 +/− 5 | Q32 | 6 +/− 2 | 0.07 +/− 0.02 | |
| Un-named southern | Chilik | 45 +/− 10 | Q2d | 72 +/− 30 | 0.8 +/− 0.5 |
| Un-named northern | Chilik | 30 +/− 10 | Q2d | 40 +/− 20 | 0.43 +/− 0.3 |
- a Ages reported in Figure 9.
- b Errors reported with 95% confidence interval.
- c Sinistral strike-slip displacement.
- d Assumed age of Q2: 100 +/− 30 ka to encompass Q2 ages of Thompson et al. [2002], Bowman et al. [2004], and this study.
[38] Fault slip rates are highest in the western Kungey Range where sinistral motion along the Toguz-Bulak fault feeds into a series of east-west trending faults and folds along the Toru-Aygir river valley (Figure 10). An overall gradient in slip rates, decreasing from west to east, is observed along the range front. Along the Toguz-Bulak fault, sinistral slip rates decrease from 1.5 ± 0.2 mm/yr to 1.1 ± 0.2 mm/yr across the intersection with the Balikche fault. Total north-south shortening rate along parallel east-west trending faults and folds in the Toru-Aygir river valley is approximately 1 mm/yr. Slip rates along the Ak-Teke fault diminish slightly along strike from west to east from ∼0.4 mm/yr to <0.3 mm/yr. Late Quaternary deformation rates continue to decrease to the east, where dip-slip reverse motion along the Cholpon-Bosteri fault diminishes from ∼0.25 mm/yr to <0.1 mm/yr (Figure 10).
[39] Slip rates were estimated along two thrust faults in the eastern Zailiskey Range front near Chilik (Figure 10). Offset was measured from deformed Q2 surfaces. We assume an age of 100 ± 30 ka for this terrace, which encompasses ages for Q2 elsewhere in the Tian Shan from this study, Thompson et al. [2002], and Bowman et al. [2004]. For the southern fault we find a rate of 0.43 ± 0.3 mm/yr. The slip rate of the northern fault is about twice this amount, 0.8 ± 0.5 mm/yr.
5. Discussion
[40] The growth history of the Kungey and Zailiskey Ranges about the inherited Kemin-Chilik fault is discussed in the context of the northward propagation of the Tian Shan orogen.
[41] By interpreting the structural evolution of these ranges in map-view and in cross-section, it is inferred that these have grown as a mega-flower structure, with the Kemin-Chilik fault at its core.
5.1. Stratigraphic Evidence for Uplift History
[42] Preceding uplift, a foreland flexural basin occupied the region of the Kungey and Zailiskey Ranges. Facies of the Shamsi formation in the Chu Basin [Bullen et al., 2001], western Kungey Range, and Terskey Range [Thomas et al., 1993; Bazhenov et al., 1994] record subsidence of the foreland basin and burial of the northern Tian Shan prior to the onset of uplift and exhumation ca. 11 Ma [Bullen et al., 2001, 2003; Sobel et al., 2006; Glorie et al., 2010]. Absence of the Shamsi formation in present-day eastern Kungey Range stratigraphy defines the lateral extent of its depositional basin, interpreted as the flexural basin north of the growing southern Tian Shan. Eastward expansion of this depocenter is recorded by the Chu (Terskey) Fm., present in outcrops along both the western and eastern Kungey Range front (Figure 4).
[43] Inception of uplift of the Kungey Range and initial closure of the Issyk-Kul basin occurred during deposition of the Chu Fm., ostensibly between 7 and 4 Ma. This transition is indicated by a reversal of paleocurrents and a change in clast lithology. Paleocurrent indicators in the Chu (Terskey) Fm. record a south-to-north transport of distally derived sediments. This transitions up-section to locally derived grus of the Chu (Kungey) Fm. Production of grus via erosion of exposed bedrock typically occurs at rates of <0.01 mm/yr [Small et al., 1997], thus it is interpreted that the thick section (400–600 m) of fine-grained sediments with thin gravel conglomerates records initial slow uplift and erosion of the Kungey Range.
[44] Upward-coarsening of the Chu (Kungey) Fm. into the conglomerates of the Sharpyldak Fm. signifies increasing relief and erosion of its source area within the Kungey Range [e.g.,Burbank et al., 1988]. Within the Sharpyldak Fm., the absence of observed growth strata (e.g., fanning dips) indicates deposition of these conglomerates prior to the inception of folding along the present southern margin of the growing Kungey Range. Stratigraphy present along the Zailiskey Range front displays similar attributes as the Kungey Range stratigraphy. The Zailiskey Chu Fm. equivalent also exhibits a coarsening-upward sequence into equivalent Sharpyldak conglomerates. Likewise, these locally derived conglomerates suggest a south-to-north growth of the Zailiskey Range from the KCF into the Kazakh Platform. The timing of influx of coarse detritus into these flanking basins is poorly constrained, as the depositional age of the Sharpyldak Fm. is unlikely to be the same as in the better-studied Chu basin section ofBullen et al. [2001]. The best estimate for the age of onset of significant uplift of the Kungey and Zailiskey Ranges is sometime after the Shamsi-Chu transition, circa 7 Ma [Abdrakhmatov et al., 2001; Bullen et al., 2001].
5.2. Structural Evolution
[45] Subsurface geometries of faults responsible for the uplift of the Kungey and Zailiskey Ranges are inferred from structural measurements of Neogene stratigraphy and exposures of the deformed unconformity surface. The Kungey Range front and portions of the Zailiskey Range front are dominated, respectively, by uniformly south- and north-dipping spatially extensive panels of the exhumed unconformity surface. While there are numerous ways to produce such a dip panel [e.g.,Suppe, 1983; Erslev, 1986; Johnston and Yin, 2001], the subsurface fault geometries of the Kungey and Zailiskey Ranges must account for a steeply dipping, active fault at the core of the range [Tapponnier and Molnar, 1979; Tibaldi et al., 1997; Molnar and Ghose, 2000; Delvaux et al., 1999, 2001; Crosby et al., 2005, 2007]. Rough seismicity of the northern Tian Shan shows events approximating a steeply dipping feature extending to ∼20 km depth in the region of the KCF [Zubovich et al., 2001].
[46] Reverse faults responsible for growth of the Kungey and Zailiskey Ranges have roughly linear map traces, especially where exposed high within the ranges. These linear traces are most consistent with dips >45° near the surface. Only the Ak-Teke fault, which appears to detach the Neogene section from Paleozoic basement, shows clear evidence of a gentle (∼30°) dip where it breaks the surface.
[47] Based on these observations of the deformed unconformity surface and steep fault dips near the surface, the Kungey and Zailiskey Ranges are modeled using a positive flower structure fault geometry with curviplanar reverse faults that steepen at depth [e.g., Erslev, 1986] to root into the KCF. To investigate the overall structure of the Kungey Range, balanced cross-sections through the Toru-Aygir River valley (Figure 11) and Cholpon-Ata syncline (Figure 13) were constructed south of the KCF using line-length balancing techniques with the program Move 2D 2010.1. Though these structural cross-sections are not unique, through presentation of these sections and their key structural observations insights are gained into overall range structure and its relation to the inherited strike-slip fault at the range core.
5.2.1. Western Kungey Range
[48] A cross-section through the western Kungey Range was constructed along a transect following the Toru-Aygir river valley and north through the range (Figure 11). Deformation stepped southwards over time along a series of steeply dipping reverse faults within the range. Proximal to the KCF these reverse faults are responsible for basement uplift but account for only minor amounts (<1 km) of shortening. To the south, the unconformity surface is preserved on both limbs of the Jai-lo syncline, where shortening is being accommodated via active faulting involving both Neogene sedimentary units and Paleozoic bedrock.
[49] Deformation exposing Paleozoic basement ceases south of the Kyzyl-Kultor fault, with shortening occurring only in overlying Neogene strata. Based on the observed structural geometry of the Ak-Teke anticline, a trishear model [Erslev, 1991; Allmendinger, 1998] was applied to analyze the geometry of the forelimb of the fold and fault at depth (Figure 12). This model results in ∼550 m of slip and, because the fault soles into a horizontal decóllement interpreted as the gypsum-rich Kokturpak Fm., an equivalent amount of shortening across the structure.
[50] Shortening and uplift estimates across the Western Kungey Range are calculated from the cross-section using the unconformity surface as a deformation marker. Shortening and uplift estimates have been made relative to the present position of the unconformity in the footwall of the Ak-Teke fault at ∼500 m elevation (Figure 11). Uplift calculated for the core of the range adjacent to the KCF is a minimum because the unconformity surface is not preserved at the range crest. Restoration of the cross-section and including folding across the Ak-Teke anticline shows that ∼4 km of shortening has been accommodated across the western Kungey Range. Uplift of the range is highest closest to the KCF, where ∼5 km of rock uplift has occurred relative to the undeformed unconformity surface beneath the Ak-Teke anticline. This amount decreases to the south, where the basement-cored Kyzyl-Kultor hills have only experienced <2 km of rock uplift.
5.2.2. Central Kungey Range
[51] A second cross-section was constructed through the central Kungey Range east of the town of Cholpon-Ata (Figure 13). Sub-surface fault geometries are estimated from well-exposed Neogene strata and exposures of tilted unconformity surface both at the range front and high within the range. With the exception of the southern limb of the Cholpon-Ata syncline, the exhumed unconformity surface dips south 25–30°, indicating a major concave-down fault at depth [Erslev, 1986]. Rooting into this fault is a set of north-vergent backthrusts bounding the northern limb of the Cholpon-Ata syncline.
[52] The unconformity surface is again used as a deformation marker to restore the cross-section through the Central Kungey Range. Because it is not directly possible to measure the position of the unconformity surface in the footwall of the central Kungey Range, the same position as the footwall unconformity surface in the western Kungey Range is assumed at ∼500 m elevation. Restoration of the cross-section indicates ∼4.5 km of shortening across the Central Kungey Range. Though this amount is similar to the shortening in the Western Kungey Range, higher topographic elevations indicate that a greater amount of uplift is likely to have taken place. From restoration of the cross-section ∼5 km of uplift is estimated relative to the inferred position of the unconformity surface at the northern margin of the Issyk-Kul basin. Fission track studies of the Kungey Range [De Grave et al., 2007] show an absence of reset ages between the western and central Kungey cross-sections. Assuming a closure temperature of 110°C for apatite fission tracks and a geotherm of 30°C/km [Sobel et al., 2006; De Grave et al., 2007], a maximum of ∼4 km of incision into the unconformity surface is allowed. This places an upper limit on the amount of rock uplift at the range crest.
5.2.3. Range Growth
[53] The KCF is one of many inherited structures that play a role in the growth of the Tian Shan [e.g., De Grave et al., 2007]. Inheritance of the KCF is demonstrated by offset of Paleozoic basement terranes [Delvaux et al., 1999]; minor late Cenozoic motion is of the same sense as Paleozoic offsets and does not by itself accomplish range uplift. Growth of the northern Tian Shan has instead taken place via a newly formed reverse fault network that surrounds the inherited strike-slip fault. Evidence that these faults are newly formed includes basement offsets that are consistent with late Quaternary deformation and no observed juxtaposition of contrasting Paleozoic terranes across members of this fault network.
[54] The cross-sections (Figures 11 and 13) demonstrate the pattern of structural growth characteristic of the Kungey Range. The low-relief pre-Cenozoic unconformity surface is used as a deformation marker from which to track integrated shortening and uplift during growth of the Kungey Range. Deformation initiated on high-angle reverse faults proximal to the KCF, and progressively stepped southward over time. As new faults form, previously active reverse faults passively deform (deformation sans slip) in the hanging wall of the newly active fault (Figure 11). Continued southward propagation of faulting and passive deformation of inactive reverse faults explains significant amounts of uplift with small amounts of slip on faults in the range core. The Kyzyl-Kultor fault at the southern margin of the Kungey Range shows the greatest amount of slip, and it is inferred that the majority of shortening (∼2.5 out of 4 km) has taken place through motion on this fault. This pattern of greater uplift in the range core and shortening along the margin is most consistent with a network of concave-down faults rooting into a steeply dipping shear zone coincident with the KCF at depth (Figure 11).
[55] The southward-stepping pattern of deformation interpreted from the cross-section is consistent with observations of stratigraphy and active structural geometries. The most denuded portions of the range lie in its core adjacent to the KCF, and are likely also the earliest-formed parts of the range, providing sediments with a granitic provenance that make up the Kungey member of the Chu Fm. in exposures uplifted along the range margin. The Kyzyl-Kultor Fault that bounds the southern limb of the Jai-Lo syncline does not deform any member of the preserved terrace chronosequence, indicating that this fault has not been active in the past ∼140 ka. Instead, motion on the Kyzyl-Kultor fault has propagated south into the Issyk-Kul basin, where the Ak-Teke fault and Tekren anticline deform Neogene strata and late Quaternary alluvial surfaces. Active deformation also occurs in the Jai-Lo syncline, as the Paleozoic rocks and overlying Neogene sediments are folded over a bend in the underlying active Kyzyl-Kultor- Ak-Teke fault system.
[56] Deformation in the central Kungey Range displays a similar pattern of north-to-south propagation, although here active faults are also recognized within the range (Figure 13). Early deformation took place proximal to the KCF and later stepped southwards to the range front with southward propagation of the Cholpon-Bosteri fault. Active faults today include both the KCF and Chon-Aksu fault within the range [Delvaux et al., 2001; Arrowsmith et al., 2004; Crosby et al., 2005, 2007], and the range-bounding Cholpon-Bosteri fault.
5.3. Comparison of Long-Term and Late Quaternary Deformation
[57] Comparison of late Quaternary deformation rates to net shortening further illustrates the pattern of uplift that formed the Kungey and Zailiskey Ranges. Using structural shortening estimates and the stratigraphic interpretation that growth initiated between 7 and 4 Ma gives long-term shortening rates between 0.5 and 1.4 mm/yr for the western Kungey Range and 0.6 to 1.1 mm/yr in the central Kungey Range. Summing north-south late Quaternary shortening rates along the Toru-Aygir river valley in the western Kungey Range and allowing for minor shortening on additional structures in the Jai-Lo syncline adds to modern shortening of ∼1.2 ± 0.1 mm/yr, similar to the long-term rate. Since not all faults in the western Kungey Range are currently active and the shortening rate appears to have remained approximately constant over time, shortening in the western Kungey Range is interpreted to be partitioned on to separate sets of faults with activity that varies over ∼100 ka timescales, and generally steps outward from the range over time. In contrast to the western Kungey Range, long-term and late Quaternary shortening rates may not agree in the central Kungey Range. Modern shortening along the central Kungey Range front is ∼0.3 ± 0.1 mm/yr. Activity along the Chon-Aksu fault [Delvaux et al., 1999] could account for the discrepancy between modern and long-term shortening rates.
[58] Opposing gradients in deformation rate are observed along the Kungey and Zailiskey range fronts. Shortening rates in the Kungey Range are highest in the west and decrease to essentially zero in the east. Though thick loess cover obscures much of the Quaternary record along the central and eastern Zailiskey Range front, deformation rates inferred from deformed terrace markers and exposures of Neogene strata appear highest in the eastern half of the range front.
5.4. The Role of Strike-Slip Faulting in Shortening of the Northern Tian Shan
[59] Intracontinental strike-slip faults in convergent orogens have generally been thought to act as transform faults, shunting deformation from one part of the orogen to another [e.g.,Molnar and Tapponnier, 1975]. Major strike-slip faults bounding the Tibetan plateau, such as the Altyn-Tagh and Karakorum faults have been held as type examples, though fierce debate surrounds this interpretation [e.g.,Lacassin et al., 2004; Chevalier et al., 2005; Cowgill, 2007]. The pattern of shortening in the Kungey and Zailiskey Ranges is geometrically opposite of what would be expected for the absorption of slip at the ends of a sinistral transform fault. Instead, shortening is absorbed via east-west striking reverse faults located along and stepping outward from the axial strike-slip fault. Thus, rather than acting as a transform, it appears that the Kemin-Chilik fault acts as a central member of a dominantly north-south contractional system. Because the Kemin-Chilik fault remains embedded within the Kungey and Zailiskey Ranges, what sinistral motion does take place along the fault must be absorbed somehow along its length. We suggest that instead of shortening at the fault tips, sinistral motion is absorbed by adjacent crustal thickening [e.g.,Kirby et al., 2007], commensurate with gradients in shortening along the range margins (Figure 10). Since inherited strike-slip faults are common within the basement-cored uplifts of the Tian Shan, generation of high-angle reverse faults via reactivation of nearby, obliquely oriented, inherited faults may be a general feature of the orogen [Burtman, 1975; Sengor et al., 1993; Delvaux et al., 1999; Burtman et al., 1996; Mikolaichuk, 2000; Abdrakhmatov et al., 2001; Buslov et al., 2003].
[60] To apply a first-order test of our hypothesis of the KCF as a nucleation point for shortening structures, we compare the observed surface and interpreted cross-sectional fault geometries in the Kungey Range with the predictions from a mechanical model of slip partitioning [e.g.,Bowman et al., 2003]. Because fault geometries and slip are better understood in the Kungey than the Zailiskey Range, only the faulting associated with the uplift of the Kungey Range is considered. We use the program Coulomb 3.2 [Lin and Stein, 2004; Toda et al., 2005] to model the western Kungey Range as a slip-partitioned system. This model gives the orientations of failure planes based on instantaneous strains from slip on a dislocation at depth [Okada, 1985, 1992], allowing a general comparison with observed fault geometries. The orientations of model-predicted failure planes roughly mimics the observed strike of thrust and reverse faults which root into bedrock north of the Toru-Aygir River valley and in the central Kungey Range (Figure 14). Overall cross-sectional geometry through the western Kungey Range agrees with our cross-sectional fault geometry, producing results consistent with a network of concave-down faults rooting into a central shear zone co-located with the KCF at 20 km depth (Figure 14).
[61] The pattern of reverse faulting surrounding an inherited strike-slip fault we observe in the Kungey and Zailiskey Ranges appears elsewhere in BITB outside the Tian Shan with embedded strike-slip faults. For example, numerous strike-slip faults embedded within the Zagros Plateau in Iran absorb shortening via networks of reverse faults that have developed surrounding the axial strike-slip fault [Allen et al., 2006]. Another example is the Kunlun Fault in northern Tibet, where Kirby et al. [2007] show that slip is absorbed gradually along strike through formation of shortening structures.
5.5. Implications for the Development of High-Angle Basement-Involved Reverse Faults
[62] The reactivation of pre-existing, steeply dipping faults oriented oblique to the shortening direction could be a common mechanism for the production of new high-angle reverse faults in basement-involved thrust belts (BITB). A pre-existing, high-angle fault zone in the crust may preferentially re-activate despite non-optimal orientation to overall compression [Webb and Johnson, 2006]. Following initial re-activation, a network of high-angle reverse faults rooting into the central steeply dipping fault may develop to accommodate continued shortening. This geometry is commonly observed in transpressional regimes where restraining bends along major strike-slip faults produce steeply dipping reverse fault networks [e.g.,Cunningham et al., 1996; Ding et al., 2004; Vassallo et al., 2007]. However, this geometry has not been proposed in regions where lateral motion along the strike-slip fault only plays a minor role. Within the tectonic framework of the northern Tian Shan, sinistral motion along the KCF has not played a major role in the shortening of the Kungey and Zailiskey Ranges. What little late Cenozoic slip along KCF that has occurred is absorbed along its length. The more important role of the KCF appears to be that it accommodates transpression at depth, and acts as a nucleation point for a slip-partitioned system composed of numerous high-angle reverse faults. This fault network analogous to a positive flower structure is ultimately responsible for range growth and crustal shortening.
6. Conclusions
[63] The development of high-angle reverse faults commonly found in basement-cored uplifts poses problems, as these faults violate mechanical predictions for the initiation of thrust or reverse faults, and absorbing shortening on high-angle structures is kinematically unfavorable [Anderson, 1951; Sibson, 1985]. Reactivation of pre-existing strike-slip faults and the initiation of new faults in the surrounding crust may provide an explanation for the existence of these high-angle reverse faults. Inherited strike-slip faults can act as nucleation points, where oblique deformation at depth can propagate upwards as a partitioned system of reverse and obliquely slipping faults [Molnar, 1992; Bowman et al., 2003]. This network of neoformed faults can be described as a positive mega-flower structure with concave-down faults emanating from a central shear zone.
[64] The Kungey and Zailiskey Ranges of the northern Tian Shan are dominated by curviplanar reverse and oblique faults whose dips increase at depth to root into the central Kemin-Chilik fault (KCF). Growth of the ranges, deduced from structural and stratigraphic evidence, took place as these fault networks developed, propagating outward from the KCF. Because many other ranges within the Tian Shan are cored by inherited strike-slip faults, this growth model may apply elsewhere within the orogen, and provides a viable explanation for the origin of high-angle reverse faults in basement-involved uplifts.
Acknowledgments
[65] This research was supported by the Civilian Research and Development Foundation (CRDF) and American Chemical Society Petroleum Research Fund (PRF). The authors would like to thank Joseph Goode, Ray Weldon, and Reed Burgette for help in the field; and Eitan Shelef for assistance with processing 10Be samples. Comments from Richard Heermance and an anonymous reviewer greatly aided revisions of this manuscript.
