Geosphere; February 2008; v. 4; no. 1;
p. 292-314; DOI: 10.1130/GES00111.1
© 2008 Geological Society of America
Geometry, kinematics, and regional significance of the Chong Shan shear zone, Eastern Himalayan Syntaxis, Yunnan, China
Sinan Akciz*,1,
B. Clark Burchfiel1,
James L. Crowley1,
Yin Jiyun2 and
Chen Liangzhong2
1 Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
2 Yunnan Institute of Geological Sciences, 131 Baita Road, Kunming, Yunnan, China
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ABSTRACT
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The geology east of the Eastern Himalayan Syntaxis is poorly known, although it figures prominently in many models for the Cenozoic tectonics of the India-Eurasia collision and subsequent intracontinental deformation. The little known Chong Shan shear zone a 
250-km–long and 10-km–wide metamorphic belt composed mainly of mylonitic augen gneisses and migmatites, forms a major shear zone developed during early Cenozoic extrusion of the Indochina crustal (lithospheric?) fragment. Foliation within the shear zone is moderately to steeply west dipping, and stretching lineations are subhorizontal, consistent with dominantly strike-slip transport. Kinematic indicators including rotated porphyroclasts, S-C fabrics, and asymmetric folds provide evidence for both dextral and sinistral movements. Our preliminary geo-chronological studies indicate that the Chong Shan shear zone has been active since at least ca. 34 Ma, and perhaps as early as 41 Ma. Strike-slip shearing continued at least until ca. 29 Ma, perhaps as late as ca. 24 Ma, and terminated by ca. 17 Ma. The Chong Shan shear zone, therefore, is not a belt of Precambrian metamorphic rocks as previously interpreted, but a Cenozoic shear zone of great significance, which was contemporaneous with movement on the left-lateral Ailao Shan shear zone and the right-lateral Gaoligong Shan shear zone two shear zones that bound the Indochina fragment on the east and west, respectively. Our data from the Chong Shan shear zone along with the data presented elsewhere from the Gaoligong Shan shear zone indicate that while the region between the Gaoligong Shan shear zone and Ailao Shan shear zone extruded to the southeast, it did not extrude as a single rigid block, but rather it was dismembered into at least two major fragments, the Baoshan to the west and the Lanping-Simao to the east separated by the Chong Shan shear zone. Our study of the Chong Shan shear zone suggests it and the other major early Cenozoic shear zones formed part of a broad major shear zone that passes into eastern Tibet between the Qiang-tang and Lhasa tectonic units.
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INTRODUCTION
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One of the best natural laboratories to study intracontinental indentation and consequent bending of folded belts, extrusion of crustal material, and rotation of large crustal fragments (see also Van der Voo, 2004) is the young and active India/Eurasia collision zone. Although the latest Cenozoic deformation around the Eastern Himalayan Syntaxis is becoming reasonably well understood from geodetic data (King et al., 1997; Chen et al., 2000; Zhang et al., 2004), the early Cenozoic deformation remains less clear. Following the start of collision between India and Eurasia at ca. 50 Ma (Rowley, 1996, 1998; Zhu et al., 2005—although the time of the start of the collision at the Eastern Syntaxis remains uncertain), India has moved north 3200 km at the Eastern Syntaxis. The processes involved in such a large amount of intracontinental deformation should be obvious; however, the characteristics of the deformation remain controversial. Most of the interpretations of the processes envisioned for the deformation are based on models, because the geology of this huge area is complex and still poorly known.
The earliest attempt to understand the tectonics around the Eastern Syntaxis involved applying slip-line theory of indentors from engineering mechanics (Tapponnier and Molnar, 1976). The use of several different indentor geometries produced patterns of slip lines by which some presently active faults could be explained. However, these were instantaneous solutions and difficult to apply to the temporal and spatial evolution of early Cenozoic deformation. Two other models for the tectonic evolution show an extreme range from southeastward extrusion of the region between the Sagaing Fault in Burma and a discontinuous belt of mylonitic rocks in the Ailao Shan, Diancang Shan, and Xuelong Shan (collectively referred to as the Ailao Shan shear zone complex [ASSZC]) in Yunnan (Fig. 1), China, as a single lithospheric block with minimal internal deformation and minor rotation (e.g., Replumaz and Tapponnier, 2003; Tapponnier et al., 1986; Tapponnier et al., 1982), to lateral spreading of thickened viscous lithosphere with little eastward extrusion (Houseman and England, 1993). Alternate models argue for eastward flow of lower crust accompanied by large-scale clockwise rotation and crustal thickening (Royden et al., 1997) and extrusion of crustal material accompanied by significant internal deformation and large-scale rotation of smaller crustal fragments (Wang and Burchfiel, 1997).
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Figure 1. Generalized map of selected tectonic elements around the Eastern Himalayan Syntaxis. The Indochina and Sibumasu crustal fragments form the main body of rocks extruded to the SE during early Cenozoic time. Their position relative to the Qiangtang element of central Tibet is discussed in the text. Major suture zones and shear zones that bound these fragments are shown in red: JS—Jinsha suture, BS—Bangong suture, NS—Nujiang suture (coincident with the Gaoligong shear zone [GSSZ]); L-CM-NU-B-R: L—Lancangjiang, CM—Changning-Mengliang, NU—Nan-Uttaradit, B—Bentong, R—Raub suture zones combined. SF—Sagaing fault; ASSZ—Ailao Shan shear zone, CSSZ—Chong Shan shear zone, DSSZ—Diancang Shan shear zone, and XSSZ—Xuelong Shan shear zone, BaS—Baoshan element that forms the northern part of the Sibumasu regional tectonic element. Box indicates the location of Figure 2.
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In particular, the concept of lateral extrusion during early Cenozoic time has played an important role in tectonic interpretations of Asia, especially since the publication of numerous detailed studies conducted along the Ailao Shan shear zone complex (e.g., Gilley et al., 2003; Harrison et al., 1996; Harrison et al., 1992; Leloup and Kienast, 1993; Leloup et al., 1995; Leloup et al., 2001; Schärer et al., 1994; Schärer et al., 1990; Tapponnier et al., 1986; Tapponnier et al., 1982). However, numerous important questions still remain unanswered regarding the nature and the style of deformation induced by the indentation of India into Asia. One of these questions that we address here in our study of the Chong Shan shear zone is whether extrusion tectonics has been largely accomplished by rigid block motion (Leloup et al., 1995; Leloup et al., 2001; Replumaz and Tapponnier, 2003), or by a series of internally deformed crustal fragments that translated and rotated differentially (Wang and Burchfiel, 1997).
Our detailed field mapping and structural and geochronological studies were focused on several transects across a poorly documented linear metamorphic belt located west of the Ailao Shan shear zone complex—Chong Shan Metamorphic Belt (hereafter the Chong Shan shear zone) in Yunnan, China (Figs. 2 and 3). The Chong Shan shear zone is a long (250 km), narrow (<10 km) belt of metamorphic rocks that divides the proposed rigid Indochina crustal unit of the extrusion model into two different tectonic fragments. The Chong Shan shear zone is rarely considered in syntheses of the Cenozoic tectonics of the area because regional studies, in general, have considered the metamorphic rocks that form the Chong Shan shear zone to be Precambrian basement of the Baoshan tectonic element. In this paper, we describe seven transects across the Chong Shan shear zone, and our preliminary 40Ar/39Ar and U/Pb results show that the Chong Shan shear zone is a zone of early Cenozoic shearing, both left- and right-lateral, and was active coevally with the strike-slip shear zones that bound the extruding Indochina crustal unit, Gaoligong Shan shear zone, and the Ailao Shan shear zone (Figs. 2 and 3). Two recent publications (Wang et al., 2006 and Socquet and Pubellier, 2005) described mylonitic rocks with mainly sinistral shear-sense indicators south of our study area and concluded that these rocks form a strike-slip shear zone of early Cenozoic age.
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Figure 2. Tectonic map for the area shown in Figure 1 along the Three Rivers in southeastern Tibet and its adjacent foreland to the northeast. The major tectonic elements are labeled. The extruded Indochina fragment lies between the Ailao Shan shear zone on the east and the Gaoligong shear zone on the west; the Chong Shan shear zone (CSSZ) lies between them. Some important fault lines are shown in red. Broad red zones are belts of mylonitic rocks discussed in the text.
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Figure 3. Generalized geological map of the Gaoligong Shan and the Chong Shan shear zones (GSSZ and CSSZ) and the rock units they separate. Main tectonic elements are labeled: BaS—Baoshan, L—Lhasa, T—Thengchong. The location of the geological cross sections that are discussed in the text are lettered a through g and are circled. Major towns that are easy to locate on any road map of Yunnan Province, rather than the small villages that the cross sections are closest to, are also labeled. Block diagrams schematically represent structural setting of the samples dated. All of the rocks forming the Chong Shan shear zone (d in the schematic block diagrams), except for the discontinuous bands of marble, regardless of their degree of metamorphism, have been intruded by cm- to dm-scale pegmatitic, leucogranitic, and aplitic sills of granitic composition. While some of the granitic intrusions within the migmatitic section of the Chong Shan shear zone contain a folded foliation, the intrusions into the granitoid and pelitic gneisses show evidence for only one deformational event and can be grouped by their degree of deformation as follows: (A) unfoliated dikes that crosscut the foliation, (B) unfoliated, foliation-parallel sill, and (C) foliated, foliation-parallel sill. None of these sills and dikes contains a lineation, even though the host rocks they intrude display a prominent subhorizontal, mylonitic lineation associated with the strike-slip shearing along the Chong Shan shear zone. Structural setting for each sample is shown to the right of the sample number. mnz—monazite, M—muscovite, B—biotite.
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OVERVIEW OF THE GEOLOGY OF WESTERN YUNNAN
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Geologic (summarized in Leloup et al., 1995) and paleomagnetic evidences (Geissman et al., 2001) indicate that the Indochina crustal unit has been extruded southward 500–1000 km. Within China, this unit consists of four structurally and paleogeographically distinct tectonic elements—the Lanping-Simao, Linchang, Chengling-Mengliang, and Baoshan elements (Fig. 2).
The Lanping-Simao tectonic element lies west of South China and is separated from it by the Ailao Shan shear zone complex. Most of the Lanping-Simao tectonic element consists of a thick (structural thicknesses up to 7.5 km) succession of mostly nonmarine redbeds of Jurassic to early Cenozoic age (Bureau of Geology and Mineral Resources of Yunnan Province, 1990), which extends into Laos, Cambodia, and Thailand. Triassic rocks that are found mainly along its west side contain abundant volcanic rocks in its lower part and lie unconformably on Paleozoic strata. The Paleozoic strata are as old as Silurian and crop out in the cores of anticlines. They also form a belt of ophiolite-bearing mélange along part of the east side of the Lanping-Simao element (Bureau of Geology and Mineral Resources of Yunnan Province, 1990).
The Baoshan tectonic element (Fig. 2) lies to the west of the Lanping-Simao tectonic element and forms the northernmost continuation of the more regional Sibumasu continental fragment as defined by Metcalfe (1988) (Fig. 1). It contains a thick section, 10 km (probably a structural thickness and not a true stratigraphic thickness), that begins with Upper Proterozoic to Middle Cambrian slightly metamorphosed siliceous clastic and carbonate rocks at the base of the Baoshan stratigraphic sequence in the study area (Bureau of Geology and Mineral Resources of Yunnan Province, 1990; Jin, 1994). Ordovician and Silurian units consist mainly of fossiliferous shallow-water siliciclastics and some argillaceous limestone and shale. Fossiliferous carbonate units of Middle Devonian to Early Carboniferous age are overlain by diamictite, turbidite, conglomeratic sandstone, and siltstone of Late Carboniferous age (Jin, 1994). Lower Permian basaltic lava flows with pillow lavas and tuffaceous intercalations are overlain disconformably by redbeds of Early Permian (?) age. The Mesozoic strata rest paraconformably on the Paleozoic rocks, consist of Triassic limestone and Jurassic marine strata and redbeds, and are generally restricted to the margins of the Baoshan tectonic element. Cenozoic rocks are rare and are characterized by folded upper Eocene-Oligocene conglomerate and sandstone that rest unconformably on deformed older rocks.
The internal structure of the Baoshan tectonic element, bounded by the right-lateral Gaoligong Shan shear zone and the left-lateral Chong Shan shear zone, is complex, and the ages of the major folds and thrusts are poorly determined. We follow the suggestion of Wang and Burchfiel (1997) and consider this fold and thrust belt structure to have formed in at least two events, one following deposition of middle Jurassic rocks but before deposition of Upper Eocene strata, and a second, less intense folding following deposition of the Upper Eocene and Oligocene strata in the central and southeastern part of the Baoshan tectonic element. Folding was followed by transverse strike-slip faulting of early and late Cenozoic age, and many of the faults are presently active.
The Baoshan tectonic element contains a very characteristic and distinct Paleozoic sedimentary and volcanic sequence that is different from the Lanping-Simao tectonic element of the Indochina continental fragment. Within the map area, rocks that would establish the existence and location of a suture bounding the east side of the Baoshan tectonic element are missing because of intense Cenozoic shearing and dislocation along the Lancangjiang section of the Lancangjiang–Changning–Menglian–Nan–Uttaradit–Bentong–Raub suture zone (Fig. 1), hereafter called the Lancangjiang suture zone. The Lancangjiang suture in the study area is cryptic, but its existence is proposed to lie in the Chong Shan shear zone along the northern part of the Lancang River and is based only on the regional analysis of suture zones to the north in Tibet and to the south in southern Yunnan and Thailand.
The Ailao Shan shear zone complex is commonly taken as the eastern boundary of the early Cenozoic extruded Indochina continental fragment (Figs. 1 and 2; Leloup et al., 1995; Leloup et al., 2001). The Ailao Shan shear zone, the longest and widest of the three mylonitic belts that form the Ailao Shan shear zone complex, is also the most thoroughly studied of all the shear zones in this region. It has been interpreted to be a major left-lateral shear zone of middle Cenozoic age (e.g., Harrison et al., 1996; Harrison et al., 1992; Leloup and Kienast, 1993; Leloup et al., 1995; Leloup et al., 2001; Schärer et al., 1994; Schärer et al., 1990; Tapponnier et al., 1986; Tapponnier et al., 1982). Ailao Shan shear zone complex has accommodated 700 ± 200 km of displacement of Indochina to the southeast relative to South China (Leloup et al., 1995) since at least 34 Ma, based on dating of monazite inclusions in synkinematic garnet (Gilley et al., 2003). U-Pb ages of accessory minerals, from late syntectonic leucogranites parallel to the foliation and affected by left-lateral shearing, cluster around 24 Ma (Leloup et al., 1995), indicating that the left-lateral shearing continued at least until this time and possibly as late as 17 Ma (Briais et al., 1993).
The Gaoligong Shan shear zone is the western boundary of the extruded Indochina continental fragment (Figs. 2 and 3). The Gaoligong Shan shear zone consists of gneisses, in two parts, a section of plutonic origin to the west, and a section of metasedimentary origin to the east, both of which consist of amphibolite-grade rocks. Most of the foliations in the Gaoligong Shan shear zone metamorphic rocks dip steeply (
60°) both to the west and east and show well-developed mylonitic textures. The mylonitic foliation contains a prominent subhorizontal lineation, interpreted as a stretching lineation, formed by the elongation of quartz, feldspar, and biotite. Consistent shear criteria, such as asymmetric tails on deformed feldspar porphyroclasts, S-C surfaces, asymmetric boudins, and cm- to dm-scale asymmetric folds observed in horizontal sections perpendicular to the foliation and parallel to the lineation, indicate a right-lateral sense of shear. Syntectonic pegmatite dikes and leucogranite bodies generally form concordant veins, layers, and lens-shaped bodies and contain a foliation formed by micas indicating they have been affected by same ductile deformation as their host mylonitic metamorphic rocks. Our preliminary geochronological work indicates this shear zone was active during early Cenozoic time and was the main western boundary for the extruding Indochina crustal fragment (Akciz, 2004).
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STUDIED CROSS SECTIONS OF THE CHONG SHAN SHEAR ZONE
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In this heavily forested area, seven structural sections across the Chong Shan shear zone were studied from Baoshan to Gongshan; all sections had varying exposure quality (see Fig. 3 for locations of the geological cross sections illustrated in Fig. 4
). Unlike the better documented Ailao Shan shear zone (e.g., Leloup et al., 1995) and the Gaoligong Shan shear zone (Akciz, 2004), the rock units that form the Chong Shan shear zone change from south to north and cannot be easily connected between cross sections, probably due to different protoliths in different sections of the shear zone assembled during Cenozoic displacement. Therefore, we describe the various different lithologies and structures observed along each section separately.
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Figure 4. Geological cross sections across the Chong Shan shear zone (CSSZ). Vertical and horizontal scales are equal; no vertical exaggeration. Schmidt diagrams (lower hemisphere) show the foliation planes and stretching lineations in Chong Shan shear zone rocks at the corresponding cross sections. QFB–quartz-feldspar-biotite.
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Wayao Section (Fig. 4A)
The Wayao section is the most accessible section across the Chong Shan shear zone, but it is not very representative of the shear zone as a whole. The Lancang River makes a sharp bend across the Chong Shan shear zone at Wayao and exposes a narrow, nearly complete section of the southern shear zone. The lower Paleozoic sedimentary rocks of the Baoshan tectonic element can be traced from west to east into progressively higher grade rocks of the Chong Shan shear zone. The high-grade rocks of the shear zone consist of from west to east: quartz-feldspar-biotite (QFB) metasedimentary gneiss intruded by weakly foliated to unfoliated leucogranitic sills, muscovite, and biotite schist with thick units of impure quartzite, calcsilicate rocks, marble, an intensely folded impure quartzite interlayered with pure quartzite, and QFB gneiss and biotite schist. Observations of large outcrops with fresh surfaces suggest that some of the QFB gneisses may in fact be the melanosome, and the leucogranites may be the leucosomes, of local migmatitic sections. Unlike the rest of the Chong Shan shear zone, most of the foliation along this transect dips moderately (40°–60°) west and is mylonitic. A subhorizontal stretching lineation associated with the mylonitic fabric is only present within the QFB gneisses and the micaceous schists. The schistosity is marked by biotite, ±chlorite, ±actinolite and quartz, ±feldspar ribbons. No garnet or aluminosilicate minerals are present that would indicate metamorphism at high temperatures. Quartz usually defines the lineation, forming long polycrystalline ribbons generally made of recrystallized grains. The grains have an irregular shape, sutured boundaries, often with undulatory extinction, subgrains, and deformation bands.
The dominant east-vergent folds, west-dipping thrust faults, and the increasing metamorphic grade from the unmetamorphosed to weakly metamorphosed sedimentary rocks of the Baoshan sedimentary sequence in the west to the higher-grade metamorphic rocks that form the Chong Shan shear zone to the east, suggest the protoliths of the Chong Shan shear zone at the Wayao section were Baoshan sediments and their metamorphic basement. The contact between the Chong Shan shear zone and the Baoshan tectonic element is not exposed along this road section, which is obscured by thick vegetation. We interpret the sharp and steeply west-dipping contact between the high-grade rocks of the Chong Shan shear zone and the unmetamorphosed Paleozoic rocks of the Baoshan tectonic element to be a normal fault. The age of this fault, which is referred to as the Chaojian fault, as well as its sense of displacement and continuation to the north, remains to be resolved. Along its east side, the Chong Shan shear zone was thrust east over the Jurassic and Cretaceous sedimentary rocks of the Lanping-Simao tectonic element. The contact between the Chong Shan shear zone and the Lanping-Simao tectonic element is exposed only here and along the Chaojian road section (see below), is foliation parallel, and is referred to as the Lancangjiang fault.
Chaojian Road Section (Fig. 4B)
The Chaojian section exposes a nearly continuous cross section of the Chong Shan shear zone composed of mainly migmatite, mylonitic granite, augen gneiss, quartzofeldspathic gneiss, and QFB gneiss as well as strongly foliated impure quartzite. All foliations dip steeply (60°) to the west. The mylonitic schistosity is marked by biotite, quartz and/or feldspar ribbons, and a prominent subhorizontal lineation interpreted as a stretching lineation formed by the elongation of quartz, feldspar, and biotite. Unlike the Wayao section, the metamorphic rocks of the Chaojian section contain garnet, staurolite, and rutile, indicating metamorphism at higher temperatures (Fig. 5). All shear-sense indicators, including C–C' fabrics (Fig. 6) and rotated plagioclase porphyroclasts, indicate left-lateral shear. Weakly to strongly foliated, pegmatite and tourmaline-bearing, micaceous leucogranite dikes, interpreted to be syndeformational, intrude all the other units within the shear zone. The contact between the Chong Shan shear zone and the Lanping-Simao belt, the Lanchangjiang fault, is foliation parallel, and can be located to within a few meters.
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Figure 5. Photomicrograph of two gneiss samples from the Chaojian section. (A) Garnet in a groundmass of very fine-grained quartz and feldspar with mylonitic seams marked by biotite and muscovite (labeled B). (B) Thin seams of muscovite and biotite define the mylonitic foliation from a somewhat weakly foliated section in otherwise well-foliated quartz, plagioclase, biotite gneiss with traces of muscovite and a relict staurolite grains. Quartz and plagioclase are mostly recrystallized, but large relict grains are still recognizable.
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Figure 6. Two examples of mylonitic foliation cut by left-lateral shear band fabric (C–C') observed along the Chaojian section of the Chong Shan shear zone. Pictures are of (A) a polished rock slab and (B) an outcrop. Tip of the pen visible in (B) is 2 cm long. C' planes are highlighted on A' and B'.
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Tuer Section (Fig. 4C)
The Tuer section consists only of limited exposures of the easternmost part of the Chong Shan shear zone. The rocks consist of grayish-green actinolite schist and ultramylonite (Fig. 7) and highly weathered leucogranite sills sheared at the decimeter scale. The actinolite schist consists mostly of alternating bands of medium- to coarse-grained actinolite with quartz and trace amounts of titanite and epidote. The ultramylonite matrix is fine grained and composed of quartz, mica, and plagioclase. Feldspar and quartz porphyroclasts mostly have symmetric tails of recrystallized grains. Foliations dip steeply (70°) to the west and contain a subhorizontal lineation. All rock types contain C–C' fabrics and rotated plagioclase porphyroclasts that indicate left-lateral, strike-slip shearing. To the east, the contact between the Chong Shan shear zone metamorphic rocks and the very weakly metamorphosed, fine-grained Jurassic-Cretaceous sedimentary rocks of the Lanping-Simao belt is foliation parallel and can be located within a few meters, although the contact is never exposed.
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Figure 7. Thin sections showing mylonitic (A and B) and ultramylonitic (C and D) fabrics from the Tuer cross section. (A) biotite-chlorite mylonitic schist, (B) actinolite schist, (C) and (D) ultramylonite. Several K-feldspar porphyroclasts are found in the ultramylonitic matrix composed of quartz-mica-feldspar.
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Bijiang Road Section (Fig. 4D)
The Bijiang section is along a dirt road constructed for timber trucks that starts at the Nujiang River, passes through the small village of Bijiang, and continues farther east. The Bijiang section passes through the metasedimentary rocks of the Gaoligong Shan shear zone; these rocks consist mainly of isoclinally folded, narrow (50 to 100 m) bands of marble, calcsilicate rocks and impure quartzite, none of which contain good shear-sense indicators, even though they are well foliated. These metasedimentary rocks are juxtaposed against mylonitic granite that contains a steep mylonitic foliation and subhorizontal stretching lineation. Both left- and right-lateral shear-sense indicators are present that include C–C' fabrics and rotated plagioclase porphyroclasts (Fig. 8). In the eastern part of the section, the Chong Shan shear zone metamorphic rocks consist mainly of foliated mylonitic granite and QFB gneiss that dip 60° both to the east and west. No right-lateral, shear-sense indicators were observed 5 km east of the town of Bijiang, although the left-lateral, shear-sense indicators are abundantly present. Foliation-parallel leucogranite and pegmatite dikes are abundant and are either weakly foliated or undeformed, compared to their strongly foliated host rocks. No mineral lineations were observed in the leucogranite sills.
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Figure 8. (A) Handsample example of asymmetric deformation of rotated feldspar porphyroclast along the Bijiang section showing left-lateral shear sense. (B) Near the contact with the Gaoligong Shan shear zone, however, right-lateral shear senses become abundant.
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Fugong Granite Section (Fig. 4E)
South of the Bijiang section (Fig. 3), the Baoshan unit wedges out between the Gaoligong Shan and the Chong Shan shear zones, and along the section north of the town of Fugong are limited exposures, all of which make the location of the boundary between the Gaoligong Shan shear zone metasedimentary rocks and the Chong Shan shear zone rocks difficult to locate accurately. We have placed the contact at the first occurrence of dominantly migmatitic/granitoid gneissic rocks with both right- and left-lateral, shear-sense indicators east of a metasedimentary section that contains neither granitoid gneisses nor any left-lateral, shear-sense indicators. The Chong Shan shear zone in this section contains dominantly mylonitic granite and granitoid gneiss with minor units of QFB gneiss and impure quartzite. Near the mountain summit, remnants of an undeformed granitic body are present (Fig. 9A). The contact between the granite and the orthogneisses is a zone of mylonitic granites (Fig. 9B) with vertical foliation and subhorizontal lineation containing left-lateral, shear-sense indicators.
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Figure 9. (A) Undeformed Fugong granite of Permo-Triassic age (?) near the Chong Shan ridge crest along the transect. (B) The same granite becomes more mylonitic toward the west within the Chong Shan shear zone.
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Lishinerhe Section (Fig. 4F)
The Lishinerhe valley is a tributary of the Nujiang valley and offers excellent exposures of the western part of the Chong Shan shear zone, but it nowhere crosses the entire shear zone. This section consists mainly of migmatite and augen gneiss as well as mylonitic granite, all of which contain a subvertical foliation and a subhorizontal stretching lineation marked by biotite, muscovite, sillimanite, and ribbons of recrystallized quartz and feldspar. Kinematic shear-sense criteria include asymmetric folds, pervasive S-C fabrics, mica fish, and feldspar porphyroclasts with asymmetric tails made of small, recrystallized quartz and feldspar grains (Fig. 10). These rocks contain dominantly left-lateral shear indicators, although some right-lateral, shear-sense indicators, such as S-C fabrics and rotated porphyroclasts, are also present near the contact with the Gaoligong Shan shear zone. The migmatitic core of the Chong Shan shear zone is characterized by an abundance of variably deformed leucogranite dikes and sills, whereas the remainder of the orthogneissic rocks is intruded by leucogranitic sills that are either weakly foliated or completely undeformed.
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Figure 10. (A) Mica fish in mylonitic rocks in the Lishinerhe section is highlighted in 13(A'). The evidence shows clear left-lateral shear sense. (B) Asymmetric folding indicating left-lateral shear in the same outcrops.
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Shidu Section (Fig. 4G)
The Shidu section is along a trail east from the Nujiang River and contains an almost complete section of the Chong Shan shear zone. There is a progressive increase in metamorphic grade from west to east along this section. The village of ShiDu is located on top of a thick section of brown to beige, impure to pure quartzite. Farther east, quartzite is interlayered with QFB gneiss. East of a thin marble unit, the dominant rock type is QFB gneiss with leucogranite sills and local occurrences of biotite schist and fine-grained impure quartzite. No aluminosilicate minerals or garnets were observed in this part of the cross section. A higher grade, migmatite-rich section occurs east of these low-grade metasedimentary rocks that contain abundant garnet, K-feldspar, and sillimanite, as well as muscovite and biotite, along with quartz and plagioclase. The remainder of the section to the east consists of garnet- and sillimanite-bearing QFB gneiss interlayered with cm-scale leucogranite sills as well as augen gneiss, mylonitic granite, and migmatite. The increased metamorphic grade is also well expressed to the north (see above) by the progressive transformation of the arkosic strata, quartzite, and slate (of unclear origin) into Chong Shan shear zone schist, gneiss, and migmatite. All of these rock units have a well-developed foliation, but lineation and mylonitic fabric are more abundant in the eastern part of the section. Similar to some of the southern sections, both the dextral and sinistral shear-sense indicators are abundant in the high-grade section. Sample 00JN21.4, for which U/Pb dates are presented below, is an unfoliated leucogranite that crosscuts the foliated and mylonitized QFB gneiss.
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THE CHONGSHAN SHEAR ZONE: SUMMARY
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The Chong Shan shear zone consists of an assemblage of migmatite, metasedimentary gneiss, micaschist, impure quartzite, calcsilicate rocks, marble, augen gneiss, and mylonitic granite, but protolith and shear sense vary along and across strike. Basement rocks of the Baoshan tectonic element and its lower Paleozoic sedimentary cover rocks are the protoliths of the metasedimentary and metaigneous core of the Chong Shan shear zone in the south. The basement rocks of the Qiangtang and their upper Paleozoic sedimentary cover rocks (Carboniferous?) are the probable protoliths for the northernmost section of the Chong Shan shear zone. These observations indicate that the Cenozoic shortening and horizontal shearing has brought together pieces of Qiangtang, Sibumasu, and Indochina along Chong Shan shear zone, and the suture zones associated with these tectonic units have become cryptic.
A major, penetrative ductile deformation with steep foliations and nearly horizontal lineations characterizes almost all the rock types of the Chong Shan shear zone, and is particularly well developed in the pelitic gneiss, mylonitic granite, and augen gneiss. The strike-slip deformation along the Chong Shan shear zone occurred in the ductile field above 300 °C as indicated by the recrystallization of quartz and formation of biotite, but probably not much above 500 °C, because K-feldspar porphyroclasts are generally preserved and do not show evidence for recrystallization, and muscovite, where present, forms the mylonitic foliation (Gapais, 1989; LeGoff and Ballevre, 1990). The foliation generally dips moderately to steeply to the west. A stretching lineation defined by the alignment of elongate mineral grains is consistently subhorizontal with a dominantly left-lateral, strike-slip shear sense. However, at and north of Bijiang (section d, Fig. 3), the cross sections of the Chong Shan shear zone contain asymmetric porphyroclasts and S-C fabrics that indicate both a dextral and a sinistral sense of motion. From Bijiang to Shidu (section g, Fig. 3), the dextral shear-sense indicators are generally, but not always, present near the Gaoligong Shan shear zone, and the sinistral indicators are observed farther east.
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TIMING OF DEFORMATION WITHIN THE CHONG SHAN METAMORPHIC BELT
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All of the rocks forming the Chong Shan shear zone, except for the discontinuous bands of marble, regardless of their degree of metamorphism, have been intruded by cm- to dm-scale pegmatitic, leucogranitic, and aplitic sills of granitic composition. The mineralogy of the sills is dominated by quartz, plagioclase, ±microcline, ±garnet, ±tourmaline, ±biotite, ±muscovite. While some of the granitic intrusions within the migmatitic section of the Chong Shan shear zone contain a folded foliation, the intrusions into the granitoid and pelitic gneisses show evidence for only one deformational event and can be grouped by their degree of deformation as follows: (1) foliated sills, (2) foliated and boudinaged sills, (3) unfoliated sills, (4) unfoliated but boudinaged sills, and (5) unfoliated dikes that crosscut the foliation. None of these sills and dikes contains a lineation, even though the host rocks they intrude display prominent, subhorizontal, mylonitic lineations associated with the strike-slip shearing along the Chong Shan shear zone. We interpret the smaller degree of deformation observed in the granitic sills and dikes compared to the strongly foliated and lineated host rocks to indicate they were intruded during the later stages of the strike-slip shearing.
We conducted a preliminary U/Pb and 40Ar/39Ar geochronological study on four different rock types to constrain the age of the large-scale, strike-slip deformation along the Chong Shan shear zone. These rock types are: (1) mylonitic orthogneiss, (2) foliated and boudinaged leucogranite sills, (3) unfoliated leucogranite sills, and (4) unfoliated leucogranite dikes that crosscut the mylonitic foliation.
Sample locations and schematic block diagrams showing their structural setting are shown in Figure 3. Table 1 lists conventional U-Pb, and Table 2 is a summary of 40Ar/39Ar analytical data. Figures 11–13 present the U-Pb analyses of monazite in the form of Concordia plots. Figure 14 shows the plateau isochron plots for micas from orthogneisses and also leucogranites intruding these gneisses. The main findings of this reconnaissance geochronological study of the crystalline core of the Chong Shan shear zone are summarized in a chart of U-Pb and 40Ar/39Ar age results in Figure 15.
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Figure 11. (A) Backscattered electron images of the monazite grains from sample 00JN25.1 that were dated. (B) U-Pb Concordia diagram of the sample.
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Figure 14. Plateau plots from argon data for micas from granitoid gneiss and leucogranite samples. See text for sample descriptions.
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U-Pb Monazite Geochronology: Sample Description and Results
Foliated Leucogranite (Sample 00JN25.1—Fig. 11)
Sample 00JN25.1 is from a 50-cm–thick foliated leucogranite sill within the Wayao section that intrudes mylonitic QFB gneiss that contains well-developed, left-lateral, shear-sense indicators. We interpret this leucogranite sill to be syntectonic because it has a well-developed foliation parallel to the mylonitic foliation of its host rock, even though it lacks the subhorizontal stretching lineation. The mineralogy of this sill is dominated by quartz, plagioclase, K-feldspar, tourmaline, and muscovite. Accessory minerals include abundant monazite and zircon. Seven fragments from four monazite grains (m1a, m1b, m2a, m2b, m3a, m3b, and m4a) yielded reversely discordant dates (Fig. 11). Reverse discordance is a common feature of young monazites and is thought to indicate 230Th disequilibrium (Schärer et al., 1984; Parrish, 1990); as a consequence, we use the 207U/235Pb dates in our age interpretations (errors given at 2
). Backscattered electron images (BSE) of monazite grains m1, m2, and m3 show moderately strong, oscillatory zoning that is typical of magmatic monazites, whereas grain m4 shows no discernable zoning. Fragments from grain m2 yielded the youngest dates of 33.60 ± 0.04 Ma and 34.38 ± 0.03 Ma. The two fragments from grain m1 yielded the next youngest dates of 36.55 ± 0.05 Ma and 37.78 ± 0.10 Ma. Three fragments from grain m3 yielded older dates of 38.49 ± 0.06 Ma, 40.63 ± 0.07 Ma, and 41.00 ± 0.07 Ma with a larger spread, spanning 2.5 my relative to the dates of the fragments from grains m1 and m2, which span only 1 myr. One fragment from grain m4 yielded the oldest date of 41.25 ± 0.07 Ma.
Unfoliated Leucogranite (Samples 98JL18.4 and 99JU27.1—Fig. 12)
Samples 98JL18.4 and 99JU27.1 are from 20-cm–thick undeformed leucogranite sills that intrude the QFB gneiss along the Bijiang and Chaojian sections, respectively. We interpret the lack of foliation, lineation, and boudinage to indicate that these sills intruded the mylonitic QFB gneiss after a time when the shearing ceased. The mineralogy of these sills is dominated by quartz, plagioclase, K-feldspar, garnet, tourmaline, biotite, and muscovite. Three monazite grains from sample 98JL18.4 (m1, m3, and m4 [m2 has not been analyzed yet]) and four monazite grains from sample 99JU27.1 (m1, m2, m3, and m4), none of which were BSE imaged, yielded reversely discordant dates (Fig. 12). As with sample 00JN25.1, we use the 207U/235Pb dates that range from 24.3 ± 0.06 to 29.4 ± 0.05 Ma and 24.4 ± 0.08 to 26.0 ± 0.14 Ma for samples 98JL18.4 and 99JU27.1, respectively, in our age interpretations. Data from 40Ar/39Ar study from the biotite and muscovite crystals of sample 98JL18.4 are given below.
Crosscutting Dike (Sample 00JN21.4—Fig. 13)
Sample 00JN21.4 was collected from a 50-cm–thick, undeformed, leucogranite dike in the Shidu section that cuts across the foliation of the QFB gneiss near the eastern boundary of the Chong Shan shear zone. The mineralogy of this dike is dominated by quartz, plagioclase, and tourmaline. Accessory minerals include monazite and zircon; however, they are less abundant and smaller in size compared to those in the other granitic rocks we sampled. Fragments from two monazite grains with little or no discernable zoning (m2b, m4a, and m4b) in a BSE image yielded reversely discordant 207U/235Pb dates of 17.01 ± 0.04 Ma, 16.51 ± 0.04 Ma, and 16.53 ± 0.11 Ma, respectively.
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Figure 13. (A) Backscattered electron images of the monazite grains from sample 00JN21.4 that were dated. (B) U-Pb Concordia diagram of the sample.
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INTERPRETATION OF THE MONAZITE DATA
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Two different types of age dispersions are observed in the leucogranite sample 00JN25.1 (foliated leucogranite): (1) a range spread of dates from ca. 33.7 to ca. 41.3 Ma, and (2) a spread of up to 2.5 my between the 2–3 fragments from a single grain. Understanding the cause of this dispersion is crucial to interpreting the age of the samples. The overall age dispersion could be produced in several ways. One possibility is that the igneous crystallization age is closely represented by the youngest monazite age, and the older grains contain an inherited component that is probably not more than several tens of millions of years older than the oldest date. A second possibility is that each date represents a crystallization age corresponding to different metamorphic and anatectic phases over the ca. 33.7 to ca. 41.3 Ma interval. A third possibility is that the oldest dates obtained from grains m3 (41.00 ± 0.07 Ma) and m4 (41.25 ± 0.07 Ma) are close to the igneous crystallization age, and the other fragments have been affected by recrystallization at a younger time than the youngest date (ca. 33.7 Ma). However, there is little indication of such recrystallization in the BSE images of the grains we analyzed. Also, because BSE images are controlled by Ce, La, Th, and Nd, it is possible that minor recrystallization did occur, but the Ce, La, Th, and Nd concentrations remained similar to those in the original magmatic monazite. Chemical mapping of several elements (e.g., Y, Ca, Si, and REE) in the grains before thermal ionization mass spectrometry (TIMS) dating would indicate whether all parts are chemically homogeneous or not, but such analyses were not made.
While these end-member interpretations imply that sample 00JN25.1 crystallized at either ca. 34 Ma, episodically from ca. 34 to ca. 41 Ma, or at ca. 41 Ma, intermediate interpretations are equally viable, if a combination of inheritance and protracted mineral growth played a role in the dispersion. Regardless, all possible interpretations of the present data indicate an important mid- to late Eocene to Oligocene magmatic and/or metamorphic event in the Chong Shan shear zone that is younger than the proposed age of the start of India-Eurasia collision (Rowley 1996, 1998; Zhu et al., 2005).
Fragments from single grains of sample 00JN25.1 also show a spread of dates, with a range of ca. 0.8 Ma for grain m2 to a range of ca. 2.5 Ma for grain m3. BSE images of the monazite grains m1 and m2 show moderately strong oscillatory zoning that is typical of magmatic monazites, although the nature of the zoning geometry is very different. The oscillatory zones of m1 are narrower and more diffuse compared to the wider and more discrete zones of m2. The morphology of the grains is also different: m2 is larger and its crystal edges form obtuse angles, whereas m1 is smaller and has pointed edges with acute angles. The contrasting morphologies and the apparent lack of any corerim zoning structures can be used to argue that these two grains crystallized during two different anatectic phases, at ca. 34 Ma and at ca. 37 Ma. However, a detailed systematic study with more grains categorized based on BSE imaging and chemical composition of the various domains is clearly needed to test the preliminary hypothesis regarding the significance of the dates from these two grains.
The spread of dates of the three fragments obtained from grain m3 is ca. 2.5 Ma, significantly more than the ca. 0.9 Ma spread observed in grains m1 and m2. A closer examination of the BSE image reveals that m3 has a core with a patch zoning and a mantle with good oscillatory zoning. Therefore, these monazite dates may represent a mixture of an older inherited core and a younger magmatic mantle.
Note that the constraint provided by the age of the foliated leucogranite sill does not address the timing of the initiation of strike-slip shearing, but rather the time during which strike-slip shearing was occurring, even though none of these sills are lineated. It is premature to say that because the deformed sills are older than the undeformed sills, they date all the shearing within the Chong Shan shear zone. Shearing may have been temporally and spatially partitioned across the width of this broad shear zone; some of the leucogranites might have escaped the deformation at the same time as others were being sheared. While this is an important issue to be addressed, our preliminary geochronological data set is too limited to test all the possibilities.
207U/235Pb data from the unfoliated leucogranite sill sample 98JL18.4 suggest that the strike-slip shearing ended before ca. 29 Ma, and perhaps as late as ca. 24 Ma, if the lack of any deformation is interpreted to indicate that the intrusion postdates the strike-slip shearing. While the spread of 207U/235Pb ages is less than that reported for sample 00JN25.1, these single grain analyses, which were obtained before the need for BSE imaging was recognized, should be interpreted with caution considering the possible internal complexity of these monazite grains.
The U-Pb data from sample 00JN21.4 from the leucogranite dike that crosscuts mylonitic gneiss of the Chong Shan shear zone indicate that the major fabric-forming deformational episode in this part of the Chong Shan shear zone had ceased by ca. 17 Ma. Even though this particular sill has intruded QFB gneiss that contains macroscopic left-lateral, shear-sense criteria, it is not clear whether the U-Pb data constrain the termination of the hypothesized earlier left-lateral or subsequent right-lateral shearing. Nevertheless, this is our best temporal constraint on the termination of the strike-slip shearing along the Chong Shan shear zone, and it is the only direct geochronological data from any one of the three mylonitic shear zones (Gaoligong Shan shear zone, Chong Shan shear zone, and Ailao Shan shear zone complex) that document the termination of strike-slip shearing.
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40Ar/39Ar GEOCHRONOLOGY: SAMPLE DESCRIPTION AND RESULTS
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Granitoid Gneiss (Samples 99JU4.2 and 99M18.6—Fig. 14)
Two granitoid gneisses with well-preserved mica were analyzed—99JU4.2 (from the Bijiang section; Fig. 5D) and 99M18.6 (from the Chaojian section; Fig. 5B) (see Fig. 3 for locations). As with most of the deformed rocks along the Chong Shan shear zone, the samples contain a vertical foliation and a subhorizontal stretching lineation, and they were sheared sinistrally. Recrystallized quartz, plagioclase, and micas form the foliation. None of the infrequent K-feldspar grains display plastic deformation features. Some feldspar grains, especially plagioclase, show brittle fractures. The contrast between quartz and feldspar deformation indicates temperatures were probably between 350° and 500 °C. Muscovite from 99JU4.2 yielded an age of ca. 19.4 Ma, and biotite from sample 99M18.6 yielded an age of ca. 17.3 Ma.
While the 40Ar/39Ar ages on micas are interpreted as cooling ages, the ca. 17 to 19 Ma age from these greenschist facies orthogneiss samples can also potentially be interpreted as the time of mica growth during the shearing, and thus may represent the minimum age of the shearing event.
Foliated Leucogranites (Samples 00JN25.1 and 99JU4.6—Fig. 14)
The locations of the two selected samples, 00JN25.1 and 99JU4.6, from foliated leucogranite sills, are shown in Figure 3. The mineralogy and the texture of sample 99JU4.6 is similar to that of sample 00JN25.1, which was described in the U-Pb sample description and results section above, except that it contains muscovite instead of biotite. 40Ar/39Ar analysis of biotite from sample 00JN25.1 and muscovite from sample 99JU4.6 yielded cooling ages of ca. 15.8 Ma and ca. 23.1 Ma, respectively.
Unfoliated Leucogranites (Samples 99JU18.4, 98JL18.4, and 99JU4.3—Fig. 14)
The locations of the three selected samples from unfoliated leucogranite sills, samples 99JU18.4, 98JL18.4, and 99JU4.3, are shown in Figure 3. All of these samples are from 30-cm–thick undeformed leucogranite sills that intrude QFB gneiss of the Chong Shan shear zone. We interpret the lack of foliation, lineation, and boudinage as an indication that these sills intruded the mylonitic QFB gneiss either at a time when shearing ceased or when strike-slip shearing was coming to an end. The mineralogy of these sills is dominated by quartz, plagioclase, garnet, tourmaline, biotite, and muscovite. Samples from the undeformed leucogranitic sills consistently yielded young cooling ages. Biotite from sample 99JU18.4, muscovite from 99JU4.3, and biotite from sample 98JL18.4 yielded ages of ca. 20.1 Ma, ca. 19.6 Ma, and ca. 13.6 Ma, respectively.
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INTERPRETATION OF THE 40Ar/39Ar DATA
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Cooling ages of the samples from the Chong Shan shear zone are spread between ca. 13.5 Ma (98JL18.4) and ca. 23.1 Ma (99JU4.6), and there is no clear correlation between the cooling ages and the intensity of shearing. In other words, the cooling ages of all the unfoliated leucogranite samples are not all younger than the cooling ages of the foliated leucogranites or the orthogneisses. This is perhaps not surprising, if we consider the length (>250 km) and width (<10 km) of the Chong Shan shear zone. It is likely that the strike-slip shearing, as well as exhumation, was probably not uniform, and may have been partitioned temporally and spatially across and along the shear zone, similar to the evolution of the well-documented Great Slave Lake shear zone in the Canadian Shield (e.g., Hanmer, 1988; Hanmer et al., 1992). As a result, different generations of leucogranites probably experienced deformation (strike-slip shearing and exhumation related) at different times, regardless of the time of their initial intrusion and crystallization.
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DISCUSSION
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The results of this study indicate that while the region between the Gaoligong Shan shear zone and Ailao Shan shear zone complex extruded to the southeast, it did not extrude as a single rigid block, but rather it was dismembered into at least two major fragments that separated two tectonic units, the Baoshan to the west and the Lanping-Simao to the east. These new structural and geochronological data permit us to interpret several important tectonic aspects of the style of early Tertiary deformation in the region east of the Eastern Himalayan Syntaxis:
(1) Although sinistral strike-slip shear has been recognized in the structural development of the Chong Shan shear zone (Wang and Burchfiel, 1997), the role and importance of dextral shearing, which is observed in the Chong Shan shear zone only near the Gaoligong Shan shear zone contact zone, have not been extensively explored (the subject of a later paper on the Gaoligong Shan shear zone). According to our preferred explanation, left-lateral, strike-slip shearing, which decoupled the Baoshan tectonic element from Simao tectonic element, was active during the earlier stages of the extrusion. This left-lateral motion along the Chong Shan shear zone came to an end before ca. 17 Ma. However, the motion along the Gaoligong Shan shear zone and Ailao Shan shear zone continued after 17 Ma, causing the now coupled Baoshan/Simao tectonic elements to continue to extrude to the south. This continued, right-lateral motion along the Gaoligong Shan shear zone, the main western boundary for the extruded Indochina fragment during early Cenozoic time, overprinted the earlier left-lateral structures of the Chong Shan shear zone.
(2) Because the Chong Shan shear zone occurs along a suture zone or follows a volcanic arc, piercing points to determine the amount of strike-slip offset cannot be identified. If the crustal material between the Chong Shan and the Gaoligong Shan shear zones (Baoshan tectonic element) is assumed to have extruded to the south, the space it created behind was closed as the two bounding shear zones "sutured," overprinting shortening structures on the earlier strike-slip faults. Because we interpret the region from which the Indochina fragment extruded as a zone of complex strike-slip faulting, it is not possible with the present state of knowledge to assign a magnitude of SE movement of the extruded crust based on geological data.
(3) The continuation of the Chong Shan shear zone to the north and south remains unclear because detailed mapping has not been done. However, certain inferences can be made based on the information gathered from the 1:200,000 Chinese geological maps of the region and our extensive field excursions (Fig. 16). To the south, the southernmost locality where Chong Shan shear zone mylonitic rocks are observed is near the major bend in the Lancang River at the Wayao section (W in Fig. 16). Here, intensely folded sedimentary rocks of
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ABSTRACT
INTRODUCTION
