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Geometry, kinematics, and regional significance of the Chong Shan shear zone, Eastern Himalayan Syntaxis, Yunnan, China

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

View larger version (114K): [in this window] [in a new window]   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.

View larger version (63K): [in this window] [in a new window]   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.

View larger version (50K): [in this window] [in a new window]   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.

View larger version (39K): [in this window] [in a new window]   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.

View larger version (50K): [in this window] [in a new window]   Figure 4 (continued). GSSZ—Gaoligong Shan shear zone.

View larger version (144K): [in this window] [in a new window]   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.

View larger version (129K): [in this window] [in a new window]   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'.

View larger version (136K): [in this window] [in a new window]   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.

View larger version (71K): [in this window] [in a new window]   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.

View larger version (120K): [in this window] [in a new window]   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.

View larger version (37K): [in this window] [in a new window]   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.

View larger version (29K): [in this window] [in a new window]   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.

View larger version (13K): [in this window] [in a new window]   Figure 12. U-Pb Concordia diagrams for samples (A) 98JL18.4 and (B) 98JU27.1.

View larger version (28K): [in this window] [in a new window]   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.

View larger version (12K): [in this window] [in a new window]   Figure 14. Plateau plots from argon data for micas from granitoid gneiss and leucogranite samples. See text for sample descriptions.

View larger version (13K): [in this window] [in a new window]   Figure 15. Summary chart of all of the U-Pb and 40Ar/39Ar age results, sample numbers, and rock types.

View larger version (59K): [in this window] [in a new window]   Figure 16. Simplified tectonic map of the Three Rivers area. Legend is the same as Figure 2. Cenozoic shear zones (black dashed lines) and the Paleozoic-Mesozoic suture zones (red dashed lines) are highlighted.

View larger version (29K): [in this window] [in a new window]   Figure 17. Maximum elevations from a 20-km–wide SWATH profile based on GTOPO30 data. High elevations are underlain by major mylonitic shear zones. Major rivers and tectonic features are also labeled. Inset shows the location of the profile. GSSZ—Gaoligong Shan shear zone, CSSZ—Chong Shan shear zone, DSSZ—Dianchang Shan shear zone, NB—Namche Barwa, RRF—Red River fault.

View larger version (28K): [in this window] [in a new window]   Figure 18. Schematic structural interpretation of relations between the Gaoligong Shan shear zone, Sibumasu, Chong Shan shear zone, and the Lanping-Simao structural units also shows the structures and the morphological features that bound the Chong Shan shear zone and have played an important role in its exhumational history. The Chong Shan shear zone is bounded to the west by a normal fault, and to the east, by a thrust fault and a major river, the Mekong/Lancangjiang (blue), which follow this boundary very closely. Contractional structures observed on either side of the shear zone indicate that both pure shear and simple shear likely have affected the Chong Shan shear zone rocks at the same time. We think that vertical stretching was also one of the agents that contributed to the exhumation of the Chong Shan shear zone (inset block diagram).