Structural Evolution of the Yushu-Nangqian Region and Its Relationship to Syncollisional Igneous Activity, East-Central Tibet

Structural evolution of the Yushu-Nangqian region and its relationship to syncollisional igneous activity, east-central Tibet

Matthew S. Spurlin† An Yin‡ Department of Earth and Space Sciences, and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567, USA Brian K. Horton Department of Earth and Space Sciences, University of California, Los Angeles, California 90095-1567, USA Jiangyu Zhou Jianghai Wang Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, People’s Republic of China

ABSTRACT INTRODUCTION removal of Tibetan mantle lithosphere may have raised the plateau over a couple of million years Field mapping, geochronological analy- The formation of the Tibetan Plateau has from an elevation of 3.5–4 km to its current and ses, and cross section construction reveal been attributed to continent-continent collision possibly maximum elevation of ~5 km (England a protracted deformation history and a since ca. 70–50 Ma (e.g., Dewey and Burke, and Houseman, 1989; Harrison et al., 1992; minimum of 61 km of Cenozoic NE-SW 1973; Molnar and Tapponnier, 1975; Patriat and Molnar et al., 1993). In contrast to the Cenozoic shortening (in present coordinates) across Achache, 1984; Besse et al., 1984; Dewey et al., models, the role of Mesozoic crustal shortening, the Yushu-Nangqian thrust belt in north- 1988, 1989; Le Pichon et al., 1992; Klootwijk et either due to construction of an Andean-type mar- ern Tibet. Cenozoic contraction started al., 1992, 1994; Beck et al., 1995; Rowley, 1996; gin (England and Searle, 1986) or pre-Cenozoic prior to 51 Ma and was followed fi rst by Yin and Harrison, 2000; Tapponnier et al., 2001; collision (Arne et al., 1997; Murphy et al., 1997; northwest-striking right-slip faulting and DeCelles et al., 2002). Because of the important Worley et al., 1997; Hildebrand et al., 2001), has later by northwest-striking left-slip faulting. implications for continental dynamics and global also been emphasized. Renewed NE-SW contraction is expressed climate change, mechanisms for Tibetan uplift Growth of the Tibetan Plateau has long been by folding of Neogene strata and thrusting, and growth have been hotly debated. Models considered to propagate northward (e.g., Burch- which again was followed by northwest- emphasizing the role of contraction during Ceno- fi el and Royden, 1991; Meyer et al., 1998). But striking left-slip faults. Late Neogene defor- zoic Indo-Asian collision include: (1) distributed this conclusion is mainly based on studies in the mation is expressed by local north-striking crustal shortening (Dewey and Burke, 1973) or northernmost part of the plateau and has not normal faults. Shortening across the Yushu- vertically uniform contraction of the Tibetan been tested for the central and southern plateau. Nangqian belt appears to be accommodated lithosphere (England and Houseman, 1986; Related to this issue is whether syncollisional by thin-skinned thrusting, which raises Dewey et al., 1988), (2) northward underthrust- igneous activity in the plateau was related to the question of how the deformation was ing of either the whole Indian crust (Argand, continental subduction (Deng, 1989; Tappon- accommodated in the lower crustal levels. 1924; Powell and Conaghan, 1973; Ni and Bara- nier et al., 1990, 2001; Meyer et al., 1998; To resolve this problem, we perform geo- zangi, 1984) or only its lower portion (Owens Roger et al., 2000; Wang et al., 2001), convec- chemical analysis of igneous rocks dated as and Zandt, 1997; DeCelles et al., 2002), with tive removal of mantle lithosphere (England 51–49 and 38–37 Ma. The rocks exhibit geo- possible assistance from southward underthrust- and Houseman, 1989; Molnar et al., 1993), or chemical signatures characteristic of sub- ing of Asia (Powell and Conaghan, 1973; Willett rollback of the subducted Indian slab (DeCelles duction, which implies that coeval crustal and Beaumont, 1994) or intraplateau continental et al., 2002; Ding et al., 2003). The continental- thickening in northeastern Tibet was most subduction (Meyer et al., 1998; Yin and Harrison, subduction model predicts coeval syncollisional likely induced by continental subduction. 2000; Wang et al., 2001), (3) injection of the igneous activity and crustal shortening, the Indian continent into Tibetan ductile lower crust convective-removal model predicts coeval igne- Keywords: Asia, China, Tibetan Plateau, (Zhao and Morgan, 1987; Royden, 1996; Royden ous activity and extension, and fi nally, the slab thrust belts, strike-slip faults, syncollisional et al., 1997; Beaumont et al., 2001), and (4) dis- rollback model predicts southward younging of magmatism. tributed crustal shortening coupled with discrete igneous activity. The distinctions of the models left-slip faulting and subduction of continental require that fi eld tests be conducted in regions mantle lithosphere (Mattauer, 1986; Meyer et al., where temporal relationships between Cenozoic 1998; Roger et al., 2000; Tapponnier et al., 2001). deformation and igneous activities can be estab- †E-mail: [email protected]. The thermal effect in the mantle for Tibetan uplift lished. The Fenghuo Shan–Nangqian thrust ‡E-mail: [email protected]. has also been considered. Specifi cally, convective belt in central Tibet (Fig. 1) is ideal because

GSA Bulletin; September/October 2005; v. 117; no. 9/10; p. 1293–1317; doi: 10.1130/B25572.1; 15 ﬁ gures; 4 tables; Data Repository item 2005149.

80° E 90° E 100° E

Gobi-Alashan Pa 200 km mir Basin 38° N s NSTB ATF Qaida Tarim Basin m Basin

KF JS KFS 34° N FSTB Fig. 10 SONGPAN- BNS QIANGTANG GYF GANZI Amdo Fig. 9 X XF B STB SGA thrust LF L Tertiary Quaternary Fig. 2 sedimentary basin system 30° N LHASA JF Tertiary sedimentary Lhasa Gangdese Sichuan basin thrust S Basin Him IT Paleogeneigneous ala TTB rocks (60–30Ma) yan Renbu-Zedong fold SOUTH -thru thrust st belt Thrust fault H CHINA ima lay 26° N Strike-slipfault an A Fore S land B R Suture asin GF R INDIA

Figure 1. Simplifi ed tectonic map of the Tibetan Plateau. Shown are sutures (JS—Jinsha suture; BNS—Banggong-Nujiang suture; ITS— Indus Tsangpo suture), major faults (KF—Karakorum fault; ATF—Altyn Tagh fault; KFS—Kunlun fault system; GYF—Ganzi-Yushu fault; XXF—Xianshuihe-Xiaojiang fault system; LSTB—Longmen Shan thrust belt; ASRR—Ailao Shan–Red River shear zone; GF— Gaoligong fault; JF—Jiali fault: BLF—Batang-Lijiang fault system), general locations of the fi ve major Cenozoic thrust belts within the Himalaya-Tibet orogen (TTB—Tethyan thrust belt; Gangdese and Renbu-Zedong thrusts; SGA—Shiquanhe-Gaize-Amdo thrust system; FSTB—Fenghuo Shan thrust belt; NSTB—Nan Shan thrust belt), Tertiary-Quaternary sedimentary basins within and along the periphery of the Tibetan Plateau, and the distribution of Paleogene igneous rocks in northern Tibet (modifi ed from Yin and Harrison, 2000).

it preserves both syncollisional structures and the Ganzi-Yushu fault lie Triassic strata locally Group (T3jz). Triassic strata decrease in thick- igneous rocks (Coward et al., 1988; Leeder et overlain by Tertiary sediments that are possibly ness from ~4400 m in the Nangqian area in the al., 1988; Pan et al., 1990; Zhang and Zheng, bounded by thrusts as the northern extension of south to ~3200 m in the Shanglaxiu area in the 1994; Yin and Nie, 1996; Chung et al., 1998; the Yushu-Nangqian thrust belt (Qinghai BGMR, north (Fig. 2) (Qinghai BGMR, 1983a, 1983b, Yin and Harrison, 2000; Liu and Wang, 2001; 1991; Fig. 2). The Ganzi-Yushu fault is an en 1991), which may be the cause for the later Horton et al., 2002; Liu et al., 2003). Below echelon left step-over of the ~1400-km-long development of the Cenozoic Yushu-Nangqian we report our new fi eld observations from this Xianshuihe-Xiaojiang fault system in eastern thrust wedge (Fig. 3A).1 thrust belt in three areas: the Yushu-Nangqian Tibet (Fig. 1) (Ding, 1984; Allen et al., 1991; We observe no direct depositional contact traverse, the Dongba area, and the Zhaduo area Wang et al., 1998; Wang and Burchfi el, 2000; Xu between the Carboniferous and Triassic strata in (Fig. 1). Results of our geochronological and and Kamp, 2000), which has a total left slip of the study area. Whether a major unconformity geochemical analyses are also documented. 78–100 km and may have been active for at least exists between the two units omitting the whole the past 4 m.y. (Wang and Burchfi el, 2000). Permian unit is not clear. Because Permian GEOLOGY OF THE YUSHU-NANGQIAN strata lie unconformably over a Carbonifer- AREA Stratigraphy ous sequence ~130 km west of Nangqian in the Zhaduo area (Qinghai BGMR, 1991), we An ~115-km-long, north-south traverse was Major stratigraphic units exposed along assume that the Permian strata lie beneath the mapped between Yushu and Nangqian (Fig. 2).1 the Yushu-Nangqian traverse are Carbonifer- Nangqian out-of-sequence fault zone (Fig. 3A). The area is bounded to the north by the active ous-Triassic marine carbonates and minor The Cenozoic strata unconformably overlie left-slip Ganzi-Yushu fault (Allen et al., 1984; clastic units overlain by Jurassic, Cretaceous, Carboniferous-Triassic strata and may be in Wang and Burchfi el, 2000) and to the south by and Paleogene red beds (Liu, 1988; Qinghai conformable contact with underlying Upper the Cenozoic Nangqian basin (Fig. 2). North of BGMR, 1991) (Fig. 2). The southern map area Cretaceous strata (Qinghai BGMR, 1983a, consists mostly of the Carboniferous Zhaduo 1983b, 1991). They were deposited in four

1 Figures 2 and 3 are on a separate sheet accompa- Group (C1zd), whereas the northern map area is distinctive Paleocene-Eocene basins (i.e., Shan- nying this issue. dominated by the younger Upper Triassic Jieza glaxiu, Xialaxiu, Niuguoda, and Nangqian)

1294 Geological Society of America Bulletin, September/October 2005 PALEOCENE-EOCENE DISTRIBUTED SHORTENING IN EAST-CENTRAL TIBET

(Fig. 2) during Paleogene contraction (Horton Yushu-Nangqian traverse, based on restoration strain. These folds are probably Cenozoic in age et al., 2002). of broad pre-Cenozoic folds. because they are parallel to footwall Cenozoic folds and have vergence consistent with north- Structural Geology Shanglaxiu Thrust System eastward thrusting. The Shanglaxiu thrust system consists of Pre-Cenozoic versus Cenozoic Deformation (1) the southwest-dipping Shanglaxiu fault, (2) a Shanglaxiu Back Thrust Pre-Cenozoic deformational history may be 5–8-km-wide zone of folds in the Shanglaxiu The Shanglaxiu back thrust places older 1 2 established by examining unconformable rela- basin in the footwall of the Shanglaxiu fault, Triassic units (T3jz , T3jz ) over younger Trias- 3 4 tionships between Cenozoic and pre-Cenozoic and (3) several low-angle north-and-south–dip- sic units (T3jz , T3jz ) (Figs. 3 and 4), with a strata. The angular discordance between Ceno- ping thrusts located between the Shanglaxiu maximum stratigraphic separation of ~4 km. zoic and underlying Paleozoic-Triassic strata basin to the south and the Ganzi-Yushu fault to This fault dips at an angle <35° to the north- is most prominent in the northernmost end of the north (Figs. 2, 3A, 4, 5A, and 5B). The trace east and is locally overlain by Lower Eocene the mapped area against the Ganzi-Yushu fault, of the Shanglaxiu fault is overlain by Lower volcanic ﬂ ows dated as 51–49 Ma (Roger et where lower Tertiary strata and underlying Tri- Eocene strata, which are themselves deformed al., 2000; this study, see following) (Fig. 2). The assic beds have opposite dip directions (Fig. 2). by northwest-trending folds coaxial with earlier volcanics overlie the Shanglaxiu back thrust and The discordance is also displayed along the folds in the hanging wall and footwall of the are deformed by northwest-trending folds. northern edge of the Shanglaxiu basin, where thrust (Figs. 2 and 4). Fault striations on the main fault surface pre-Cenozoic strata dip 10°–40° more steeply The Shanglaxiu fault dips 20°–30° to the (fault 15 in Fig. 6) and asymmetric folds in the than the overlying Cenozoic strata. Despite south and juxtaposes a Triassic carbonate fault zone indicate SW-directed motion. The 2 the difference in dip angles, the two sequences sequence (T3jz ) over Paleocene–Lower Eocene back thrust is parallel to a zone of tight folds 1 generally share similar strikes (Fig. 2). The strata (ES ). Due to the reconnaissance nature of in Triassic strata in both the hanging wall and discordance between Cenozoic strata and our mapping, we did not examine the kinemat- footwall. In the footwall, these folds are locally underlying Triassic beds in the Xialaxiu basin ics of the Shanglaxiu fault zone in detail. Nev- overlain by gently folded Tertiary strata, which is less obvious—dips of the two sequences ertheless, we interpret the fault to be a major are in turn cut by a southwest-dipping fault above and below the unconformity are quite northeast-directed thrust based on: (1) the con- preserving subhorizontal striations (fault 10 in similar (Fig. 2). Variable discordant relation- sistent older-over-younger relationship (Fig. 4), Fig. 6). ships between Jurassic and Cenozoic strata and (2) a parallel relationship between the fault and between Carboniferous and Cenozoic strata are northeast-verging and locally overturned folds Xialaxiu Thrust also observed along the southern margin of the exposed immediately above and below the The Xialaxiu fault is folded and roots to the 1 2 Nangqian basin. fault (Fig. 4), (3) its low-angle geometry, and south. It juxtaposes Triassic strata (T3jz , T3jz ) We found no clear evidence for the existence (4) locally preserved growth-strata relationship over Paleogene deposits of the Xialaxiu basin 2 of major pre-Cenozoic faults across the region. between the Shanglaxiu fault and its footwall (EX) and Triassic unit T3jz (Fig. 2). We have The only possible candidate for pre-Cenozoic folds (Horton et al., 2002). no constraints on the thrust transport direction, faults is the Jiangda thrust exposed ~15 km North- and south-dipping thrusts north of the but based on its similar strike to the Shanglaxiu west of Nangqian (Fig. 2). The fault juxtaposes Shanglaxiu thrust juxtapose the lowest Triassic thrust, an older-over-younger relationship, 1 2 an older Carboniferous unit over a younger units (T3jz sandstone and T3jz carbonate) over parallelism between the fault and footwall Carboniferous unit and itself is overlain by equivalent or younger Triassic strata. They also folds, and the low-angle geometry, we suggest the uppermost part of the Paleocene-Eocene thrust Triassic strata over the distal facies of the that this is also a northeast-directed thrust. In Nangqian basin strata (Fig. 2), suggesting Paleogene Shanglaxiu basin (Figs. 2 and 3A). cross-sectional view, we interpret the fault to be 1 motion between the Carboniferous and Eocene. Fault striation on one thrust trends southwest a hanging-wall ﬂ at following the base of T3jz However, even for this fault the evidence for (fault 18 in Figs. 2 and 6). sandstone (Fig. 3A). In map view, the fault cuts major motion in the Paleocene is strong, as In map view the Shanglaxiu thrust makes an upsection in hanging-wall strata laterally to the indicated by the distribution of proximal-facies abrupt change in strike of ~70° from a northwest southeast. sedimentation and a growth-strata relationship trend to nearly north-south direction (Figs. 2 and The hanging wall and footwall of the Xia- with folds against the fault (Horton et al., 2002). 4). Correspondingly, the footwall folds make a laxiu thrust are folded together (Fig. 2), which Because of the lack of evidence for major pre- similar turn in the trend of fold axes. This map may be attributed to motion on a minor north- Cenozoic faults in the region, we assume that the pattern may result from oroclinal bending under east-directed thrust (Figs. 2 and 3A). Minor, Cenozoic strata were unconformably deposited distributed right-slip shear across a broad (>10- northeast-directed thrusts are present a few on top of broadly folded Carboniferous-Triassic km-wide) northwest-trending deformation zone. kilometers north of the northern margin of the strata (Fig. 3A). The pre-Cenozoic deformation The inferred oroclinal bending may explain the Xialaxiu basin, ~4 km south of the Shanglaxiu along the northern margin of the Qiangtang ter- peculiar observation that fault striations along back thrust (Fig. 2). rane may have been related to Late Triassic clo- the north-striking segment of the Shanglaxiu sure of the Songpan-Ganzi remnant ocean (Yin fault trend westward (Fig. 4) (see fault 11 in Niuguoda Thrust and Nie, 1993, 1993; Nie et al., 1994; S¸engör Figs. 2 and 6). The northeast-dipping Niuguoda fault jux- 2 and Natal’in, 1996; Zhou and Graham, 1996), The Triassic strata in the Shanglaxiu hanging taposes a Triassic unit (T3jz ) over Paleogene because the Cenozoic and Jurassic-Cretaceous wall are folded, with a wavelength of 1–1.5 km deposits of the Niuguoda basin (ENU) and 3 strata are generally conformable in the area. In and amplitude of several hundred meters Triassic unit T3jz (Figs. 2 and 3A). Similar to any case, pre-Cenozoic folding appears to have (Figs. 2 and 4). The northeast-verging folds the thrusts described above, this fault is also accommodated rather minor NE-SW shorten- close to the Shanglaxiu thrust are locally over- folded producing an irregular map pattern. At ing, perhaps no more than 2–3 km along the turned and accommodated ~50% of shortening one location, the fault exhibits north-trending

Geological Society of America Bulletin, September/October 2005 1295

SPURLIN et al. Figure 4. Geologic map of the Shanglaxiu area (see Fig. 2 for regional location). Location of photos (Fig. 5A–B) also shown. location). Location of photos (Fig. regional 2 for (see Fig. 4. Geologic map of the Shanglaxiu area Figure

1296 Geological Society of America Bulletin, September/October 2005 PALEOCENE-EOCENE DISTRIBUTED SHORTENING IN EAST-CENTRAL TIBET ) in the hanging wall of 2 jz 3 nd lacustrine limestone of the Niuguoda graph and (B) line drawing of the north- graph and (B) ly folded as a result of slip on the Niuguoda as a result ly folded ic carbonate (T e east-northeast. See Figure e east-northeast. See Figure photo location. 4 for Figure 5. Paleocene-Eocene thrust structures within the northern and central portions of the Yushu-Nangqian traverse. (A) Photo (A) traverse. Yushu-Nangqian and central portions of the within the northern thrust structures 5. Paleocene-Eocene Figure Triass Shanglaxiu thrust and associated syncontractional deposits of the basin. Note truncation east-directed Shanglaxiu thrust and a well-developed footwall syncline (with partially overturned strata) associated with thrust motion to th syncline (with partially overturned footwall Shanglaxiu thrust and a well-developed a mudrock consist of intercalated rocks Niuguoda thrust. Footwall line drawing of the southwest-directed Photograph and (D) (C) photo location. 2 for thrust. See Figure basin. A local synorogenic unconformity attests to the syncontractional nature of the underlying footwall rocks, which are high which are rocks, of the underlying footwall attests to the syncontractional nature unconformity A local synorogenic basin.

Geological Society of America Bulletin, September/October 2005 1297 SPURLIN et al. striations along its north-striking segment (fault 12 in Fig. 6). This observation together with the Fault-B system in the Nangqian fault zone Fault-C system older-over-younger relationship indicates that Fault-C system in the Nangqian in the Nangqian fault zone the fault transport direction was southwestward fault zone in present coordinates. Fault-C system in the Nangqian fault zone Nangqian Fault Zone The northwest-trending Nangqian fault zone is a composite structural assemblage formed fault 1 fault 2 fault 3 fault 4 during several phases of deformation. For the convenience of description, we break the fault Fault-B system zone into three small fault systems (Figs. 2 Fault-B system in the Nangqian in the Nangqian fault zone. and 7), each of which in turn consists of small Fault-C system fault zone in the Nangqian splays and complex arrays that may have differ- fault zone ent kinematics and may have been formed at different times. Proposed ages of fault activity are based on assigned stratigraphic ages (Qing- hai BGMR, 1983a). Where possible, we utilize fault 5 fault 6 fault 7 fault 8 our new age determinations on igneous rocks. However, timing along the Fault-A and Fault-B A minor fault systems is based on age assignment of Upper Fault-B in the north of Xialaxiu A north-striking Nangqian A north-striking segment 3 basin folded segment of Eocene unit E by Qinghai BGMR (1983a), for fault zone of the folded Shanglaxiu the Niuguoda N thrust thrust which we do not have independent veriﬁ cation. Fault-A System. Fault-A in the northwestern end of the fault zone strikes east and dips 40°S. Fault striations plunge gently (17°) to the west, indicating dominant strike-slip motion fault 9 fault 10 fault 11 fault 12 and a minor normal-slip component (Fig. 7). The fault places younger Upper Eocene strata Left-slip (E 3) over older Triassic units (Fig. 7). Although Fault-C system fault in N in the Nangqian Dongba area 3 fault zone the Fault-A system cuts EN , the Eocene unit directly against the fault exhibits growth-strata Main trace of Fault-A in the Shanglaxiu relationship, suggesting that motion along the the Nangqian fault back thrust fault and deposition of the unit were related. To system the southeast, this fault splits into two strands. 3 The northern strand cuts unit EN , whereas the fault 13 fault 14 fault 15 fault 16 3 southern strand dies out into a fold in unit EN . The southern strand is high-angle and preserves mostly subhorizontal striations (fault 14 in 3 Footwall Fig. 6). The offset pattern of unit E (Fig. 7) Northern of Jiangda N Shanglaxiu thrust window Dongba thrust system normal fault indicates the Fault-A system to be right-slip Fault-C system in the Nangqian with total slip <2 km. fault zone Fault-B System. The northwestern end of the Fault-B system immediately south of the Fault- A system consists of a single strand striking east. The fault dips north and places a younger fault 17 fault 18 fault 19 fault 20 2 Triassic unit (T3jz ) over older Carboniferous strata, omitting several younger Carboniferous Small transfer fault 1 below the Jiangda units and the basal Triassic unit T3jz (Fig. 2). thrust Fault-C in the To the southeast the fault changes strike to the Nangqian fault zone northwest and is overlain by a folded Upper Footwall 3 of Fault-B Eocene unit EN (Fig. 7), suggesting that it had in Nangqian ceased motion by late Eocene time. The Fault-B fault zone system is older than the Fault-A system, because 3 fault 21 fault 22 fault 23 the latter cuts EN . To the southeast Fault-B splits into two splays. The northern branch is nearly vertical and exhib- Figure 6. Fault kinematic data. See Figure 2 for location and text for details. Note that some its a sharp contact between Tertiary red beds to striations are not exactly on the corresponding fault surfaces. This is due to the fact that the northeast and Triassic carbonate in the south. fault surfaces are highly irregular in places and only average measurements are used to The fault dies out into the Triassic strata. The represent fault attitudes.

1298 Geological Society of America Bulletin, September/October 2005 PALEOCENE-EOCENE DISTRIBUTED SHORTENING IN EAST-CENTRAL TIBET Figure 7. Geologic map of the Nangqian area (inset from Fig. 2). Location of photos (Fig. 8) also shown. 2). Location of photos (Fig. Fig. (inset from 7. Geologic map of the Nangqian area Figure

Geological Society of America Bulletin, September/October 2005 1299 SPURLIN et al. fault zone is observed at locations of fault 1 and Fault-C System. The northwest-striking along the northern fault branch of the Fault-B fault 8 (Figs. 2 and 6). Fault 8 is east-dipping Fault-C system in the southwestern part of the system that produced en echelon folds and and preserves both nearly down-dip east-trend- Nangqian fault zone consists of several nearly east-trending folds at the southwest fault termi- ing lineation and subhorizontal north-trending vertical fault branches striking northwest nation, (3) left-slip faulting along the southern lineation. Fault 1 exhibits subhorizontal stria- (Fig. 8). They mainly follow the course of the fault branch of the Fault-B system that locally tion, and Tertiary beds within 1–3 m from the Lacang River. For this reason we also refer to reactivated the northern fault branch of the fault contact are consistently offset left laterally this fault system as the Lacang fault system. The Fault-B system, produced north-trending folds for ~20–50 cm, suggesting left-slip motion on main fault strand juxtaposes Paleogene red beds at its southwestern termination and deformed fault 1. Our local observation of fault kinemat- to the northeast and highly folded Triassic strata the older northwest-trending folds, (4) depo- 3 ics appears to be incompatible with the regional to the southwest. Its strike changes southeast- sition of Upper Eocene unit EN , which was map pattern for the northern fault strand, which ward from northwest to north. In contrast to the subsequently folded due to NE-SW contraction, seems to indicate right-lateral motion for the parallel fault-fold relationship along the Fault-B and (5) development of the Fault-A system that 1 3 fault. First, Tertiary unit EN is offset for ~1.5 km system, the Fault-C system cuts and offsets sev- cuts EN . The proposed sequence of deformation right laterally. Second, an en echelon fold pat- eral northwest-trending folds, which may have implies two distinctive phases of right-slip fault- 1 tern in EN directly against the fault is consistent formed during the development of the Fault-B ing and two phases of NE-SW contraction. with right-slip shear. Third, the fault terminates system. The Fault-C system also cuts the 37 Ma into a series of east-trending folds at its south- Nangqian igneous complex (Fig. 8). Striations Jiangda Thrust eastern end, a relationship requiring the fault measured in the Fault-C system (faults 2, 3, 4, The Jiangda thrust juxtaposes an older 2 to be right-slip. The contradictory observations 5, 6, 13, 19, and 23 in Fig. 6) are all subhori- Carboniferous unit (C1zd ) over a younger 3 may be explained by the fault zone experiencing zontal, indicating dominant strike-slip motion. Carboniferous unit (C1zd ) (Figs. 2 and 8). A both left-slip and right-slip motion. Perhaps the One fault branch in the Fault-C system offsets folded thrust window juxtaposing the same right-slip motion is more dominant, producing the unconformity between Carboniferous and units is exposed ~3 km southwest of the frontal the map-scale structures and followed by minor Paleogene strata right laterally for 120–130 m trace of the Jiangda thrust, where the fault zone and local left-slip reactivation. Alternatively, (Fig. 8C and 8D). Several vertical northwest- consists of a 40-m-thick cataclastic zone. The folds adjacent to and at the termination of the striking faults directly north of Nangqian can frontal trace of the thrust is overlain to the south 1 fault could be unrelated to faulting. In this be traced into east-striking low-angle thrusts, by Nangqian basin strata (EN ) (Fig. 7), which 1 case, Tertiary unit EN was deposited during indicating right-slip motion on the vertical faults constrains the thrust to predate the late Eocene. left-slip faulting and the apparent right-lateral (Fig. 7). However, left-slip fault motion has also Sedimentologic investigations along the south- 1 offset pattern of EN is due to a complex original been observed. For example, fault 19 and fault 5 ern margin of the Nangqian basin suggest that distribution of individual localized basins. This are parts of a major vertical fault striking north- the Jiangda thrust was active during the early latter hypothesis can be tested by further detailed west. They juxtapose Tertiary strata against stage of basin development in the Paleocene sedimentological studies. Carboniferous strata. Near the fault 19 location, (Horton et al., 2002). The Jiangda thrust may The southern branch of the Fault-B system minor faults in the fault zone are north-strik- reappear again ~15 km due south of Nangqian, dips moderately to steeply (50°–70°) northeast- ing and preserve subhorizontal striations. Drag where a southwest-dipping thrust juxtaposes 2 ward and juxtaposes Carboniferous units over folds and offset of beds by small faults next to Carboniferous unit C1zd over Jurassic and Cre- 1 Paleogene strata (EN in Fig. 2). Imbricate faults the main fault zone all indicate left-slip kinemat- taceous strata (Fig. 2). in its footwall cut 37 Ma Eocene volcanic fl ows ics. The preceding observations suggest that the Folds and thrusts involving Carboniferous 2 (EN ) (Fig. 2). Tight folds involving Tertiary Fault-C system may have also experienced two strata are also present between the Jiangda strata are directly below the main trace of the phases of strike-slip faulting. However, it is not thrust in the south and the Nangqian fault zone southern branch of Fault-B and parallel to the clear if the left-slip faulting is older or younger in the north (Fig. 2). A broad northwest-trend- 4 fault (Fig. 7). Small thrusts are also preserved than right-slip faulting in the fault system. ing antiform defi ned by unit C1zd is located in the fold zone, such as fault 7 (Figs. 2 and Summary. The preceding observations 7 km northwest of Nangqian (Fig. 2). This fold 6); striations on these faults dominantly trend suggest a complex deformation history. In may be associated with blind thrusting at depth northeast, consistent with NE-SW contraction. particular, the Fault-B system may have been (Fig. 3A). Directly north of Nangqian, Carbon- 4 3 In contrast, gentle (13°) to moderate (45°) reactivated several times in the past. The close iferous units (C1zd , C1zd ) are thrust along two north-plunging striations are observed on the association between the southern fault strand in south-dipping imbricate thrusts over lacustrine 1 main trace of the southern fault branch (Fig. 7), the Fault-B system with parallel folds and its deposits of the Nangqian basin (EN ) and a local indicating the fault has a signifi cant strike-slip consistent north dip suggest that it may have apron of Paleogene conglomerate related to 3 component. Fault 9 (Figs. 2 and 6) in the main originated as a thrust. Contradictory kinematics thrusting (EN ). southern fault zone could be a minor conju- suggest that this fault zone has experienced both Kinematic data on the Jiangda faults are gate fault. The sense of slip on the southern right-slip and left-slip faulting, with the former sparse. One small left-slip high-angle fault is fault strand is unclear. However, based on the occurring fi rst, possibly coeval with NE-SW observed directly below the Jiangda thrust (fault regional map pattern, we suggest that it is left- contraction. The crosscutting relationship with 22 in Fig. 6). If this fault is a transfer structure 3 slip. This is because the fault terminates in the respect to unit EN also helps bracket deforma- in a contractional system, it implies that the southeast into a series of north-trending folds in tional events. Based on the fi eld observations Jiangda thrust is a northeast-directed structure. the Paleogene Nangqian basin. The north-trend- and preceding discussion, a possible sequence In the Jiangda thrust window to the southwest, ing folds caused northward concave bending of of deformation in the Nangqian fault zone is two mesoscopic high-angle faults lie below the a prominent NW-trending anticline, suggesting proposed: (1) NE-SW contraction that produced low-angle Jiangda thrust. They preserve subho- that left-slip motion along the southern fault northwest-trending folds in the Nangqian basin, rizontal striations and right-slip-sense kinematic branch postdates the NE-SW contraction. (2) coeval or slightly younger right-slip faulting indicators. It is not clear how these high-angle

1300 Geological Society of America Bulletin, September/October 2005 PALEOCENE-EOCENE DISTRIBUTED SHORTENING IN EAST-CENTRAL TIBET ne rocks in the footwall of the Nangqian in the footwall ne rocks uth-dipping Tertiary unconformity between unconformity Tertiary uth-dipping drawing of the southwest-directed Nangqian drawing of the southwest-directed his photo during a younger episode of south- his photo during a younger Figure 8. Paleocene-Eocene fault structures in the Nangqian area. See Figure 7 for photo locations. (A) Photograph and (B) line 7 for See Figure in the Nangqian area. fault structures 8. Paleocene-Eocene Figure of the Carboniferous-Paleoge Folding of the Nangqian area. margin along the northeastern thrust, a major basin-bounding feature thrust resulted from footwall imbrication and surface-rupture of an additional thrust located ~1 km to west (to the right) of t of an additional thrust located ~1 imbrication and surface-rupture footwall from thrust resulted west-directed thrusting. (C) Photograph and (D) line drawing of right-lateral offset an upper middle Eocene intrusion and so Photograph and (D) (C) thrusting. west-directed Nangqian basin. clastic strata of the lower red Paleogene strata and overlying underlying Carboniferous

Geological Society of America Bulletin, September/October 2005 1301 SPURLIN et al. faults below were related to motion on the low- over strata that are mapped as Neogene in age The maximum total stratigraphic throw across angle Jiangda thrust. If the high-angle faults are (Qinghai BGMR, 1988). Neogene strata in the the Dongba thrust is ~4–5 km, assuming a small transfer structures, the thrust transport Dongba basin generally dip homoclinally to the continuous Carboniferous to Triassic section direction for the south-dipping Jiangda fault is north. However, as they approach the Dongba directly above the thrust. If we further assume to the south. thrust, the strata become folded and form a series that the Dongba thrust dips ~30°, its maximum of short-wavelength (<1 km) synclines and slip is at least 8–10 km. Taking the Neogene age GEOLOGY OF THE DONGBA AREA anticlines parallel to the thrust. In the northern assignment by Qinghai BGM (1988) at face Dongba area, Neogene strata rest directly on top value, the ﬁ eld relationships imply signiﬁ cant We conducted reconnaissance mapping in the of Carboniferous strata, missing Permian and contractional deformation in the Yushu-Nang- Dongba area to examine the lateral continuity of Triassic rocks. Because the age of this uncon- qian thrust belt that occurred either episodically the Yushu-Nangqian thrust belt (Figs. 1 and 9). formity is unknown, it is not clear whether the or continuously from the Paleogene to Neogene The area exposes the north-dipping Cenozoic Permian and Triassic strata were eroded away over a time span of perhaps more than 30 m.y. Dongba thrust that puts Carboniferous units in the Mesozoic or during Cenozoic thrusting.

Figure 9. Geologic map of the Dongba area.

1302 Geological Society of America Bulletin, September/October 2005 PALEOCENE-EOCENE DISTRIBUTED SHORTENING IN EAST-CENTRAL TIBET

A prominent left-slip fault occurs along the total fusion age. A block of glacial fl oat presum- by a pipette system), abundance sensitivity, and southern margin of the Dongba basin (Fig. 9). ably derived from the Dadaomayang granite radioactive decay. Correction of 40Ar/39Ar ratios Striations on minor faults parallel to the main ~15 km east of the Xialaxiu volcanic fi eld was for nuclear interferences and atmospheric argon, fault surface are subhorizontal and left-slip also dated by Roger et al. (2000), which yields a and calculation of apparent ages were carried (fault 16 in Fig. 6). Because it lies entirely in U-Pb age of 48.5 ± 1.0 Ma and an Ar/Ar biotite out as described in McDougall and Harrison the Jurassic strata, we are uncertain whether total fusion age of 48.6 ± 1.1 Ma. Our mapping (1999) using conventional decay constants and this fault is a Cenozoic or Mesozoic structure. shows that the Dadaomayang granite truncates isotopic abundances. A summary of 40Ar/39Ar Immediately west of the town of Dongba, we a Tertiary thrust (Fig. 2). In the Nangqian area ages is given in Table 1; reported ages are at the observe a west-dipping high-angle normal fault (Fig. 7), volcanic rocks unconformably overlie 1σ uncertainty level. that cuts the Dongba Neogene strata (Fig. 9; and are interbedded locally with the uppermost fault 17 in Fig. 6). This fault appears to merge levels of the Nangqian basin strata (Horton et U-Pb Methods with the left-slip fault to the south. However, we al., 2002). Intrusive rocks crosscut Cenozoic are uncertain whether the two faults join each strata in the Nangqian basin zone (Fig. 7). One volcanic sample [MS-99–4-18-(1)] from other or crosscut one another. the Nangqian basin was dated using the UCLA GEOCHRONOLOGY CAMECA IMS 1270 ion microprobe. The zir- GEOLOGY OF ZHADOU AREA cons were mounted with a standard in epoxy, 40Ar/ 39Ar Methods polished to 0.25 µm with diamond paste, and The Zhaduo area ~130 km west of Nangqian coated with 100 Å of Au for analysis. Analyses (Figs. 1 and 10) consists of a series of northwest- Sixteen volcanic and plutonic samples were utilized a 4–8 nA primary O– beam focused to striking thrusts that dip both to the northeast and analyzed at the University of California–Los an ~20 µm spot. Of the mounted zircon grains, southwest. Tertiary strata rest unconformably on Angeles (UCLA) using 40Ar/39Ar thermochro- three were analyzed. A single U-Pb age was top of Cretaceous and Permian strata. Tight folds nology. Hand-selected biotite (~1–10 mg) and determined by comparison with a working are present in the Carboniferous units above or whole-rock aliquots (~18–23 mg) were wrapped curve defi ned by measurement of zircon stan- below the major thrusts. It is not clear whether in Cu foil and packed along with Al-wrapped dard AS-3, which yields concordant 206Pb/238U these folds formed during the Cenozoic or Meso- Fish Canyon sanidine (FCT-1: 27.8 ± 0.3 Ma) and 207Pb/235U ages of 1099.1 ± 0.5 Ma by the zoic. The style of deformation in the Zhaduo area fl ux monitors in 6 mm ID quartz tubes that were U-Pb conventional method (Paces and Miller, generally mimics that along the Yushu-Nangqian evacuated and sealed. Samples were irradiated 1993). Additional details of analytical proce- traverse, indicating that the observed thrust belt at the University of Michigan’s Ford reactor dures can be found in Quidelleur et al. (1997) is laterally continuous for at least 130 km west of (L67 position). McDougall and Harrison (1999) and Dalrymple et al. (1999). the Yushu-Nangqian region. provide more information regarding this facility The Zhaduo area preserves the best evidence and 40Ar/39Ar irradiation procedures. Correction Results for Cenozoic out-of-sequence thrusting. In the factors for reactor-produced K- and Ca-derived northeastern corner of the mapped area, a north- argon were determined by measuring K2SO4 Xialaxiu Volcanics dipping thrust places a younger Triassic unit and CaF2 salts included with each irradiation. A rhyolite and a small granitic body found 2 2 40 39 (T3jz ) over a Permian unit (P1z ). The same fault The irradiation history, data of Ar/ Ar analy- within the Xialaxiu volcanic sequence were 40 39 also thrusts Triassic strata over a Paleocene unit sis, and all irradiation parameters (J, Ar/ ArK, sampled for analysis (Fig. 4). Another rhyolite 38 39 36 37 39 37 (Ed), clearly indicating Cenozoic thrusting on Ar/ ArK, Ar/ ArCa, and Ar/ ArCa) are avail- sample was collected from an isolated volcanic the fault. This younger-over-older relationship is able in the GSA Data Repository.2 fl ow that unconformably overlies the basal part similar to what we observed along the Nangqian Biotites and whole-rock aliquots were incre- of the Shanglaxiu basin (Fig. 4). The 40Ar/39Ar fault zone (i.e., the southern branch of the Fault-B mentally heated in a double-vacuum Ta furnace ages for rhyolite and granitic intrusive within the system) and lends support to our speculation that (see Lovera et al., 1997). Evolved gas was trans- Xialaxiu volcanic fi eld yield 40Ar/39Ar ages of the Permian unit is continuous along the Yushu- ferred by expansion and purifi ed with an SAES 49.5 ± 0.2 Ma and 49.7 ± 0.2 Ma, respectively, Nangqian traverse, but is buried by an out-of- ST-101 10 l/s getter pump. Gas delivery to the while the isolated rhyolite sample yields a 40Ar/ sequence thrust near Nangqian (Fig. 3A). mass spectrometer was governed by splitting, 39Ar age of 51.2 ± 0.2 Ma (Table 1; Fig. 11). using precalibrated procedures. Argon isotopic These ages are the same within uncertainty as CENOZOIC IGNEOUS ACTIVITIES measurements were performed with an auto- those obtained by Roger et al. (2000) from the mated VG1200S quadrupole mass spectrometer. same volcanic fi eld. The age of the Xialaxiu Cenozoic igneous rocks are widely distrib- This instrument is equipped with a Baur-Signer igneous rocks constrain the Shanglaxiu basin and uted in central and eastern Tibet (e.g., Liu, 1988; source and an electron multiplier (see Quidel- the Shanglaxiu back thrust to predate 51 Ma. Deng, 1989, 1998; Pan et al., 1990; Turner et leur et al., 1997; Harrison et al., 2000). al., 1993; Roger et al., 2000; Wang et al., 2001). Values of 40Ar/39Ar, 38Ar/39Ar, 37Ar/39Ar, and Niuguoda Basin In the Yushu-Nangqian region, Paleogene igne- 36Ar/39Ar ratios were corrected for total system Two volcanic clasts were collected from a ous rocks were known at least two decades ago backgrounds, mass discrimination (monitored small (800 m × 400 m) outcrop of Paleogene (Qinghai BGMR, 1983a, 1983b, 1991). In the by measurement of atmospheric Ar introduced strata unconformably overlying the upper sec- Xialaxiu area, Roger et al. (2000) dated Xialaxiu tion of the Niuguoda basin fi ll (Fig. 2). Whole- volcanic fl ows using the Rb-Sr method. Their rock 40Ar/39Ar analyses were conducted, owing 2 isochron defi ned by whole rock, K-feldspar, GSA Data Repository item 2005149, sum- to a lack of coarse-grained, dateable mineral mary of argon isotopic results from the Yushu- and four biotite fractions yields an age of 51.4 Nang qian region, is available on the Web at http:// phases. Both samples yielded identical ages of 40 39 ± 0.6 Ma. The Ar/ Ar dating of the volcanic www.geosociety.org/pubs/ft2005.htm. Requests may 101.6 ± 1.3 Ma and 101.6 ± 3.1 Ma (Table 1). fl ow by the same authors yields a 48.6 ± 1.1 Ma also be sent to [email protected]. These dates provide a lower age boundary for

Geological Society of America Bulletin, September/October 2005 1303 SPURLIN et al. Figure 10. Geologic map of the Zhaduo area. Figure

1304 Geological Society of America Bulletin, September/October 2005 PALEOCENE-EOCENE DISTRIBUTED SHORTENING IN EAST-CENTRAL TIBET

TABLE 1. SUMMARY OF U-Pb AND 40Ar/39Ar AGES FOR IGNEOUS ROCKS IN THE the deposition of the Niuguoda basin. We infer NANGQIAN-YUSHU REGION that the 102 Ma clasts were derived from a Sample Rock type Phase U-Pb age† 40Ar/39Ar age‡ proximal volcanic source that was produced by (Ma) (Ma) subduction of the Indian slab. Cretaceous volca- Xialaxiu volcanic fi eld nic rocks in the northern Qiangtang terrane are MS 99–5-12-(1a) Rhyolite Biotite - 49.5 ± 0.2 also reported ~700 km to the west by Yin et al. MS 99–5-12-(2a) Granite Biotite - 49.7 ± 0.2 (1999a) and Kapp (2001). MS 99–5-10-(1) Rhyolite Biotite - 51.2 ± 0.2 Niuguoda basin Nangqian Basin MS 00–5-8-(1a) Volcanic clast Whole rock - 101.6 ± 0.9 Eleven samples were collected from igne- MS 00–5-8-(1b) Volcanic clast Whole rock - 101.6 ± 0.8 ous rocks in the Nangqian basin (Fig. 6). Eight Nangqian basin samples were trachydacite to trachyandesite MS 00–5-22-(2) Nepheline-syenite Biotite - 37.0 ± 0.2 tuffs and fl ows are either interbedded with or MS 99–4-27-(1b) Trachyandesite Biotite - 37.2 ± 0.1 unconformably overlie the Nangqian basin MS 99–4-18-(1) Trachydacite Zircon, Biotite 36 ± 3.0 37.3 ± 0.2 strata. Another three samples are nepheline- MS 00–5-9-(2) Monzonite Biotite - 37.3 ± 0.3 syenite and monzonite hypabyssal intrusions MS 00–5-9-(4) Trachydacite Biotite - 37.4 ± 0.1 cutting the Nangqian basin sequence. The eight MS 00–5-25-(1) Trachydacite Biotite - 37.6 ± 0.1 volcanic samples yield 40Ar/ 39Ar ages ranging MS 99–4-16-(2) Basaltic trachyandesite Biotite - 37.7 ± 0.2 MS 00–5-9-(3) Trachydacite Biotite - 37.7 ± 0.2 from 38.2 ± 0.1 Ma to 37.2 ± 0.1 Ma (Table 1). MS 99–4-17-(1) Monzonite Biotite - 37.7 ± 0.2 Secondary Ionization Mass Spectrometry MS 99–4-15-(3) Trachyandesite Biotite - 37.8 ± 0.1 (SIMS) analyses for one volcanic sample, dated MS 99–4-28-(1a) Trachydacite Biotite - 38.2 ± 0.1 as 37.3 ± 0.2 Ma by 40Ar/39Ar, yielded a single †Reported weighted mean 206Pb/238U age based on comparison with zircon standard AS-3 U-Pb age of 36 ± 3.0 Ma (Table 2). The three (1099.1 ± 0.5 Ma; Paces and Miller, 1993); refer to Table 2. 40 39 ‡Reported ages are weighted means and age uncertainties refl ect analytical errors; ages plutonic samples yielded Ar/ Ar ages ranging based on comparison with FCT-1 sanidine (27.8 ± 0.3 Ma; McDougall and Harrison, 1999). from 37.7 ± 0.2 Ma to 37.0 ± 0.2 Ma.

GEOCHEMISTRY

Geochemical Analyses Xialaxiu volcanic field One sample from the Xialaxiu volcanic A B 60 55 ﬁ eld and nine samples from the Nangqian 40 39 MS 99-5-10-1 (biotite) MS 99-5-12-1a (biotite) basin dated by Ar/ Ar and U-Pb methods

55 ) 50 were analyzed for major and trace element and isotopic compositions from whole-rock 50 45 analyses (Tables 3 and 4). Major element compositions were determined by wet chemistry,

45 arent age (Ma Total Gas Age = 51.0 ± 0.2 Ma Total Gas Age = 49.2 ± 0.1 Ma 40 and the trace element data were obtained by

Weighted Mean = 51.2 ± 0.2 Ma App Weighted Mean = 49.5 ± 0.2 Ma Apparent age (Ma) 40 inductively coupled plasma–mass spectrometry (ICP-MS) at the Guangzhou Institute of 35 020406080 100 020406080 100 Geochemistry, China. Cumulative39 Ar released (%) Cumulative39 Ar released (%) Results Nangqian basin D Xialaxiu Volcanics C 45 45 A sample from an Eocene rhyolite (49.5 ± MS 00-5-22-2 (biotite) MS 99-4-15-3 (biotite) 40 40 0.2 Ma) was collected from the Xialaxiu vol- 35 35 canic ﬁ eld. The major element geochemistry 30 30 of the sample indicates that it is calc-alkaline

25 25 (Table 3). The chondrite-normalized rare

arent age (Ma) earth element (REE) pattern shows a slightly 20 Total Gas Age = 36.7 ± 0.1 Ma 20 Total Gas Age = 37.6 ± 0.1 Ma

Apparent age (Ma) Weighted Mean = 37.0 ± 0.2 Ma App Weighted Mean = 37.8 ± 0.1 Ma negative Eu anomaly, which may indicate that 15 15 plagioclase is a major phase involved in frac- 10 10 tional crystallization (Fig. 12B). The pattern of 020406080 100 020406080 100 normalized trace elements exhibits pronounced 39 39 Cumulative Ar released (%) Cumulative Ar released (%) negative anomalies in Nb, Ta, Ti, and P, typi- Plutonic Volcanic cally indicative of subduction-related magmatism (Fig. 12C) (see discussion by Wang et al., Figure 11. 40Ar/39Ar age spectra for selected igneous rocks within (A–B) the Xialaxiu volca- 2001). Our results are broadly similar to those nic ﬁ eld and (C–D) the Nangqian basin. obtained by Roger et al. (2000) for the same

Geological Society of America Bulletin, September/October 2005 1305 SPURLIN et al.

TABLE 2. U-Pb ISOTOPIC DATA Isotopic ratios Apparent ages ± 1σ (Ma) Spot ID1 206Pb*/ 238U±1 207Pb*/ 235U±1 207Pb/ 206Pb* ±1 206Pb* % 206Pb*/238U 207Pb*/235U 207Pb*/ 206Pb*

MS 99–4-18-(1) Nangqian trachydacite 2,4,1 5.52E-03 6.33E+00 2.54E-02 1.42E+02 3.34E-02 1.39E+02 79.4 35 ± 2 25 ± 36 neg 2,4,2 5.62E-03 8.98E+00 –1.21E-03 –4.76E+03 –1.57E-03 4.76E+03 75.8 36 ± 3 neg neg 2,9,1 5.64E-03 2.75E+00 1.98E-02 4.60E+01 2.55E-02 4.49E+01 95.9 36 ± 1 20 ± 9 neg 2,9,2 5.61E-03 2.55E+00 1.02E-02 1.25E+02 1.32E-02 1.23E+02 96.6 36 ± 1 10 ± 13 neg 2,9,3 5.07E-03 7.36E+00 2.53E-02 1.73E+02 3.62E-02 1.69E+02 78.0 33 ± 2 25 ± 43 neg 2,9,4 5.88E-03 9.22E+00 4.75E-02 1.14E+02 5.86E-02 1.10E+02 75.4 38 ± 3 47 ± 52 552 ± 2400 2,11,1 5.72E-03 4.06E+00 2.24E-02 8.41E+01 2.84E-02 8.22E+01 92.5 37 ± 2 23 ± 18 neg 2,11,2 5.62E-03 4.12E+00 2.12E-02 7.89E+01 2.73E-02 7.67E+01 95.9 36 ± 1 21 ± 16 neg 2,11,4 6.10E-03 1.03E+01 3.61E-02 1.99E+02 4.29E-02 1.94E+02 67.9 39 ± 4 36 ± 70 neg 2,11,5 5.70E-03 5.75E+00 2.56E-02 1.29E+02 3.26E-02 1.25E+02 85.5 37 ± 2 26 ± 33 neg 1Spot identiﬁ cation. #,#,# = row number, grain number, spot number. *Radiogenic Pb, corrected for common Pb. neg, negative apparent age due to reverse discordance analyses corrected for common Pb using measured 204Pb and Pb evolution models of Stacey and Kramers (1975).

volcanic fl ows. Given the similar petrology, ages of 40–36 Ma are distributed along the of normalized trace elements exhibit pro- geochemistry, and age of the Lower Eocene entire length of the Ailao Shan–Red River shear nounced negative anomalies in Nb, Ta, Ti, and (ca. 51–49 Ma) igneous rocks in the Xialaxiu zone and its northern extension, the Batang- P (Fig. 12C), which is a signature of subduction area, they are most likely cogenetic. These Lijiang fault system (e.g., Ratschbacher et al., as discussed by Roger et al. (2000) and Wang et rocks differ geochemically from the younger 1996) that links the Yushu-Nangqian thrust belt al. (2001) for Eocene rocks along the same belt. late middle Eocene (ca. 37 Ma) igneous rocks (Fig. 1) (Leloup et al., 1995; Roger et al., 2000; The content of SiO2 varies widely in the range found in the Nangqian basin to the south; that is, Wang et al., 2001). The initiation and termina- of 49.5%–68.5%. We estimate an ~3%–5% Eocene igneous rocks near Xialaxiu have higher tion of igneous activity in the Nangqian area volumetric crustal contamination of the Nang- 87Sr/86Sr values (~0.7072) and slightly lower occurred at 51 and 37 Ma, which is earlier than qian volcanics, roughly equaling a 90% isotopic 143Nd/144Nd values (~0.5125–0.5124) (Roger et along the southern segment of the active Red contribution by crustal rocks. Isotopic analyses al., 2000) (Table 4; Fig. 12A). River fault between 40 and 24 Ma (e.g., Wang et for the high-potassic Nangqian volcanics yield Cenozoic high-potassic igneous rocks in al., 2001 and references therein). Since fraction- 143Nd/144Nd values of ~0.5126 and 87Sr/86Sr val- northern and eastern Tibet have 143Nd/144Nd and ation of isotopes is a time-dependent process, ues that range from 0.7051 to 0.7057 (Table 4). 87Sr/86Sr isotopic compositions that lie in the the lower 87Sr/86Sr and higher 143Nd/144Nd values enriched extension of the mantle array, but fall of younger igneous rocks in the Red River area STRUCTURAL EVOLUTION far outside the fi elds of mid-ocean-ridge basalt to the south (Fig. 12A) may have resulted from (MORB), Hawaiian, and Kerguelen basalts continued continental subduction along the Style of Deformation (Fig. 12A). The high 87Sr/86Sr and low 143Nd/ eastern margin of the Tibetan Plateau associated 144Nd isotope signatures require mantle sources with upper crustal shortening as we observed in Because of the lack of involvement of with a time-integrated history of enrichment in the Yushu-Nangqian thrust belt. It is possible crystalline basement in our mapped areas, we light (L) REEs and Rb. A plausible explanation that the isotopic difference between the Yushu- hypothesize that the Cenozoic thrust system is to ascribe these combined trace-elemental and Nangqian area and the Red River region results is of a thin-skinned style (e.g., Price, 1981) isotopic observations to a metasomatized sub- from diachronous cessation, from west to east, as shown in our cross section (Fig. 3A). This continental lithospheric mantle before the onset of Tertiary continental subduction. style of deformation is continuous at least for of partial melting (Arnaud et al., 1992; Turner et ~90 km west of the Yushu-Nangqian traverse, as al., 1993; Wang et al., 2001). Alternatively, the Nangqian Basin demonstrated by our observation in the Zhado same geochemical signatures may be explained Nine of the late middle Eocene igneous area. However, the thin-skinned hypothesis may by partial melting of granulitic or eclogitic samples (37.0 ± 0.2 Ma to 38.2 ± 0.1 Ma) were not be valid laterally to the east of the traverse, Tibetan lower crust (Hacker et al., 2000; Cooper collected from interbedded volcanic fl ows in because a small gneissic unit occurs within et al., 2002). the upper Nangqian basin. The major element Ordovician strata ~30 km southeast of Yushu Cenozoic igneous rocks in eastern Tibet geochemistry of the Nangqian volcanic rocks (Roger et al., 2003). The gneiss was dated by (western Sichuan) and Yunnan lie along the is calc-alkaline, similar to Gangdese plutonic the U-Pb zircon method that yields a discordia same Eocene magmatic belt as those we stud- rocks in the southern Lhasa block (Harris et al., age with a lower intercept at 682 ± 25 Ma and ied in the Yushu-Nangqian area (Roger et al., 1988) and those studied by Roger et al. (2000) an upper intercept at 2460 ± 74 Ma (Roger et 2000; Wang et al., 2001). However, the former in the Shanglaxiu area and the area to the west in al., 2003). It is possible that a lateral thrust ramp have lower 143Nd/144Nd (<0.5126) and higher north-central Tibet. Geochemical analyses show exists east of the Yushu-Nangqian traverse. That 87Sr/86Sr (0.7055–0.7070) values compared to enrichment in the major elements (particularly is, the basal thrust cuts downsection eastward the Nangqian volcanics (Zhu et al., 1992; Wang Na and K; Na = 0.98–4.43; K2O = 7.61–4.12; into the basement east of Yushu exposing the et al., 2001; cf. Fig. 12A). This difference may K2O/Na2O = 5.06–1.07), LREEs, and large ion basement rocks. Some 500–700 km to the west be explained by the wide age spread of igneous lithophile elements (LILEs; i.e., Rb, Sr, Cs, Ra, of our map area, Kapp et al. (2000, 2003) show rocks across the region. Igneous rocks with etc., see Table 3 and Fig. 12B–C). The patterns that the basement of the central Qiangtang

1306 Geological Society of America Bulletin, September/October 2005 PALEOCENE-EOCENE DISTRIBUTED SHORTENING IN EAST-CENTRAL TIBET 3) MS 99–5-12-(1a) TABLE 3. GEOCHEMICAL ANALYSES OF MAJOR (%) AND MINOR (PPM) ELEMENTS FOR IGNEOUS ROCKS IN THE NANGQIAN-YUSHU REGION IN OF MAJOR (%) AND MINOR (PPM) ELEMENTS FOR IGNEOUS ROCKS GEOCHEMICAL ANALYSES 3. TABLE 4.22 4.551.321.23 4.46 0.43 0.42 1.85 0.98 0.78 2.81 0.42 0.32 2.22 0.43 0.43 4.19 0.44 0.29 2.11 0.85 1.18 3.68 0.42 0.24 1.57 0.59 0.68 0.37 0.23 49.6915.18 63.47 16.92 57.71 17.27 68.53 15.48 64.83 16.35 65.47 16.53 50.87 15.07 67.72 15.17 55.36 17.85 71.53 15.02 ‡ † 3 3 5 2 2 O 2.43 4.33 4.25 2.96 2.64 4.43 0.98 3.88 1.85 3.07 O 2 O Major elements are scaled; FeO omitted. FeO Major elements are scaled; coupled plasma–mass spectrometry. ICP-MS is inductively Standards GBPG-1 and AMH-1 used in the analyses. 2 OO 5.90 4.12 4.89 7.61 6.60 4.75 4.95 4.42 7.56 4.13 2 † ‡ 2 2 TiO Sample MS 00–5-22-(2) MS 99–4-18-(1) MS 00–5-9-(2) MS 00–5-9-(4) MS 00–5-25-(1) MS 00–5-9-(3) MS 99–4-16-(2) MS 99–4-17-(1) MS 99–4-15-( ICP (%) SiO ICP-MS (ppm) BaHfNbPbRbSrTa 2684Th 13.01U 26.89Y 31.44Zn 151.00Zr 1703 2660 6.59 1.56 13.49La 35.52 13.12Ce 115.25Pr 7.36 29.20Nd 103.33 2184 18.05Sm 465.78 26.33 9.35 0.81Eu 26.82 64.16 98.51Gd 159.35Tb 283.97 6.28 20.71 26.69 2479Dy 2391 240.19 35.82 26.25 10.49Ho 137.70 40.97 152.64 1.54 58.07Er 104.61 23.30Tm 202.62 5.08 27.06Yb 9.00 84.52 14.57 356.90 1944 2185Lu 21.56 16.31 78.34 128.76 7.86 (ppm) 1.59Total 39.43 161.87 43.55 1.55 12.44 6.38 290.69 1.02 22.76 389.80 57.48 2.69 9.00 2.64 673.97 1810 8.09 2215 130.42 35.88 0.34 124.91 14.47 124.71 32.33 3.95 0.94 1.96 39.75 20.29 250.76 0.96 3.98 0.27 297.45 0.67 20.73 71.20 439.90 4.40 12.01 6.80 2934 1.82 170.89 96.51 27.23 1728 17.85 127.08 0.27 26.64 242.86 76.07 8.02 1.29 15.88 1.63 0.88 5.49 143.93 0.25 668.28 18.42 36.00 0.91 94.47 105.56 91.99 3.33 25.70 1454 9.46 5.20 2.33 5003 11.81 196.32 42.53 0.32 45.32 14.42 5.81 1.04 2.10 300.99 1.03 538.75 4.61 0.29 39.54 92.91 147.48 0.77 263.86 72.00 20.16 2670 19.42 3.04 546.32 1152 8.75 8.61 2.07 21.65 0.32 46.47 11.10 215.48 523.34 9.05 0.95 1.79 0.69 13.70 4.06 48.09 108.15 0.28 70.36 260.46 0.66 145.10 67.83 19.94 1829 3186 2.42 43.87 6.86 4.94 1.79 329.56 30.38 41.68 0.26 422.45 3.98 0.77 1.56 31.97 43.02 1.11 177.95 3.55 0.22 58.53 345.87 15.88 0.55 25.19 24.66 142.84 1233.36 8.85 864 9.07 1.47 9.18 0.21 2.45 20.97 1.29 24.73 56.22 174.85 107.94 1.20 9.19 0.20 46.16 1.38 313.16 5.78 2.12 5.58 3.41 29.15 0.40 37.76 0.65 2.54 10.98 2.75 0.33 813.01 0.42 17.19 6.44 1.11 6.54 0.16 1.80 0.97 234.59 7.14 0.16 1.03 4.64 1.16 2.72 0.34 0.70 2.11 3.63 0.28 0.69 1.94 0.28 1.87 0.24 Al Fe MnOMgOCaONa P 0.12Total 7.59 10.01 0.05 1.48 97.68 3.17 0.09 3.51 98.95 5.32 0.03 99.25 0.53 1.48 0.05 99.21 1.62 4.00 99.78 0.06 1.69 3.27 99.16 0.13 5.39 14.08 97.70 0.04 1.69 3.26 98.95 0.10 3.16 8.01 98.83 0.03 0.95 2.97 99.87 K

Geological Society of America Bulletin, September/October 2005 1307 SPURLIN et al.

TABLE 4. WHOLE ROCK Rb-Sr AND Sm-Nd ISOTOPIC DATA FOR IGNEOUS ROCKS IN THE NANGQIAN-YUSHU REGION Samples Age Rb Sr 87Rb/87Sr 87Sr/86Sr ±2σ Sm Nd 147Sm/144Nd 143Nd/144Nd ±2σ (Ma) (µg/g) (µg/g) (µg/g) (µg/g) MS 00–5-22-(2) 37.0 ± 0.2 100.4 3237 0.08980 0.705090 15 21.66 136.34 0.09598 0.512607 7 MS 99–4-18-(1) 37.3 ± 0.2 110.5 1918 0.1669 0.705662 12 11.97 79.574 0.09098 0.512703 9 MS 00–5-9-(2) 37.3 ± 0.3 93.20 3856 0.07000 0.705300 11 19.26 132.21 0.08812 0.512679 8 MS 00–5-9-(4) 37.4 ± 0.1 218.6 1417 0.4467 0.705156 13 5.894 45.612 0.07816 0.512681 7 MS 00–5-25-(1) 37.6 ± 0.1 205.4 1071 0.5555 0.705458 11 3.519 21.839 0.09747 0.512675 9 MS 00–5-9-(3) 37.7 ± 0.2 142.3 1790 0.2303 0.705248 12 2.740 18.423 0.08996 0.512676 9 MS 99–4-16-(2) 37.7 ± 0.2 90.59 5139 0.05104 0.705390 13 40.78 280.43 0.08796 0.512668 8 MS 99–4-17-(1) 37.7 ± 0.2 133.9 1107 0.3501 0.705100 14 8.603 56.528 0.09206 0.512694 5 MS 99–4-15-(3) 37.8 ± 0.1 126.0 2352 0.1551 0.705305 13 27.18 172.83 0.09513 0.512692 6 MS 99–5-12-(1a) 49.5 ± 0.2 139.1 234.1 1.721 0.707168 12 4.658 31.913 0.08829 0.512543 9

terrane was composed of Triassic blueschist- northeast-directed Shanglaxiu thrust ceased The north-striking normal fault in the Dongba bearing mélange that may have been subducted motion by 51 Ma. The thrust together with the area is the only extensional structure we mapped from the Songpan-Ganzi terrane beneath the overlying Eocene strata was subsequently folded in the fi eld. Because it cuts Neogene strata, it Qiangtang in the latest Triassic. Refl ection and by NE-SW contraction. In the footwall of the must signifi cantly postdate (older than 15– refraction seismology indicates that the fi rst- Shanglaxiu back thrust, strike-slip faulting post- 20 Ma) Paleogene igneous activity in the region. order crustal shortening in the Yushu-Nangqian dates NE-SW contraction. Cessation of major However, when east-west extension terminates region is accommodated mainly in the upper Cenozoic thrusts in the Eocene followed by in the Dongba basin is unclear. The normal fault crust (Galve et al., 2002). This result is consis- continued NE-SW contraction is also observed may have formed synchronously during left-slip tent with our assumption that deformation in the along the Xialaxiu and Niuguoda thrusts. In the faulting along the Ganzi-Yushu fault, similar to Yushu-Nangqian traverse is thin-skinned. Seis- above area, we could not distinguish whether structural development of Late Cenozoic rifts mic studies by the receiver-function method and NE-SW contraction was continuous during and strike-slip faults in central Tibet (Molnar the analysis of teleseismic P waves indicate that and after motion on major thrusts or episodic. and Tapponnier, 1978; Armijo et al., 1986, the Moho is offset along the southern margin of However, observations from the Nangqian 1989; Yin et al., 1999a; Yin, 2000; Taylor et the Qaidam basin, but no offsets are observable area strongly suggest that NE-SW contraction al., 2003). However, the location of the Dongba along the Jinsha suture and the active left-slip occurred in two distinctive phases, between normal fault is considerably east (>250 km) of Kunlun fault (Vergne et al., 2002). These obser- which major phases of strike-slip faulting (both the easternmost known rift in Tibet (Armijo et vations suggest that if the Paleogene crustal right-slip and left-slip) occurred. Specifi cally, al., 1986, 1989; Taylor et al., 2003). shortening along the Fenghuo Shan–Nangqian the Nangqian fault zone has experienced at least By integrating the aforementioned crosscut- thrust belt involves continental subduction, the fi ve phases of deformation: (1) NE-SW contrac- ting relationships, we suggest the following offset mantle has been “healed,” possibly by tion (~60 km) as expressed by northwest-trend- deformational history for the Nangqian-Yushu ductile fl ow of the lower crust. ing folds, minor thrusts, and possible initiation region (Fig. 13): (1) development of the Shan- Because the detailed structures in the crust of the main trace of the Fault-B system as an glaxiu (including its back thrust), Xialaxiu, along the Yushu-Nangqian traverse are not out-of-sequence thrust; (2) right-slip faulting Niuguoda, and Jiangda thrusts and associated known, our cross section shown in Figure 3A (<3–4 km) coeval or slightly younger than NE- folds followed by the initiation of the Nangqian should be viewed as one of the simplest solu- SW contraction, development of the associated fault zone (the southern branch of the Fault-B tions for downward projection of the observed en echelon folds, and right-lateral offset of early system) as an out-of-sequence thrust, (2) devel- surface geology. We note that Cenozoic thrusts Eocene basins; (3) development of left-slip opment of right-slip faults terminating into east- are widespread in the Qiangtang terrane (Kapp, faults that terminate into north-trending folds trending folds and causing development of en 2001; Kapp et al., 2005), which is in sharp (<3–4 km); (4) renewed NE-SW contraction echelon folds and right-lateral offset of lower contrast to the lack of Cenozoic shortening in (>5–10 km) causing folding and thrusting in Eocene basin fi ll, (3) development of left-slip the Lhasa terrane to the south (e.g., Murphy et the Upper Eocene strata; (5) right-slip faulting faults that terminate into north-trending folds al., 1997). This may be due to the fact that base- (< a few km); and (6) active left-slip motion and reactivate early thrusts and right-slip faults, ment of the Qiangtang terrane is composed of along the Ganzi-Yushu fault (78–100 km, Wang (4) right-slip faults that cut the Eocene strata mechanically much weaker mélange complexes and Burchfi el, 2000). overlying the early left-slip faults, (5) renewed than the crystalline basement of the Lhasa ter- The youngest phase of NE-SW contraction NE-SW contraction causing folding of Upper rane that is most likely similar to the Indian cra- in the Yushu-Nangqian thrust belt probably Eocene strata, and (6) development of left-slip ton as the two were linked in Paleozoic times. occurred during the Neogene, because mapped and normal faulting. Neogene strata are deformed by northwest- Sequence and Timing of Deformation trending folds and cut by northwest-striking Regional Correlation thrusts in the Dongba area. Out-of-sequence Although all major faults are northwest-strik- thrusting is also evident in the thrust belt as The complex Cenozoic deformational history ing, the Yushu-Nangqian area has experienced convincingly demonstrated by our mapping in of the Yushu-Nangqian area is not unique in multiple episodes of Cenozoic deformation with the Zhaduo area where a major Cenozoic thrust the eastern Indo-Asian collision zone. Directly distinctively different fault kinematics and strain locally juxtaposes Triassic strata over Carbonif- south of our study area along the Batang-Lijiang patterns. In the Shanglaxiu area, the Paleogene erous and Permian strata. fault zone, Ratschbacher et al. (1996) show the

1308 Geological Society of America Bulletin, September/October 2005 PALEOCENE-EOCENE DISTRIBUTED SHORTENING IN EAST-CENTRAL TIBET

Figure 12. Isotope and trace element data MORB normalized to chondrite values for Cenozoic ca.49 Ma (this study) high-potassic igneous activities. (A) Sr-Nd 0.5130 ca. 49 Ma [Roger et al. (2001)] isotopic diagram. Plotted are Cenozoic Hawaii 42 - 24 Ma [Wang et al. (2001)] volcanic rocks from the Yushu-Nangqian 38 - 37 Ma (this study) area (Roger et al., 2000; this study) and 0.5128 16 - 0 Ma [Wang et al. (2001)] elsewhere in eastern Tibet and west Yunnan, China (Wang et al., 2001). Fields shown

144 0.5126 are for mid-ocean-ridge basalt (MORB), Hawaiian, and Kerguelen basalts (Zindler

Nd/ Nd

143 Kerguelen and Hart, 1986) (EM2 enriched mantle end 0.5124 member). (B) Rare earth element (REE) and (C) spider diagrams of trace element data normalized to chondrite values for rep- EM 0.5122 2 resentative analyses of volcanic rocks of the Yushu-Nangqian area. A 0.5120 0.702 0.704 0.706 0.708 0.710 87Sr/ 86 Sr

1000 region underwent east-west contraction during

MS99-4-15-(3) the Paleogene and early Miocene followed by MS99-4-16-(2) strike-slip faulting. In the northern Yunnan prov- MS99-4-17(1a/b) ince of south China ~400 km east of the eastern ite MS99-4-18-(1) MS00-5-9(2) Himalayan syntaxis and north of the Red River MS00-5-9(3) fault, Lacassin et al. (1996) documented activity 100 MS00-5-9(4) of a major ductile décollement accommodating MS99-5-12(a) SSW-directed thrusting at ca. 36 Ma. This con- MS00-5-22(2) MS00-5-25(1) tractional event was followed immediately by the initiation of left-slip motion along the Ailao

Rocks/Chondr Shan–Red River shear zone that was coeval with

10 emplacement of leucocratic melts at 22–26 Ma eous and east-west contraction as expressed by

Ign development of the distributed thrust belts on both sides of the left-slip shear zone (Leloup et al., 1995, 2001; Lacassin et al., 1996; Wang and B Burchﬁ el, 1997). The Ailao Shan–Red River 1 shear zone subsequently experienced rapid cool- La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu ing between 22 and 17 Ma as a result of left-slip transtensional shear (Harrison et al., 1996).

1000 Finally, the right-slip Red River fault has reac- C tivated the left-slip Ailao Shan–Red River shear zone since ca. 5 Ma, with coeval north-trending

ite normal faults (Allen et al., 1984; Leloup et al., 1995). The sequence and timing of deformation established by this study match well those along 100 the Ailao Shan–Red River shear zone. That is, both areas experienced Eocene contraction in the NE-SW or NNE-SSW direction, followed

Rocks/Chondr ﬁ rst by left-slip faulting and associated east- west contraction and then by right-slip faulting

10 and east-west extension.

eous MS99-4-15-(3) MS99-4-16-(2) The initiation and termination of major faults MS99-4-17(1a/b) MS99-4-18-(1) Ign in the eastern Indo-Asian collision zone are dia- MS00-5-9(2) MS00-5-9(3) chronous. For example, the left-slip Wang Chao MS00-5-9(4) MS00-5-22(2) fault zone in Indochina probably terminated MS00-5-25(1) MS99-5-12(a) by 30 Ma (Lacassin et al., 1997), signiﬁ cantly 1 older than the termination age of ca. 17 Ma for Rb Ba Th U K Nb Ta La Ce Sr P Nd Sm Zr Hf Ti Tb Dy Y Yb Lu the Ailao Shan–Red River left-slip shear zone to the north. This northward decrease in fault

Geological Society of America Bulletin, September/October 2005 1309 SPURLIN et al.

1. Paleocene-late Eocene Xialaxiu thrust Shanglaxiu basin 37 Ma volcanics Paleocene-early Xialaxiu basin Eocene strata Niuguoda basin Paleocene-early Nangqian Eocene strata basin Paleocene-early Eocene strata 49-51 Ma middle Eocene strata 37 Ma volcanics volcanics Niuguoda thrust

Nangqian thrust

Paleocene-late Eocene strata AY 01/15/04

oroclinal bending 2. Late Eocene of older thrust due to right shear Xialaxiu thrust Fault-B system Shanglaxiu basin

Niuguoda Xialaxiu basin Fault-C system basin Nangqian basin

49-51 Ma middle Eocene strata volcanics Niuguoda thrust

Nangqian thrust

Old right-slip fault 3. Late Eocene reactivated to become left-slip fault Xialaxiu thrust Shanglaxiu basin Fault-B system Xialaxiu basin Niuguoda basin

49-51 Ma middle Eocene strata volcanics Niuguoda thrust Nangqian basin Old thrust reactivated to become left-slip fault

Figure 13. Possible sequence of deformation across the Yushu-Nangqian traverse. (1) NE-SW contraction creating northwest-trending folds and major thrusts and initiation of the Nangqian thrust. (2) Right-slip faulting terminating at east-trending folds and causing development of en echelon folds and right-lateral offset of early Eocene basins. (3) Development of left-slip faults that terminate into north-trending folds. Some of the left-slip faults reactivated early thrusts or right-slip faults. The left-slip faults are overlain by Upper Eocene strata. (4) Development of right-slip faults that offset Upper Eocene strata overlying the left-slip fault zone. (5) Renewed NE-SW contraction causing folding in the Upper Eocene strata. (6) Left-slip and normal faulting. Dashed lines are structures to be developed in the next stage. (Continued on following page.)

1310 Geological Society of America Bulletin, September/October 2005 PALEOCENE-EOCENE DISTRIBUTED SHORTENING IN EAST-CENTRAL TIBET

4. Late Eocene

Fault-C system Xialaxiu thrust Shanglaxiu basin late Eocene strata Paleocene-early Xialaxiu basin Niuguoda Eocene strata basin Paleocene-early Nangqian Eocene strata basin

49-51 Ma middle Eocene strata volcanics Niuguoda thrust

5. Neogene

Dongba thrust Future Ganzi-Yushu fault

Fault-A system Xialaxiu thrust Neogene Shanglaxiu basin Dongba basin Paleocene-early Niuguoda Xialaxiu basin Eocene strata Nangqian basin Paleocene-early basin Eocene strata

49-51 Ma middle Eocene strata volcanics Niuguoda thrust

6. Late Neogene-Present

Dongba thrust Ganzi-Yushu fault

Fault-A system Xialaxiu thrust Neogene Shanglaxiu basin Dongba basin

Niuguoda Xialaxiu basin Nangqian basin basin

49-51 Ma middle Eocene strata volcanics Niuguoda thrust

Geological Society of America Bulletin, September/October 2005 1311 SPURLIN et al. termination age has been related to progressive fi lls, which were deposited during NE-SW Therefore, our estimate of total Cenozoic crustal northward penetration of India into Asia (e.g., contractional deformation (Horton et al., 2002). shortening is a minimum amount of shortening Lacassin et al., 1997). However, our observa- This relationship suggests coeval contraction for the larger-scale region. tions along the Nangqian fault zone suggest that and magmatic activity in the late Eocene in the In the northern section, Triassic strata are left-slip faulting postdates the 37 Ma Nangqian Nangqian area. Crosscutting relationships in repeated by imbricate thrusts above an inter- 1 volcanics and predates deposition of Upper the Nangqian area also suggest that strike-slip preted basal décollement in the T3jz unit with Eocene strata. That is, although left-slip faulting faults postdate the emplacement of the 37 Ma a depth varying from ~5–8 km (Fig. 3A). In the in the Yushu-Nangqian region could have been Nang qian igneous complex (Fig. 8). From this southern section we use a hanging-wall ramp to initiated coeval with the Ailao Shan–Red River and the previous discussion, we conclude that explain the occurrence of older Carboniferous shear zone, its termination is perhaps >15 m.y. Eocene magmatic activity in the Yushu-Nang- strata (Fig. 3A). Thrusting and associated fault- older than the latter. The complex evolution qian region occurred during or immediately bend and fault-propagation folds north of the of fi nite-strain fi elds in the eastern Indo-Asian after a major episode of NE-SW contraction. Shanglaxiu thrust are related to slip along the collision zone has been related to diachronous basal décollement (Fig. 3A). The basal thrust lateral extrusion of large crustal blocks in the Cross Section Restoration ramps upsection in the Shanglaxiu thrust foot- 1 2 Indo-Asian collisional zone and clockwise wall from T3jz sandstone to T3jz carbonate at rotation around the eastern Himalayan syntaxis A cross section of the Yushu-Nangqian the northern end of profi le A″–A″′ (Fig. 3A). (e.g., Peltzer and Tapponnier, 1988; Lacassin traverse is shown in Figure 3A. This cross sec- In the cross section, the Xialaxiu thrust is inter- et al., 1997; Royden et al., 1997; Wang and tion cannot be strictly balanced because: (1) the preted to exhibit a hanging-wall fl at geometry at 1 Burchfi el, 1997). For complete understanding presence of late strike-slip faults, (2) some the base of T3jz sandstone (Fig. 3A). Broad fold- of regional tectonics and the growth history of contractional faults may also have strike-slip ing of the Xialaxiu thrust and the Cenozoic strata the Tibetan Plateau, systematic determination components (the Fault-B in the Nangqian fault in the Xialaxiu basin are attributed to motion on of crustal rotation should be a future research zone), and (3) out-of-sequence thrusting due to a minor northeast-directed thrust located along priority so that the observed fault kinematics either or both pre-Cenozoic folding and multiple the southeastern margin of the basin (Fig. 2). A can be interpreted in the context of the original phases of Cenozoic shortening creating unusual hanging-wall anticline-syncline pair above the coordinate systems during the occurrence of “younger-over-older” relationships across some younger thrust is projected in the down-plunge each deformational phase. of the major contractional faults in this and the direction beneath the Xialaxiu thrust system Zhaduo areas. The effect of strike-slip faulting (Fig. 3A). Minor, northeast-directed thrusts are Relationship between Deformation and on the overall estimate of shortening is prob- present a few kilometers north of the northern Syncollisional Igneous Activity ably small, because each fault has slip <2–3 km, margin of the Xialaxiu basin (~4 km south of the based on the offset of Paleogene basins and Shanglaxiu back thrust) (Fig. 2). Their develop- In the Xialaxiu region, the 49–51 Ma volcanic rapid termination of faults along strike (Fig. 2). ment may have resulted in the southward tilting fl ows overlie the trace of the Shanglaxiu back In addition, the thrust sheets we mapped are of the northern basin margin. thrust. This relationship indicates that the Xia- highly deformed. We did not consider the effect The limited fault slip along the Fault-B sys- laxiu igneous complex postdates motion along of fi nite strain in cross section construction such tem, particularly at its southeastern end, raises the Shanglaxiu thrust system. Because the Xia- as the method proposed by McNaught and Mitra the question of why younger Triassic strata are laxiu volcanic fl ows themselves are folded with (1996). This is because some of the hanging-wall locally placed over older Carboniferous strata. fold axes trending northwest, NE-SW contraction deformation could be pre-Cenozoic, and an arbi- There are at least three explanations: (1) The age must have occurred after their emplacement. A trary choice had to be made to partition the pre- assignment of units by Qinghai BGMR (1983a), critical question is whether the NE-SW contrac- Cenozoic and Cenozoic strain. Thus, in our cross which we follow in this study, may be wrong. At tion was a continuous process before, during, and section, internal deformation of the thrust sheets this point we do not have independent fossil evi- after the emplacement of the Xialaxiu volcanics is calculated by fi rst projecting fold geometry dence to test this possibility. However, given the and motion on the Shanglaxiu thrust system. As downward on the cross section and then measur- fact that Carboniferous strata were deposited in we discussed above, two distinctive phases of ing the total bed length being shortened. This a marine environment, its age identifi cation and NE-SW contraction can be established in the method implies that all folds in the Shanglaxiu regional correlation should be straightforward Nangqian area: one in the Paleogene prior to a thrust sheet were developed above a décolle- and we consider an inaccurate age assignment major strike-slip event and one in the Neogene ment zone. Due to the presence of out-of-plane unlikely. (2) There is a major unconformity 2 after the strike-slip event. Therefore, the crosscut- strike-slip faulting, out-of-sequence thrusting, at the base of Triassic unit T3jz along which ting relationship alone in the Xialaxiu area cannot and uncertainties between hanging-wall folds Carboniferous units and older Triassic units establish whether the volcanic activity was syn- and the underlying basal thrusts, the estimates of have been eroded away. In this scenario north- chronous with contraction. However, based on crustal shortening provided below are imprecise. east-dipping faults in the Fault-B system simply the Paleocene–early Eocene age of the Xialaxiu Nevertheless, we apply a line-balanced tech- form a fault fl at without duplicating or omitting basin and its coeval and genetic relationship to nique wherever possible and believe that our sections. However, as the Carboniferous and thrusts along the basin margin, we know that reconstruction captures the fi rst-order geometric even Permian strata are exposed continuous just motion along the Xialaxiu thrusts were active and kinematic features of the Yushu-Nangqian west of the Nangqian region we mapped (Qin- in the Paleocene and early Eocene (Horton et al., fold-thrust belt. Another implicit assumption in ghai BGMR, 1991), we also do not favor this 2002), immediately before the deposition of the our cross section construction is that no subho- possibility. (3) The Fault-B system is an out-of- Xialaxiu volcanics. rizontal shear has occurred at the two ends of sequence structure resulting from two phases of In the Nangqian area, the 37 Ma volcanic the cross section so that pin-lines can be used. Cenozoic shortening or Cenozoic faulting super- fl ows are interlayered with the uppermost part Because our section is drawn in the middle of posed on pre-Cenozoic folded strata. We favor of the Paleocene–late Eocene Nangqian basin a thrust belt, this assumption may not hold. this explanation because major contractional

1312 Geological Society of America Bulletin, September/October 2005 PALEOCENE-EOCENE DISTRIBUTED SHORTENING IN EAST-CENTRAL TIBET -5 5km -5 -5 5km 5km S.L. S.L. S.L. Tr Tr ); (NDB); 50 km

Tr leozoic ust

with deposition ST

Tr ST mbricate thrust, P P P Tr SB SB SB Tr

X XT SBT XB no vertical exaggeration future ST XB Tr 10 20 30 40 Tr Tr Tr Tr Tr 0 NGT

SBT XB

NGT NDB C C C Tr NDB C Tr future XT C NT C NB JT lower Paleozoic lower Paleozoic lower Paleozoic

NGT JT NB future NT back thrusts Tr NDB Tr P P future footwall imbricate thrust C unconformity Tr C C erosion level approximate erosion level approximate JT Figure 14. Possible sequence of contractional deformation across the Yushu-Nangqian traverse. S.L.—sea level. traverse. Yushu-Nangqian the across sequence of contractional deformation 14. Possible Figure P C restores (immediately north of the presenterosional Shanglaxiu denudation basin of (SB)); uplifted deposition Permian-Triassicstrata strata of to lower the Xialaxiu south basin (Note: (XB), a lower pre-existing Shanglaxiu unconformity basin, between and Permian lower strata Niuguoda and basin underlying Pa to regional dip). and Shanglaxiu thrust (ST); depositioncontinued of slip lower on to thrusts middle north Shanglaxiu of basin, Shanglaxiu upper thrust; Xialaxiu continued basin, hinterland upper erosion Nuiguoda of basin, Carboniferous-Triassic strata and in lower the Nangqian Nangqian basin area. (NB of middle to upper Nangqian basin; folding of Carboniferous strata in the Nangqian area by minor north- and south-directed thrusting. A. Stage 1: slip on the Jiangda thrust (JT), Niuguoda thrust (NGT), Niuguoda imbricate thrusts, Shanglaxiu back thrust (SBT) and a north-directed thr B. Stage 2: folding of the Jiangda thrust by footwall imbrication of pre-Carboniferous-Triassic strata; slip on the Xialaxiu thrust (XT), Xialaxiu i C. Stage 3: minor north-directed thrusting beneath the Xialaxiu thrust; thrusting on the Nangqian thrust (NT) and footwall imbricate thrusts coeval

Geological Society of America Bulletin, September/October 2005 1313 SPURLIN et al. deformation occurred in the Paleocene–early Eocene as expressed by 49–51 Ma volcanic A fl ows overlying the Shanglaxiu fault (Figs. 2 and 4). This occurred prior to motion along the Fault-B system, which must postdate 37 Ma as it cuts the 37–38 Ma Nangqian igneous complex (Figs. 2 and 8). This interpretation is used to show the relationship between the Nangqian thrust and the underlying Permian-Triassic units (Fig. 3A). The overturned fold in the footwall of the Fault-B system at its southeastern end is also projected beneath the Nangqian thrust. Our interpretation that the folded thrust window 15 km west of Nangqian is part of the Jiangda thrust requires its slip to be at least 6 km (Fig. 2). When restoring the folded fault and displaced Carboniferous units, a minimum of 14–15 km can be inferred for the Jiangda thrust. Folding of the Jiangda thrust may have been related to the development of a minor north- directed blind thrust within Carboniferous strata B at a lower structural level (Fig. 3A). Restoring the cross section shown in Fig- ure 3A requires a minimum of ~43% shortening, that is, ~61 km of shortening across the 83-km- long profi le A′–A″′. Minimum-slip estimates for individual thrusts from the restored section range from ~2 to 18 km (Fig. 3B). The Yushu-Nang- qian thrust belt appears to extend farther to the southeast for another 60 km, based on the presence of Tertiary sedimentary basins shown on regional geologic maps (Qinghai BGMR, 1991). If we extrapolate the estimated 43% shortening from our traverse to the entire width of the ~150- km-wide Yushu-Nangqian fold-thrust belt, our minimum estimated Cenozoic shortening across the northern half of the Qiangtang terrane along the longitude 96°30′°E is roughly 110 km. A possible sequence of contractional defor- Figure 15. Alternative block models for subduction of mantle lithosphere beneath Qiangtang mation in our map area is shown schematically during Paleogene crustal shortening. (A) Subduction of the Songpan-Ganzi terrane from in Figure 14. In stage 1, the Jiangda thrust initi- the north. (B) Subduction of the Lhasa terrane from the south (NYTB—Yushu-Nangqian ated as a footwall ramp and feeds slip to a fl at fold-thrust belt; FSTB—Fenghuo Shan fold-thrust belt; BNS—Banggong-Nujiang suture; along the interface between Triassic and Permian Pz-Mz—Paleozoic-Mesozoic strata). (Continued on following page.) strata (Fig. 14A). As a result, the Niuguoda and Shanglaxiu thrusts were developed above the thrust fl at. For clarity we did not show the broad triggered the development of the Nangqian related contraction lasted past the early Miocene pre-Cenozoic folds. A possible angular uncon- transpressional system (Fault-B system) as a (Leeder et al., 1988; Coward et al., 1988; Zhang formity between Permian strata, not exposed in back thrust across this ramp. The fi rst two stages and Zheng, 1994; Liu and Wang, 2001; Wang et the study area, and the underlying Carboniferous of fault development occurred during deposition al., 2002; Liu et al., 2003). This study and early units is also shown below the southern segment of the Paleocene-Eocene basins exposed along work by Horton et al. (2002) expand this tem- of the thrust fl at. This assumption helps explain our traverse. In stage 3 (younger than 37 Ma), poral framework to the Yushu-Nangqian area. the magnitude and tightness of the inferred large contraction across the Nangqian fault zone Our results provide new constraints on how the overturned ramp anticline of the Carboniferous remained active (Fig. 14C). Tibetan Plateau has been uplifted during the units in the Nangqian region (Figs. 2 and 3A). Indo-Asian collision. In stage 2, the Xialaxiu thrust was initiated as a MECHANISMS FOR THE FORMATION Our results suggest that >61 km (~45%) footwall thrust splay that cut upsection to merge AND GROWTH OF THE TIBETAN Cenozoic shortening was accommodated via with the thrust fl at along the Permian-Triassic PLATEAU thin-skinned thrusting across the 115-km-long interface (Fig. 14B). Motion across the Xialaxiu Yushu-Nangqian thrust belt. This observation thrust ramp further amplifi ed the amplitude of It has long been known that the Fenghuo requires that the basement of the Yushu-Nang- the hanging-wall ramp anticline and may have Shan thrust belt initiated in the Paleocene and qian thrust belt be involved with crustal shorten-

1314 Geological Society of America Bulletin, September/October 2005 PALEOCENE-EOCENE DISTRIBUTED SHORTENING IN EAST-CENTRAL TIBET

60-40 Ma Fenghuo Shan-Nangqian the Indus-Tsangpo suture occurred in the late thrust belt ITS BNS JS Oligocene and early Miocene (Yin et al., 1994, India 1999b; Harrison et al., 2000). In light of recent Lhasa Qiangtang Songpan- Ganzi results on the chronology of plateau growth in northern Tibet (Yin et al., 2002), it appears that the Tibetan Plateau has grown both northwards and southward from its central part along the oldest Fenghuo Shan–Nangqian thrust belt. Our study does not support convective Shiquanhe-Gaize-Amdo Fenghuo Shan-Nangqian removal of Tibetan mantle lithosphere as a 40-35 Ma thrust belt thrust belt ITS BNS JS major cause for plateau uplift (Molnar et al., 1993), because we observe no coeval exten- India Songpan- Lhasa Qiangtang sion and syncollisional magmatism in our Ganzi study area. However, this does not preclude the convective-removal process operating in other parts of Tibet (e.g., Turner et al., 1993). Coeval Paleogene-Eocene contraction and magmatism in the Yushu-Nangqian region is generally consistent with the continental subduction model of Tapponnier et al. (2001) (also see Deng, 1989; Renbu-Zedong Gangdese thrust Shiquanhe-Gaize-Amdo Fenghuo Shan-Nangqian 35-20 Ma thrust thrust belt thrust belt Roger et al. 2000; Wang et al., 2001). How- BNS ITS JS ever, our observations do not support coupled India continental subduction and lateral extrusion Songpan- Lhasa Qiangtang Ganzi envisioned by Tapponnier et al. (2001); Paleo- cene-Eocene contraction is distinctly different from the subsequent left-slip faulting event. In addition, our documented late Eocene right- C slip deformation is in contradiction with the predicted left-slip faulting at this time. Finally, Figure 15 (continued). (C) Slab rollback of Indian plate after collision between India and our observed Neogene contraction involving Asia. ITS—Indus Tsangpo suture; JS—Jinsha suture. major thrusts and intense folding (e.g., in the Dongba area) suggests that signifi cant contraction continued in the Neogene instead of reach- ing at depth. We prefer continental subduction plateau uplift in the Neogene. In fact, Royden ing completion in the Eocene as suggested by as the major accommodation mechanism for (1996) showed that lower crustal fl ow becomes Tapponnier et al. (2001). the development of the Yushu-Nanqiang thrust important only some 20 m.y. after initial Indo- belt (Fig. 15A) because it explains (1) coeval Asian collision. CONCLUSIONS development of contraction and magmatism, Because syncollisional magmatism occurred (2) the geochemical signatures of the Paleogene over our mapped thin-skinned thrust belt, the A complex sequence of Cenozoic deforma- igneous rocks (also see Wang et al., 2001), and inferred continental subduction must have been tion occurred across the Yushu-Nangqian fold- (3) the source of partial melting for the Paleo- generated by development of coeval thrust belts thrust belt that has accommodated a minimum gene magmatism in the mantle lithosphere outside our study area to the north or south, of 61 km NE-SW crustal shortening. Cenozoic (e.g., Deng, 1998; Roger et al., 2000; Wang et allowing the basement and underlying mantle contraction started prior to 50 Ma and was al., 2001). The continental subduction could lithosphere to be subducted beneath the Yushu- followed by northwest-striking right-slip and be either south-dipping along the Jinsha suture Nangqian traverse. In other words, we do not left-slip faults; their motion was brief (probably (Tapponnier et al., 2001) or north-dipping along suggest that thrusting in our study area is the <3 m.y.) postdating 37 Ma but predating the end the Banggong-Nujiang suture (Yin and Har- direct cause of the syncollisional magmatism. of the Eocene. Renewed NE-SW contraction rison, 2000; Kapp, 2001; Kapp et al., 2005) It is interesting to note that the initiation of occurred in the Neogene and was later replaced (Fig. 15B). It is also possible that rollback major contractional zones in central and south- by right-slip faulting. The youngest phase of of the subducted Indian mantle lithosphere ern Tibet migrated southward, in contrast to the deformation in the region is east-striking left- was the cause of syncollisional magmatism inferred northward progression of deformation slip faulting and associated east-west extension. in central and southern Tibet (Harrison et in northernmost Tibet (Burchfi el and Royden, Our 40Ar/39Ar and U-Pb dating of volcanic and al., 2000; DeCelles et al., 2002; Ding et al., 1991; Meyer et al., 1998). Specifi cally, the plutonic rocks in the study area yields magma- 2003) (Fig. 15C). One may also argue that the Fenghuo Shan–Nangqian thrust belt in central tism at 51–49 and 38–37 Ma. The Paleogene observed thin-skinned deformation was accom- Tibet initiated in the Paleocene-Eocene (Horton magmatism clearly predates Neogene east-west modated by lower crustal fl ow (e.g., Royden et et al., 2002; Liu et al., 2003; this study), the extension, casting doubts on the proposed link al., 1997). However, it is not obvious to us how Shiquanhe-Gaize-Amdo thrust belt along the between Tibetan extension and syncollisional this process could generate partial melting in the Banggong-Nujiang suture zone further south magmatism. Geochemical analysis supports an mantle lithosphere. It is possible that the lower started in the Oligocene (see review by Yin and interpretation that the igneous rocks were gen- crustal fl ow was an important mechanism for Harrison, 2000), and the Gangdese thrust along erated by continental subduction. When placing

Geological Society of America Bulletin, September/October 2005 1315 SPURLIN et al.

Coward, M.P., Butler, R.W.H., Chambers, A.F., Graham, Horton, B.K., Yin, A., Spurlin, M.S., Zhou, J., and Wang, J., our results in the regional context, it appears that R.H., Izatt, C.N., Khan, M.A., Knipe, R.J., Prior, D.J., 2002, Paleocene-Eocene syncontractional sedimenta- Cenozoic contractional deformation propagates Treloar, P.J., and Williams, M.P., 1988, Folding and tion in narrow, lacustrine-dominated basins of east- northward north of Qaidam basin but southward imbrication of the Indian crust during Himalayan col- central Tibet: Geological Society of America Bulletin, lision: Philosophical Transactions of Royal Society of v. 114, p. 771–786. in central and southern Tibet. London, ser. A, v. A326, p. 89–116. Kapp, P.A., 2001, Tectonic evolution of the Qiangtang ter- Dalrymple, G.B., Grove, M., Lovera, O.M., Harrison, T.M., rane and Banggong-Nujiang suture zone, central Tibet ACKNOWLEDGMENTS Hulen, J.B., and Lanphere, M.A., 1999, Age and ther- [Ph.D. thesis]: Los Angeles, University of California, mal history of the Geysers Plutonic Complex (Felsite 285 p. Unit), Geysers Geothermal Field, California: Earth and Kapp, P.A., Yin, A., Manning, C.E., and Murphy, M.A., Discussions with Paul Kapp and Mike Taylor Planetary Science Letters, v. 173, p. 285–298. Harrison, T.M., Spurlin, M., Ding, L., Deng, X., and improved our understanding of Cenozoic deformation DeCelles, P.G., Robinson, D.M., and Zandt, G., 2002, Impli- Wu, C.M., 2000, Blueschist-bearing metamorphic core and igneous activities in central Tibet. We thank T. cations of shortening in the Himalayan fold-thrust complexes in the Qiangtang block reveal deep crustal Mark Harrison and Marty Grove for their insights and belt for uplift of the Tibetan Plateau: Tectonics, v. 21, structure of northern Tibet: Geology, v. 28, p. 19–22. advice regarding 40Ar/39Ar and U-Pb analyses. This no. 6, 1062. Kapp, P., Yin, A., Manning, C.E., Harrison, T.M., Taylor, work was supported by National Science Foundation Deng, W., 1989, Cenozoic volcanic rocks in the northern M.H., and Ding, L., 2003, Tectonic evolution of the (NSF) grants EAR-0106677 and EAR-0337191, and Ngari district of the Tibet—Discussion on the concur- early Mesozoic blueschist-bearing Qiangtang metamor- rent subduction: Acta Petrologica Sinica, v. 3, p. 1–11. phic belt, central Tibet: Tectonics, v. 22, no. 4, p. 1043. a NSF graduate research fellowship. 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