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Discussion |
1 1 CNRS, UMR 5570, Université de Lyon, Université Claud Bernard Lyon1, Ecole Normale Supérieure de Lyon, Laboratoire de Science de la Terre, UMR 5570 CNRS, 2 rue Dubois, Bat géode, la Doua, 69622 Villeurbanne, France (e-mail: herve.leloup{at}univ-lyon1.fr)
2 2 Tectonique et Mécanique de la Lithosphère, Institut de Physique du Globe de Paris, and Université Paris 7, CNRS, Paris, France
3 3 Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK (e-mail: mike.searle{at}earth.ox.ac.uk)
P. H. Leloup, P. Tapponnier & R. Lacassin write: In his recent paper, Searle (2006) acknowledges that the 1000 km long Ailao Shan–Red River shear zone is a large Miocene left-lateral shear zone, but speculates that left-lateral slip started after 21 Ma and claims that the total finite offset remains unknown. From this he concludes that continental extrusion was only a relatively minor tectonic factor during the India–Asia collision, as long argued by other workers (e.g. England & Houseman 1986; Cobbold & Davy 1988; Dewey et al. 1989; Houseman & England 1993). We summarize below the field and geochronological evidence that makes us maintain a viewpoint in better accordance with facts.
Timing of left-lateral shear along the Ailao Shan–Red River shear zone. The Ailao Shan–Red River shear zone is composed mostly of high-grade metamorphic rocks and deformed granitoids with ubiquitous evidence for left-lateral shear parallel to the belt (e.g. Tapponnier et al. 1986, 1990; Leloup et al. 1993, 1995, 2001). The crystallization of the granitoids has been dated between 22 and 35 Ma (Fig. 1d–f), for example by Schärer et al. (1990, 1994) and Zhang & Schärer (1999), leading those workers to propose that left-lateral shear started at least c. 35 Ma ago. In contrast, Searle (2006) claims that all the deformed granitoids found within the shear zone predate left-lateral shear and that their crystallization ages should thus be interpreted to provide an upper limit for the onset of deformation, rather than a lower limit, thus suggesting a maximum age of 21 Ma for this deformation. A clear understanding of the P–T history and in situ deformation history of the intrusions, as well as of their relationships with surrounding paragneisses, is fundamental for interpreting correctly the geochronological data.
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c
glide system in quartz (Leloup & Kienast 1993; Leloup et al. 1995) is also diagnostic of shearing temperatures close to the granitic solidus (e.g. Gapais & Barbarin 1986). This is clearly in contradiction with Searle's assertion that all left-lateral kinematic indicators are low-temperature fabrics.
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The age of high-temperature metamorphism associated with left-lateral shear is also constrained by U–Th/Pb dating of monazites within the mylonites (Gilley et al. 2003). In the Xuelong Shan, Diancang Shan and Ailao Shan ranges, 47 of the 50 ages of matrix monazite from 10 samples are between 19 and 34.5 Ma (Fig. 1b), demonstrating that high-temperature metamorphism lasted c. 16 Ma. This is confirmed by ages of monazite inclusions within garnets from the Ailao Shan (16 ages in six samples) that span the period between 21.5 and 34.5 Ma (Fig. 1b) (Gilley et al. 2003). The matrix and garnet inclusion monazites display the same age range, and inclusion and matrix ages overlap in each sample. This could hardly be the case if a hypothetical, early metamorphic event had been preserved within the garnet cores.
Monazites give a broader U–Th/Pb age pattern in the Daynuiconvoi range, from 21 to 208 Ma and from 43 to 224 Ma for matrix and inclusion monazites, respectively (Fig. 1c), showing that these rocks experienced a poly-phase metamorphic history (Gilley et al. 2003). The oldest ages probably correspond to the Indosinian (250–160 Ma) metamorphism and subsequent cooling documented in the nearby SongChay dome (Roger et al. 2000; Maluski et al. 2001; Gilley et al. 2003) and elsewhere in Indochina (e.g. Carter et al. 2001). However, matrix monazites show a clear age peak between 21.5 and 32.5 Ma (12 data from five samples), similar to the age range from the Ailao Shan, Diancang Shan and Xuelong Shan ranges, implying partial resetting of an Indosinian metamorphic assemblage during Oligo-Miocene deformation (Gilley et al. 2003).
After peak temperatures, left-lateral shear continued during cooling below 600 °C until greenschist conditions were reached (Harrison et al. 1992, 1996; Leloup et al. 1993, 1995, 2001; Nam et al. 1998; Jolivet et al. 2001). Because 39Ar/40Ar ages generally reflect cooling below a closure temperature (from c. 510 ± 50 °C for amphiboles to <200 °C for the less retentive domains of K-feldspars), the cooling and exhumation can be dated, yielding further indirect constraints on the timing of shearing. Of the 68 published 39Ar/40Ar amphibole and mica ages from the Ailao Shan–Red River shear zone, only three give ages older than 35 Ma, but 59 are older than 21 Ma (Harrison et al. 1992, 1996; Leloup et al. 1993, 2001; Wang et al. 1998, 2000; Maluski et al. 2001; Garnier et al. 2002). All 52 K-feldspars show ages younger than 25 Ma for their less retentive domains (with 28 
21 Ma), and only two show ages older than 35 Ma for their more retentive domains (Harrison et al. 1992, 1996; Leloup et al. 1993, 2001; Wang et al. 1998, 2000). Most of these data have been summarized by Leloup et al. (2001, plate 2 and table 7). These data show that parts of the shear zone started to cool as early as c. 35 Ma, and that the temperature had dropped below c. 250 °C before 21 Ma in the Xuelong Shan, the southern half of the Ailao Shan, the FanSiPang and the Daynuiconvoi ranges. If deformation had started at 21 Ma, there should not be any evidence for left-lateral ductile deformation in any of these ranges, which is not the case.
A more detailed analysis shows that different parts of the Ailao Shan–Red River shear zone have distinct cooling histories (Leloup et al. 2001). An early cooling phase (0 in Fig. 1a) occurred soon after 35 Ma in the FanSiPang and LoGam ranges in Vietnam, whereas the main phase of rapid cooling (I in Fig. 1a) lasted from 30 to c. 28 Ma in the Xuelong Shan range, from 23 to 20 Ma in the Diancang Shan, and from 27 to 23 Ma in the Daynuiconvoi. In the Ailao Shan range, this phase was diachronous along strike, lasting from c. 28 to 25 Ma in the NW, and from 21 to 17 Ma in the SE. This pattern led Harrison et al. (1996) and Leloup et al. (2001) to propose a ‘zipper’ kinematic model linking strike-slip tectonics and exhumation. This offers a simple explanation for the cooling pattern and yields an estimate of the left-lateral slip-rate along the Ailao Shan–Red River shear zone, which is compatible with the total offset and lifespan of the shear zone and with quantitative sea-floor spreading kinematics in the South China Sea (Briais et al. 1993).
When combined with structural work, the available geochronological data summarized in Figure 1 thus leave no doubt that left-lateral shear started at least around 35 Ma and lasted until c. 17 Ma. The proposal that the onset of shear did not occur until c. 27 Ma (Wang et al. 2001) is in contradiction with the cooling histories and structural data from both the Xuelong Shan and FanSiPang ranges, and with the ages of the oldest synkinematic leucocratic dykes and metamorphic monazites elsewhere. An onset of deformation after c. 21 Ma (Searle 2006) is in contradiction with cooling histories in all ranges except the Diancang Shan and the northern part of the Ailao Shan, as well as with the ages of all the synkinematic leucocratic dykes and metamorphic monazites.
Finite offsets across the Ailao Shan–Red River shear zone. Searle (2006) states that ‘none of the features cited by Leloup et al. (1995) are reliable markers and finite geological offsets along the Red River shear zone remain unknown’. Although there is no question that the Ailao Shan–Red River shear zone shear zone is not the small-scale type of brittle fault across which structural geologists have long been used to linking piercing points, there is compelling evidence for a total offset larger than 500 km (e.g. Leloup et al. 1995, 2001; Chung et al. 1997), and we recall here briefly the most important points of our argument.
It has been recognized for over 40 years (e.g. Huang 1960) that regional geological features cannot be matched simply across the Ailao Shan–Red River shear zone. Depending on the feature considered, large-scale apparent left-lateral offsets vary between >400 km and 1050 ± 100 km (e.g. Tapponnier et al. 1986; Leloup et al. 1995; Chung et al. 1997). Searle (2006) contests the 
650 km offset between the Nan–Uttaradit suture south of the fault and the Jinsha–Benzilan north of it. Because ultramafic slivers have been mapped SW (not NW as mentioned by Searle) of the Ailao Shan (Leloup et al. 1995), he instead proposes that the Song Ma suture has to be matched with the Jinsha and that ‘the Red River shear zone may follow the suture for part of its course in South Yunnan’. This is partly incorrect because the Ailao Shan ultramafic rocks are strongly affected by left-lateral shear (Leloup et al. 1995) and correspond to smeared pieces of a suture zone cut by the Ailao Shan–Red River shear zone, whereas the Song Ma crops out 120 km south of the main shear zone in northern Vietnam. The Ailao Shan–Red River shear zone did not follow a pre-existing suture, but instead the Jinsha suture has been partly drawn into parallelism to it because of intense deformation. Furthermore, matching the Song Ma suture, instead of the Nan–Uttaradit, with the Jinsha suture would also imply an offset 650 km.
The 40–29 Ma ultrapotassic igneous rocks that crop out in Tibet north of the Ailao Shan–Red River shear zone, along the structure itself (<20 km from the gneissic core) and farther south in Vietnam (Chung et al. 1998; Wang et al. 2001; Guo et al. 2005) define an apparent sinistral offset of c. 600 km between the JianChuan and FanSiPan magmatic provinces corresponding to motion younger than c. 35 Ma (e.g. Chung et al. 1997). This distance is only a lower bound of the total offset on the shear zone because a larger motion (c. 680 km) is needed to match the eastern boundaries of the two magmatic provinces.
Numerous palaeomagnetic studies, none of which are cited by Searle (2006), are consistent with clockwise rotation and 10 ± 3° of southward motion of Indochina with respect to South China since the Late Cretaceous (see Leloup et al. 2001a, and references therein). If the strike of the shear zone is assumed to have remained constant, this corresponds to 1400 ± 400 km of left-lateral displacement. A clockwise rotation of the fault zone would imply an even larger offset. In any case, the left-lateral offset at the end of left-lateral shear (c. 17 Ma) corresponds to the present-day apparent left-lateral offset augmented by the amount of later right-lateral offset along the Red River fault, which has been estimated to be between 6 and 57 km (Allen et al. 1984; Leloup et al. 1995; Replumaz et al. 2001; Schoenbohm et al. 2006).
Conclusions. The inference of an initiation of the Ailao Shan–Red River shear zone shear zone at 21 Ma does not hold in front of the geological and geochronological data. A large number of independent datasets, including geological offsets, structural, petrological and geochronological studies within the shear zone, palaeomagnetic measurements in South China and Indochina, magnetic anomalies in the South China Sea (Briais et al. 1993), and the timing of sedimentation and tectonic style in the YinGeHai pull-apart basin (Clift & Sun 2006), are consistent with our view that the Ailao Shan–Red River shear zone had a left-lateral sense of movement between c. 34 Ma and c. 17 Ma, a total finite offset larger than 500 km and slip rates of the order of 3–5 cm a–1.
The Ailao Shan–Red River shear zone and its timing are keys to debates on the rheology of the continental lithosphere in general and the nature of the India–Eurasia collision in particular. Large-scale Oligo-Miocene left-lateral motion along the Ailao Shan–Red River shear zone and Mio-Pliocene reversal to right-lateral faulting along the Red River fault cannot be explained by continuous deformation steadily accumulating in front of India (e.g. England & Houseman 1986; Houseman & England 1993), right-lateral shear along Tibet's eastern margin (e.g. Cobbold & Davy 1988; Dewey et al. 1989), or lower crustal outflow away from Tibet.
11 June 2007
Mike Searle replies: I thank Leloup, Tapponnier and Laccasin for their discussion reiterating their belief that the Red River shear zone was a long-lasting strike-slip shear zone responsible for synkinematic metamorphism, granite melting, and over 500–1050 km of left-lateral offset since 35 Ma. Tapponnier et al. (1990) and Leloup et al. (1993, 1995, 2001) have certainly produced a large amount of excellent data from the Red River shear zone, both in Yunnan and Vietnam, but their tectonic interpretation is not wholly consistent with many of the structural data combined with U–Th–Pb geochronology from granitoids along the Red River shear zone (Schärer et al. 1990, 1994; Zhang & Schärer 1999; Carter et al. 2001; Gilley et al. 2003).
The Red River shear zone consists of four major metamorphic complexes, the Xuelong Shan, Diancang Shan and Ailao Shan in Yunnan and the DoiNuiConVoi complex in northern Vietnam. They each contain high-grade metamorphic rocks and three types of granites: (1) calc-alkaline granodiorites–granites forming the protolith of many of the Red River shear zone gneisses; (2) mantle-derived amphibole–quartz monzonites–syenites; (3) crustal melt leucogranites containing biotite with rare garnet and tourmaline. The metamorphic massifs are bounded by late, steep normal + strike-slip faults which dip at high angles away from the massifs. Our differences of opinion can be summarized as follows.
(1) Do the 35–26 Ma U–Pb zircon ages from amphibole + quartz monzonite–syenite intrusions (Schärer et al. 1994; Zhang & Schärer 1999) constrain the age of initiation of strike-slip shear along the Red River shear zone (Leloup et al. 2001, and their Comment), or are they pre-kinematic (Searle 2006)?
(2) Is high-grade metamorphism along the Red River shear zone synchronous with strike-slip shearing (Leloup & Kienast 1993; Leloup et al. Comment), or pre-kinematic with respect to strike-slip shearing (Wang et al. 1998, 2000; Jolivet et al. 2001; Searle 2006; Anczkiewicz et al. 2007)?
(3) Are the deformed leucogranites along the Red River shear zone synkinematic with respect to strike-slip shearing (Schärer et al. 1990, 1994; Leloup et al. 1993, 1995, 2001; Zhang & Schärer 1999) or pre-kinematic (Searle 2006)?
(4) Are the 500–1050 km finite geological offsets proposed by Tapponnier et al. (1986) and Leloup et al. (1993, 1995, 2001) reliable, or not?
Age of initiation of left-lateral shear along the Red River shear zone. Leloup et al. (1995, 2001, and their Comment) constrain the age of initiation of left-lateral strike-slip shear along the Red River shear zone as synchronous with the U–Pb ages from the FanSiPan alkali granite–syenite in northern Vietnam (Zhang & Schärer 1999) and a sub-volcanic monzonite intrusion at Jianchuan, north of Dali, 50 km north of the Red River shear zone in Yunnan (Schärer et al. 1994). Zhang & Schärer (1999) dated nine titanite fractions from the FanSiPan alkali granite and obtained ages of 35.2 ± 0.4 Ma (207Pb/235U) and 35.0 ± 0.3 Ma (206Pb/238U), which they interpreted as dating emplacement of the alkali granite. The FanSiPan alkali granite–syenite is located 10 km SW of the Red River shear zone in northern Vietnam and separated from it by a massive, largely undeformed granodiorite (PoSen granodiorite) and by a series of regional high-grade marbles (Sapa ruby corundum + phlogopite marble). The FanSiPan massif is mostly undeformed and shows clear igneous textures, with a few later, minor left-lateral shear zones cutting through the massif. Structural relationships clearly show that both the regional Sapa metamorphism and the FanSiPan alkali syenite are not related to the Red River shear zone either spatially or temporally (Searle 2006).
Other alkaline igneous rocks dated by U–Pb include two quartz monzonites from the Ailao Shan (26.1–26.3 Ma) and a sub-volcanic monzonite c. 20 km away from the Red River shear zone, intruded into unmetamorphosed Eocene red-beds (35.0 ± 0.1 Ma; Schärer et al. 1994). Schärer et al. (1994) and Leloup et al. (2001, and their Comment) suggested that these ages date an early stage of left-lateral shear along the Red River shear zone. However, these alkali intrusions are not restricted to the Red River shear zone, and are related to a regional phase of extensive alkali igneous activity throughout Tibet and Yunnan, with ages ranging between c. 40 and 30 Ma (Chung et al. 1997, 2005). Both of the dated monzogranite–syenites from Mount FanSiPan and Jianchuan are undeformed, and the Ailao Shan monzonites are pre-kinematic with respect to the strike-slip fabric. None have any relationship to the Red River shear zone.
Relationship between metamorphism and shearing along the Red River shear zone: fabrics. Metamorphic rocks within the Red River shear zone include amphibolites, quartz-feldspathic gneisses, sillimanite + garnet schists, and migmatites. Most rocks contain mylonite fabrics with stretching lineations parallel to the strike of the shear zone. Secondary stretching lineations are at high angles to fold axes and indicate a component of normal faulting at right angles to the shear zone (Anczkiewicz et al. 2007). In northern Vietnam, fabrics within the DoiNuiConVoi gneisses are domed, flat-lying in the core with steeper dips along the margins. All fabrics within the DoiNuiConVoi are abruptly truncated by the two steep faults that bound the metamorphic complex, clearly indicating that the fabrics formed earlier than the strike-slip faults (Jolivet et al. 2001; Searle 2006; Anczkiewicz et al. 2007).
Leloup & Kienast (1993) and Leloup et al. (1993, 1995, 2001) presumed that high-grade metamorphism along the DoiNuiConVoi, as well as the Ailao Shan, Diancang Shan and Xuelong Shan, was synchronous with left-lateral shearing along the Red River shear zone. This model requires that: (1) metamorphism is restricted to the Red River shear zone; (2) P–T conditions of metamorphism were acquired at the time of strike-slip shearing; (3) the heat source was from shear heating along the fault (Leloup & Kienast 1993). However, Gilley et al. (2003) used a time-dependent conductive thermal model to show that the heat supply from strike-slip shearing would be insufficient to raise temperatures high enough to record the P–T conditions known from the DoiNuiConVoi (c. 750 °C; 6 kbar; Nam et al. 1998; Leloup et al. 2001; Anczkiewicz et al. 2007). Jolivet et al. (2001) also showed that left-lateral fabrics within the DoiNuiConVoi gneisses were always post-sillimanite growth and synchronous with crystallization of biotite. Anczkiewicz et al. (2007) described microstructural features within the DoiNuiConVoi gneisses compatible with deformation temperatures in the amphibolite facies of 550–500 °C. It seems probably that the P–T conditions of metamorphism relate to earlier metamorphic conditions, prior to strike-slip shearing.
Leloup et al. also suggest that activation of the prismatic c glide system in quartz is diagnostic of fabrics in the Red River shear zone. However, all except one (fig. 18i) of the quartz c-axis figures of Leloup et al. (2001, fig. 18; p. 42) are compatible with lower temperature strain (middle amphibolite facies) in the a glide system (R. D. Law, pers. comm.). Anczkiewicz et al. (2007) also presented quartz preferred orientation measurements that show dislocation creep on a planes consistent with amphibolite-facies conditions. Deformation fabrics within Red River shear zone gneisses are clearly high temperature, but still record lower temperature than melting of the granites. Hence U–Pb ages of deformed granites must be pre-strike-slip shearing.
Relationship between metamorphism and shearing along the Red River shear zone: Th–Pb ages of Red River shear zone gneisses. Leloup et al. state that the age of high-temperature metamorphism is constrained by U–Th–Pb dating of monazites from Red River shear zone gneisses. However, the data from Carter et al. (2001) and Gilley et al. (2003) show evidence that metamorphism could be as old as Triassic (Indosinian orogeny). Leloup is a co-author of the latter paper, but chooses to ignore data that do not fit with the model of Leloup et al. Gilley et al. (2003) stated that matrix monazites and monazite inclusions from DoiNuiConVoi gneisses range from c. 220 to 21 Ma. Geochronology and cooling history of the DoiNuiConVoi gneisses are very different from those in the other Red River shear zone metamorphic massifs further north in Yunnan. Along the entire Red River shear zone in Vietnam there is evidence that metamorphism was older than shearing. At Bao Yen, Th–Pb ages of monazite inclusions within garnets are 168 ± 6 Ma, 128 ± 2 Ma, 81.2 ± 3.2 Ma and 27.5 ± 0.3 Ma (Gilley et al. 2003). Gilley et al. interpreted the data as reflecting partial resetting of pre-existing metamorphism. Equally, it could be that young matrix monazite ages could have resulted from diffusive Pb loss (e.g. Smith & Giletti 1997; Cherniak & Watson 2000). At Luc Yen, multiple spots on a single monazite crystal yielded a wide range of ages from 177 to 31 Ma. Tertiary monazite ages were recorded only within garnet rims. At Yen Bai, elongated (strained) garnets have monazite inclusions with ages of 169 ± 3 Ma and 66.3 ± 2.4 Ma and another with ages ranging from 75.3 to 43.5 Ma; matrix monazites were 29–23 Ma (Gilley et al. 2003). These data suggest that the garnets were pre-kinematic with relation to strike-slip shearing. High-temperature ductile shear strain after peak metamorphism resulted in elongation of the garnet. At Viet Tri, ages of monazite inclusions are 220 ± 3.5 Ma and those of matrix monazites range from 208 to 21 Ma; at Ninh Binh matrix monazites span ages of 107–32 Ma (Gilley et al. 2003). Sm–Nd garnet cooling ages of 40 Ma (Anczkiewicz & Thirlwall 2003) also suggest earlier metamorphism in DoiNuiConVoi gneisses.
It is possible that K–Ar hornblende (34–27 Ma), muscovite (33–24 Ma), and biotite (26–23 Ma) ages (Harrison et al. 1992, 1996; Wang et al. 1998, 2000; Leloup et al. 2001) plus recently reported apatite fission-track ages (30–25 Ma; Anczkiewicz et al. 2007) from Red River shear zone rocks may record timing of transpressional exhumation, if they are used as a proxy for rocks cooling above the ductile–brittle transition.
Relationship between leucogranites and shearing along the Red River shear zone. Perhaps the most contentious issue is whether the deformed leucogranites along the Red River shear zone were pre-kinematic or synkinematic with relationship to strike-slip shearing.
Schärer et al. (1990, 1994) and Zhang & Schärer (1999) presented U–Pb ages of 22.4 ± 0.2 Ma and 24.1 ± 0.2 Ma from leucogranites from the Ailao Shan and ages of 22.4 ± 0.4 Ma and 24.7 ± 0.2 Ma from the Diancang Shan. A leucogranite dyke from the Xuelong Shan is somewhat older with 10 Ti–U oxide fractions defining a concordant age of 33.1 ± 0.2 Ma (Zhang & Schärer 1999). Leloup et al. (1995, 2001, and their Comment) interpreted the U–Pb zircon and monazite ages from the leucogranites as dating left-lateral strike-slip shearing along the Red River shear zone. On the other hand, Searle (2006) suggested that the majority of the leucogranites along the Red River shear zone were pre-kinematic, and deformed at high temperatures (not low temperature as misquoted in Leloup et al.'s Comment) but in the solid-state ductile regime, after crystallization of the granite.
During subsequent fieldwork in the Diancang Shan and Ailao Shan in 2007 many examples of folded and boudinaged fabrics in leucogranites within gneisses were found. The critical observation that both amphibolite layers and leucogranite layers show similar boudinage textures suggests that high-temperature ductile strain was imposed in solid state after both metamorphism and leucogranite intrusion. However, critical outcrops at Yuanjiang in the Ailao Shan show at least three sets of leucogranites, one pre-kinematic showing elongated, boudinaged sills with fabrics and margins that parallel the surrounding gneisses, and also two later sets that cross-cut both the gneisses and early layer-parallel sills. The later set are undeformed and clearly cut the ductile fabric related to left-lateral strike-slip shearing. Thus at Yuangjiang there is good field evidence for the majority of granodiorite and leucogranite sills being pre-strike slip shearing, but there is also an important, but minor, set of late cross-cutting dykes that are clearly post-kinematic with respect to the strike-slip shearing.
Leloup et al. (fig. 2) show ‘field evidence for several generations of syn-left-lateral shear leucocratic dykes’. These fabrics, however, do not show synkinematic features at all. Most examples show high-temperature solid-state ductile fabrics that must have been imposed after crystallization of the leucogranite melt. U–Th–Pb ages of leucogranites, which record the time the first melt crystallized from a liquid, must therefore be older than the strike-slip fabric. Their figure 2a shows three generations of granite sills or dykes. Set 1 sills are boudinaged in high-temperature solid-state conditions, as are all four sets in their figure 2b and the boudinaged amphibolite layer in their figure 2c. The later sets 2 and 3 dykes in their figure 2a cut across the ductile strike-slip fabric, and are presumably younger than strike-slip shear fabrics. These late cross-cutting dykes at Yuanjiang are apparently the same as the sample (YU-4a-00) dated by Gilley et al. (2003, p. 14-10) at 21.7 ± 0.2 Ma. This suggests that the ductile shear fabric that is dominant along the Ailao Shan must have been older than 21.7 ± 0.2 Ma. In the Diancang Shan a late, cross-cutting aplitic granite has a U–Pb age of 24.7 ± 0.2 Ma from three zircon fractions (Zhang & Schärer 1999), suggesting that ductile shear had ended by that time in that massif.
Finite geological offsets along the Red River shear zone. Leloup et al. restate their case for large finite geological offsets, but provide no new data for more precise pinning points. My statement that ‘none of the features cited by Leloup et al. (1995) are reliable markers and finite geological offsets along the Red River shear zone remain unknown’ remains true. Discontinuous serpentinized ultramafic rocks provide no reliable pinning point; neither do regional 40–30 Ma alkaline intrusions (Chung et al. 1997, 2005; Wang et al. 1998, 2000). Palaeomagnetic data provide evidence for rotations, not pinning points for fault offsets. The correlation between the Sichuan and Khorat red-bed basins cannot possibly be correct; they are both >100 km away from the Red River shear zone and have different depositional histories. There remain no accurate offset data for the Red River shear zone, unlike the precise pinning points of well-mapped and dated granites along the Karakoram fault (Phillips et al. 2004; Phillips & Searle 2007; Searle & Phillips 2007).
Conclusions. Structural and U–Th–Pb geochronological data from the Red River shear zone show that the deformation history of this shear zone is considerably more complex than shown in the model of Leloup et al. (1993, 1997, 2001, and their Comment). My original conclusions (Searle 2006) remain mostly valid, except for the newer constraints on timing of ductile shear, constrained by U–Th–Pb ages of a few cross-cutting dykes. High-temperature metamorphism within the DoiNuiConVoi gneisses in Vietnam could be as old as Triassic in age, although there is some evidence for amphibolite–greenschist-facies overprint during the late Eocene–early Oligocene, depending on how one interprets the wide range of Th–Pb monazite age data. In the DoiNuiConVoi in Vietnam, Th–Pb matrix monazite ages range from c. 220 to 44 Ma, and in the Ailao Shan they range from 74 to 16 Ma (Gilley et al. 2003). These ages cannot be related to strike-slip shearing or shear heating along the Red River shear zone. Initiation of left-lateral strike-slip shear along the Red River shear zone cannot be constrained by U–Pb ages from the Jianchuan or FanSiPan alkali monzogranite–syenites, both of which are undeformed, pre-kinematic alkali igneous intrusions and are located outside the Red River shear zone. There is no need to extend the Red River shear zone down into the mantle.
All calc-alkaline granites and most leucogranites along the Red River shear zone are pre-kinematic showing high-temperature ductile fabrics, boudinage and folding imposed after crystallization of the melt (Searle 2006). However, a few narrow leucogranite dykes are undeformed and cross-cut both gneissic fabric and early deformed leucogranites. Using the U–Th–Pb age data of Schärer et al. (1990, 1994), Zhang & Schärer (1999) and Gilley et al. (2003), the age of ductile shearing along the Red River shear zone, at least in the Ailao Shan and Diancang Shan, can be constrained more accurately as occurring during a narrow time interval between early deformed leucogranites (31.9–24.2 Ma) and later cross-cutting dykes (21.7 Ma). Using the new ages of Sassier et al. (2006) reported in Leloup et al.'s figure 2d, the age of ductile shear at Yuanjiang can be constrained as between 29.9 Ma (age of deformed leucogranites parallel to the shear fabric)–26.2 Ma and 22.5 Ma (age of undeformed cross-cutting leucogranite dykes). Brittle strike-slip faulting continued afterwards, cutting all metamorphic and granitic rocks. 40Ar–39Ar–K–Ar ages could be used as a proxy for timing of cooling during transpressional–transtensional exhumation along the bounding faults of the Red River shear zone.
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28 July 2007
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type feldspars. Modified from