| Summary. Phillips et al. (2004) argue that by dating a pre-kinematic mylonitic leucogranite and a post-kinematic undeformed granite they can constrain the maximum and minimum age of fault initiation and the duration of activity in the most active strand of the Karakoram shear zone in Ladakh. This approach results in the strand being active for only less than 2 My, between 15.7 Ma and 13.7 Ma. Their conclusions imply that there are two different magmatic event spaced by 2 Ma, one pre- and the other post-tectonic, and imply movement rates larger than 20 mm/yr and more likely larger than 75mm/yr, to accommodate most of the estimated 40 to 150 km displacement on the fault. In my view, which supports Lacassin et al.'s view, the authors fail to recognize that their pre- and post-tectonic leucogranites are part of the same magmatic event, which continuously intruded an active shear zone making them a single syn-kimematic intrusion event. The dykes now display a range of strain intensities, which depends partly on the timing of intrusion and partly on grain size. Thus, the leucogranites date neither initiation nor cessation of movement on the main strand of the shear zone. Read on for more details. Magmatism and timing
of shearing Phillips
et al. (2004) argue that the
mylonitic leucogranites are pre-tectonic, pre-dating fault initiation.
Therefore, their crystallization ages should yield a maximum age for
shear zone initiation. Their
explanation for the pre-kinematic intrusion of the now mylonitic
leucogranites is given
for sample P1 in the last sentence in col. 1 p. 310: “There
is no
evidence for
high temperature deformation and there are no textural or structural
indicators
that suggest granite was emplaced coeval with fault initiation or
evolution.
Synkinematic textural criteria were not evident in the mylonitic
leucogranites
within the shear zones. We therefore assume that the granite was
emplaced
before fault initiation.” This conclusion is then extended by
analogy
to the
mylonitic granite of sample P11 (p. 311). Using a similar logic, they argue that undeformed leucogranite dykes are post-kinematic, post-dating cessation of movement on the particular strand of the shear zone they occur in. An implication of this line of thought, although never explicitly stated, is that there were two separate phases of leucogranite magmatism, one that immediately pre-dated shearing and one that post-dated shearing. This would require two separate phases of magmatism, of similar nature, and spaced in time by less than 2 My. I argue that the evidence for the syn-kinematic nature of granite intrusion is overwhelming not only at the outcrop at Tangtse (their Fig. 6a, b) and my main focus here, but also within the Tangtse section, bounded by the two bounding mylonitic strands of the shear zone: the Tangtse and the Pangong strands. In my view the authors failed to understand the syn-kinematic nature of the leucogranites for two main reasons: a) Grain size and mineralogy control strain localization during shearing.
Coarse-grained granites and pegmatites once solidiifec are generally poor recorders of deformation, in contrast to mica-rich granitic rocks, which tend to develop more obvious foliation than mica-poor granites. There is no mention of these possibilities in the text and information on the grain size or modal composition of the different dated rocks in the paper is missing. To compound this problem they do not to include scale in the photomicrographs in Fig. 7. In Tangtse, in the outcrop of their samples P8 and P11 (their Fig. 6a, b), differences in straining as a function of both grain size and modality can be demonstrated. Here, a folded dyke changes continuously from a pegmatite to a not visibly deformed pegmatite to a foliated fine- to –medium grained granite (Figure 1). Also, a large part of the outcrop is comprised of a muscovite-rich leucogranitic mylonite, with a very evident foliation, while elsewhere in the same outcrop, mica-poor leucogranites develop a less obvious foliation. |
|
Figure 1. Texturally composite folded dyke at the outcrop close to the Gompa at Tangtse (Phillips et al. 2004, Fig. 6a,b). The pegmatite section (centre) does not record a penetrative foliation, and grades to the right to a medium-grained foliated leucogranite, demonstrating the control of grain size on foliation development. |
| b)
Dykes
demonstrating a
continuum of deformation features between mylonitic and undeformed
The leucogranite intrusions in the outcrop at Tangtse (their Fig. 6a,b) record a continuum of deformation degrees from strongly to weakly deformed dykes. All degrees of deformation can be found. This is a typical feature of syn-kinematic intrusion and there are a number of features preserved in the dykes indicative of their syn-kinematic intrusion: 1. there is a full range between mylonites and weakly-deformed to undeformed dykes. This is in part a reflection of varying grain size, as argued above, but also a reflection of the time of intrusion, 2. in the outcrop of their Fig. 6a, b, there are dykes that crosscut the main foliation trend of the shear zone, but that develop shear zone-parallel foliation within them., 3. leucogranite and pegmatite dykes cut across the foliation when intruding competent amphibolites, and are disrupted into boudins, when crossing a contact into an incompetent marble, indicating solid-state deformation. However, when these dykes are coarse-grained to pegmatitic they do not show the development of foliation even though they are boudinaged (point a).
The structures preserved in the outcrop at Tangtse are complemented by those in the migmatites exposed in between the two shear zone strands in the Tangtse gorge a few kilometers away. In these migmatites, leucosomes, interpreted to represent the remains of melt, accumulate in boudin axis, form an interconnected network linking layer-parallel leucosomes to axial planar leucosomes, indicating that folding and melting occurred simultaneously. Folding in this area accommodated the pure shear component of the Karakoram transpressional shear zone.
A further aspect that argues for the magmatic intrusions to be part of the same continuous event rather than two separate magmatic phases, one before shear initiation and one after shear cessation are their close intrusion age, and similar leucogranitic composition, with similar mineralogy and modes, including garnet, muscovite and biotite as accessory phases.
Cessation
of shearing This question of strain localization is also intrinsically linked with their perception expressed in p. 317 paragraph 1 column 1 that “ the Tangtse strand must have been the dominant ductile fault strand for the vast majority of the shear zone history”. They refer in this paragraph to the Tangtse-Nubra fault strand as the one with a long history, concluding that any offset along the Pangong strand is negligible despite current activity and determined displacement rate of ~4mm/yr. Firstly there is no evidence for that either in their paper or in the references cited. Secondly, Dunlap et al. (1998) demonstrated using cooling histories that the Pangong strand has had a long and active history, and Brown et al. (2002) demonstrated that it is still active. In my view the entire 10km wide zone, inclduing the Pangong and Tangtse strands and the volume between them, accommodated shearing throughout the history of deformation. It behaved as one evolving entity and it is indeed movement within it as a whole that exhumed the upper amphibolite and granulite facies rocks of the Pangong Range, as a pop-up structure. Finally,
the main
conclusion in
the paper is that shearing in the Tangtse strand, interpreted by them
to be the
dominant ductile shear zone, started after 15.7 Ma but ceased before
13.7 Ma. Giving
it only less than 2My of activity in which to accommodate most of the
displacement on the shear zone. A very fast fault indeed! References Brown et al., 2002, Slip rates of the Karakorum fault, Ladakh, India, determined using cosmic ray exposure dating of debris flows and moraines, J. Geophys. Res., 107, Art. No. 2192. Dunlap et al., 1998, Karakoram fault zone rock cool in two phases, J. Geol. Soc. London, 155, 903-912. Lacassin et al., 2004, Large-scale geometry, offset and kinematic evolution of the Karakoram fault, Tibet, EPSL, 219, 255-269. |