Disclaimer:
This is a transcription of the notes I scribbled in my field book, with only minimal efforts to expand and correct the contents. Expect incomplete thoughts, insufficient explanations, and possibly outright mistakes! Some comments are my own, some are from the field trip leaders, and some are from other participants. I attribute comments to their speaker when it seemed relevant--e.g. when he or she proposed something that was not universally accepted by all present. There was plenty of healthy debate and discussion, and some participants held controversial views--which is part of what makes this is such an interesting region to study. I hope no one is bothered by statements I attribute to them, and I apologize for any misinterpretations on my part.
Photos:
Click photos for larger versions. Please acknowledge any use.
GPS waypoints:
A Google Earth compilation of the waypoints I took during the field trip is here. These waypoints are referenced in the text as WP###.
Acknowledgements:
I'd like to thank our hosts and the organizers of the field trip, Sandeep Singh and A.K. Jain, for putting together such an informative, thought-provoking and well-organized field excursion. My understanding of the region and its complex tectonic history grew immeasurably.
Field Guide:
The field guide companion to the trip is published as:
Geology and Tectonics of the Southeastern Ladakh and Karakoram
A. K. Jain and Sandeep Singh
Geological Society of India, P.B. No. 1922, Gavipuram, Bangalore 560 019
2009. xiv + 181 pp. Price: Rs 600/$60.
Itinerary:
Day 1: Drive Leh to Tso Morari
Day 2: Visit Tso Morari gneiss & eclogites
Day 3: Same, also Ordovician granites
Day 4: Drive Tso Morari to Tangste
Day 5: Visit Pangong Tso & Tangste outcrop of Karakoram Fault
Day 6: Visit Tangste outcrop & Tangste gorge
Start of the journey:
Drove into IYSZ (Indus Yarlung Suture Zone). Photo below shows red & green inter-bedded molasse from the Neo-Tethyan continental slope:
Drove into Ladakh Batholith. Golden granite! Some of the granite is intact and some is cut by a network of white leucogranite dikes from a second phase of magmatism.
WP002
Photo above is a conglomerate boulder from the Indus Group (with orange road paint on it). The Indus Group consists of terrestrial, fluvial sediments deposited after full closure of the Tethys at 50.5 Ma.
This conglomerate had granites in it, which must be from Asia, as well as ophiolitic material. This may imply that ophiolites were obducted onto the northern shore of the Neo-Tethys, which is a controversial view (ophiolites on the southern short are well-accepted). If the ophiolite was only obducted onto India, it would be difficult to get it into a conglomerate with granite from Asia. {diagram}
Mantle material is visible from this location.
A few minutes of driving later and we're on the other side of the hill and into mantle material. Pillow basalts on the side of the road!
Photo above is a view down the road, with approximate rock type locations, as described to me (click for larger versions).
Normal sequence: carbonates over pillow basalts over sheeted dikes over serpentine
Observed sequence: serpentine over carbonates over sheeted dikes over pillow basalts
The observed sequence in this location implies inverted stratigraphy followed by a through-cutting thrust fault. {diagram}
WP003
{diagram}
At this location we see the Zildat Fault, which is what I call a 'compressive normal fault' - a normal-sense fault in which the footwall is exhumed while the hanging wall stays under compression (e.g. the South Tibet Detachment, a normal fault in a compressive regime). In this case, serpentine from 100 km depth is exhumed to be in contact with Triassic unmetamorphosed carbonates from the the Indian margin. This serpentinized material is totally different (totally different chemical signature) from the ophiolitic serpentine we saw just a few minutes ago. It probably originated from deep in (and possibly the bottom of) the mantle wedge. I think this is the right photo:
Transit photos:
"Himalayan" terrain.
Arrival in camp.
Our camp on the shores of Tso Morari, viewed from a nearby ridge.
The metamorphic grade in this location is greenschist, thus less metamorphosed than the UHP core of the dome, which we will see later.
Albrecht Steck enthusiastically lectured on (in both senses of the word) the sheared sediments draped on top of the dome:
Top to the south shearing on the SW limb of the dome indicates exhumation of the dome (another example of 'exhumation normal faulting.') {diagram}
Eclogite: From rapid deep subduction and rapid exhumation. UHP rock derived from continental sediments (subducted Indian sediments in this case). Eclogite is ultra high pressure and moderate temperature, indicating very rapid subduction to great depth without temperature equilibration. Preservation of eclogite also requires rapid exhumation.
We are now in the Tso Morari dome, but in the relatively unstrained upper layers of the dome.
The dome is derived from the steeply and rapidly subducted leading edge of the Indian Plate, which, for some reason (buoyancy?) returned up along the subduction zone.
The Tso Morari is all Indian-derived material, stayed on the Indian side of the suture, and returned on the Indian side.
WP005
Note: Eclogite and eclogite facies are two different things. The eclogite facies is named after eclogite the rock, which is eclogitized mafic rock.
WP006
Eclogite sample containing garnet (pink dots rimmed with white):
Coesite visible, indicating burial to 100 km depth (27 kbar). Deepest structure in the whole Himalaya. Evidence of exhumation normal faulting, just like the STD.
Basalt sills injected into the old ordovician granites, then subducted. Basalt maybe from passive margin rifting? These are the source of the eclogite.
Field Guide stop D2-8
Lunch.
Same rocks as last time, more or less.
Orthogneiss: former igneous rock metamorphosed under high P-T to create extremely thin pseudo-bedding (see photo below). Paragneiss is from a sedimentary source. Orthogneiss is from an igneous source.
Shear bands show top-to-the-west shearing. N-S shearing is due to Tso Morarai exhumation. E-W shearing may have been earlier.
More eclogite:
Field Guide stop D2-9
WP007
Same rocks as last time, but gneisses are better exposed.
Large folds and extensive shear fabrics are visible:
WP008
Stefan says is has tons of volatiles (500 ppm arsenic, for example), so it is very different from the ophiolite. Brought up on the fault described at the end of Day 1--the Zildat Fault. This fault brings up mantle wedge material from ~100 km depth, after it has been enriched in volatiles from the fluids rising from the subducting Indian slab. {diagram p.19-20}
Only seen on the N side of the dome, supporting the explanation of mantle wedge material being entrained during Tso Morari exhumation.
This serpentine is not on Steck's map.
WP009
7.11 in field guide: geologic map
7.9 in field guide: description
Contact between Puga gneisses and Tanglang La.
Repairing a broken leaf spring:
Undeformed (no fabric) granites indicate lack of deformation of Tso Morari dome during either subduction or exhumation. Therefore, Tso Morari subduction and exhumation was low-stress. Deformation is localized to isolated shear bands.
Albrecht Steck says: "Crustal-type 2-mica granites intruded comagmatically with basaltic dikes." Mike Searle counters: "If true, it's extremely rare and possibly unique. Typical Himalayan granite is located nowhere near basalt, and has a 100% crustal signature with no mantle component. We are actually looking at Ordovician granite intruded by Permian basalt."
End member granites:
We see pyroxene in this granite, meaning it cannot be S-type 'Himalayan' granite. It must be an earlier A-type granite, according to Mike Searle. But it has been through the entire Himalayan deformation (UHP, silimanite-grade). To remain undeformed (as it is), it must have been effectively a massive boudin and strain was taken up by the surrounding country rock.
WP010
Controversy over shear direction: N or S? Photo below: some indicators show top-to-the-north, some show top-to-the-south [but I didn't note the orientation of photo].
Stefan's measurements were split 60/40, effectively equal amounts of top-to-the-north and top-to-the-south, possibly indicating flattening.
Field Guide stop D4/17
Pillow basalts in the Nidar ophiolite. Early Cretaceous, ~139 Ma.
WP011
Mixture of felsic & mafic magmas within the Ladakh complex.
Sandeep says no continental contribution in these rocks, or in any of the Ladakh "Batholith" (in which case it isn't a batholith, if a batholith is understood to be emplaced continentally).
So apparently these rocks are derived entirely from differentiation of oceanic material?? Keiko says yes, Kyle, too. Re-melting and melt-fractionation. {diagrams p.28-29}
Transit photo: Crossing Chang La.
Mike Searle and A.K. Jain spend some time arguing about the Karakoram Fault in this region: Jain says we have a suture zone and zero offset on the KF; M.S. says there's no evidence at all for a suture zone (no ophiolite, no deep sea sediments, etc.), and 40-120 km offset on the KF.
See: geologic map of Richard James Phillips 2004. [?]
{diagram p.32-34}
After the island arcs (Ladakh-Kohistan, Spong, plus others that may exist further east) accreted, the magmatism continues as continental granitic intrusions. These are the Trans-Himalayan Batholiths:
M.S. believes Spong ophiolite and arc accreted onto India, while the other arcs accreted onto Asia. This requires a transform fault between the Spong and Ladakh-Kohistan (Mark etc. say this is fine). Everyone agrees that Spong ophiolites obducted onto the southern shore, but it's less well accepted that the arc itself obducted onto the south.
Mike Searle: metamorphism here is ~100 Ma and is due to magmatism, not the fault. The fault does cause strain.
Four phases of metamorphism in this region, according to M.S.:
Marble.
Looked down the valley at the trace of the Karakoram Fault, as seen in Mike Searle's paper (see next stop). The trace of the fault goes straight (from this perspective) up the left side of the saddle (click for enlarged version without annotation). On the left side of the fault are granites and, further from the fault, deformed granite lenses. On the right side of the fault are dipping marbles.
WP012
Field Guide stop D5/29 (maybe)
{diagram p.41}
Everything here shows obvious dextral shear. Photo below is Karkoram Fault (top = north).
{diagram p.44-45: KF}
WP013
Field Guide stop D6/31, maybe
We are now between the two strands of the Karakoram Fault, in the uplifted Pangong block.
Photo below shows a single boulder with all stages of melt:
{diagram p.46}
Two types of granite: a dark (right side of photo below) and a light (far left of photo); the light forms dikes within the dark.
Mike Searle: "The dark granites are cretaceous, Andean-type granites subjected to metamorphism as a result of crustal thickening, turning them to orthogneisses.
"The light granites intruded as a result of Indian subduction and continental intrusion (causing a second round of heating-related metamorphism in the dark granites, resulting in granites [no idea what I intended when I wrote this])."
M.S. correlates the light granites with the Baltoro granite.
WP014
Photo below is the dike-cut granite shown on the left side of the previous photo.
a Mike Searle outcrop
WP014
An incredible exposure of the Karakoram Fault. The Karakoram Fault is more deeply exposed at Tangste than anywhere else. The brittle fault is plainly visible as cliffs, some with 20-30 degree dipping lineations (e.g. below the gompa, see photo).
From here the undeformed Ladakh batholith is to the south. To the north is a 1 km thick zone of mylonites from the ductile phase of faulting. Proto-lith is Asian plate margin.
See paper which uses this outcrop: Phillips, Parrish, Searle. Age constraints on ductile deformation and long-term slip rates along the Karakoram fault zone, Ladakh. EPSL, 2004.
From that paper:
Set 1 leucogranite dikes are 15.6 Ma.
Set 2 leucogranite dikes are 13.2 Ma.
Set 1 have shear fabric from KF ductile shear, meaning they were emplaced prior to ductile shearing, but not necessarily prior to initiation of shearing.
Set 2 are undeformed, implying that ductile shearing ended by 13.2 Ma in this region.
{diagram p.53}
These results say only that Karakoram Fault motion (mostly) ended in Tangste between 13.2 and 15.6 Ma. This says nothing about the rest of the KF.
WP016
After everyone studied the outcrop, Mark announces that may have found boudins in the set 2 granites (see above photo according to my notes, although I no longer see what I may have intended to capture), implying that the 13.2 granites are actually deformed by KF ductile shearing. [If true, this invalidates the hypothesis that KF fault motion ended here by 13.2 Ma.]
This was one of the dated samples from the paper.
Joe played devil's advocate: 'boudins and folds could occur during emplacement.' But shear fabrics are present?
After some discussion, there is no consensus either way, and in the end M.S. qualifies: "90% of ductile shearing is over by 13.2 Ma."
We are on the main/north strand of the Karakoram Fault. South of here is entirely Ladakh geology; the Ladakh granites (undated) and calc-volcanics. Strain disappears 100 m from the fault. North of here is 1 km of mylonites.
Photo below (facing north) shows lineations of the Karakoram Fault:
Photo below shows mylonites on the Searle/Phillips outcrop looking west:
Owen Green agrees with the Indians that it's chert, possibly Ordovician. Nodule chert. Chert necessitates a marine environment (Owen doesn't know of a fresh water source), but does not necessate a suture zone. Chert can come from volcanic sources, as is the explanation for the England chalks. And here we probably have volcanics (M.S. calls this the Kohistan-Dras volcanic arc; see Phillips 2004 map).
M.S. story compatible with presence of chert & absence of suture here.
WP017
Darbruk to Shyok road
Field Guide stop D7/34, probably
Entering the gorge:
South strand of the Karakoram Fault is at the mouth of the gorge. We've traveled north into the gorge into the Pangong block between the two strands of the Karakoram Fault.
Old metamorphosed granite, possible equivalent to the K2 orthogneiss. Shear fabrics visible.
Lots of talk about ductile shear fabrics.
SY44 on map 7.23 in Field Guide
Sandeep Singh: This is the Chillam granodiorite, ~44 Ma, but poorly constrained. Jain calls this the suture.
Above photo: Shear fabrics (running top to bottom in the photo [actually looks like the photo didn't rotate, so left to right]), cut by a body that disintigrates into the rock. Rick thinks this is convincing evidence of... something I didn't catch. Something broke the rock during crystallization.
M.S.: Ladakh Batholith is 103-44 Ma. Intrusion shuts off as soon as collision occurs.
Mike shows us sheared granite on the north side of the Tangste strand of the Karakoram Fault.
M.S.: Tangste granite with high temperature dextral shear fabric.
U-Pb age gives age of crystallization, 750-800 for granite.
M.S. suggests that other researchers have incorrectly used this age to date shearing. Mike says this is wrong; shearing must have occurred after crystallization.
The French counter: they agree with M.S. that at this outcrop, the shearing post-dates the 17 Ma U-Pb crystallization age.
You can determine the temperature of shearing by looking at which minerals or constituents are sheared and which are intact. For example,
Thus shearing occurred at 500 < T < 700 degrees, and if the U-Pb age corresponds to 750-800 degrees, then shearing occurred after this time.
WP018
Karkoram Fault is to the north. We are in the Ladakh block. Intruding Ladakh batholith is nearby.
This is the Indians' alleged suture zone. They want the Shyok to parallel the Karakoram Fault. This makes the Shyok suture zone continuous with no offset, which is how they determined zero offset on the Karakoram Fault.
The French want Shyok along the Karakoram Fault as well (see: Twenty million years of continuous deformation...).
Mike Searle does not want Shyok along the Karakoram Fault (see: my oversimplified diagram {p.44}, which has no Shyok adjacent to the Karakoram Fault).
Here we have pillow lavas (maybe?), conglomerates, cherts and tuffs. I.e. deep sea sediments. Karl calls this site a volcanic sequence. Chert can by hydrothermal. This is definitely sea floor. Breccias with gabbros & basalts. Turbidite. This is all reported in Rolland 2000 or 2004 in Chem. Geol., 56p! [I find the following, which must be it: Rolland, et al. The cretaceous Ladakh arc of NW himalaya-slab melting and melt-mantle interaction during fast northward drift of Indian Plate. Chemical Geology, 2002, 39p.]
M.S. says this seems convincing as a deep sea back arc.
WP019
A bit further in the Ladakh block.
Sandeep Singh says we have basalts and carbonates.
Alex says we have limestone, chert and fossils.
M.S. says this is not diagnostic of a suture.
Jim Dunlop mapped here, Kardun to ___________. Ar dating of granite. See: Dunlop & W________
WP020
Just a few meters up the road from Stop 2.
Gabbro here, which is the bottom of the oceanic crust sequence. More possible support for suture zone. "Suggestive" of ophiolite, according to Rick, but not necessarily so.
Two end-member scenarios:
What about a middle scenario...
We are seeing patches of possible suture material, although M.S. claims--perhaps perfectly correctly, but I wouldn't know--that this stuff is all over Pakistan and is not diagnostic of a suture.
So "B" may still be correct, but "A" or "C" are looking feasible (well, less so "A"). However, isolated roadside sampling without proper regional context is dangerous.
Mark says "C" is feasible. Entrainment of material in shear zones occurs at all scales. This would cause significant deformation--it seems to me--so again some good regional mapping is required.
Upon further reflection, "C" is not possible. We are too far away from the fault, and we don't see these rocks in the fault.
WP021
Definite mafic & ultramafic rocks: serpentine, dunite.
M.S. says this is definitely an ophiolite. But we are only 5-10 km from the Indus suture. Perhaps this is actually an extension of the IYS, intruded by the Ladakh batholith, separating it from the main IYS.
Owen's dating of the fossils may answer this question.
Goodbye, Leh.
Goodbye, Ladakh.
Goodbye, Himalaya.
