Recent interpretations of Himalayan–Tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the Tibetan plateau1,2,3, and that ductile extrusion of high-grade metamorphic rocks between coeval normal- and thrust-sense shear zones can explain exhumation of the Greater Himalayan sequence4,5,6,7. Here we use coupled thermal–mechanical numerical models to show that these two processes—channel flow and ductile extrusion—may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low-viscosity material. Our models provide an internally self-consistent explanation for many observed features of the Himalayan–Tibetan system8,9,10.

Himalayan tectonics explained by extrusion of a low-viscosity crustal channel coupled to focused surface denudation

http://www2.ess.ucla.edu/~yin/05-Publications/papers/091-Yin-2006-ESR.pdf

Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation

https://stuff.mit.edu/afs/athena.mit.edu/course/12/12.115/www/lister%201989%20the%20origin%20of%20metamorphic%20core%20complexes%20and%20detachment%20faults%20formed%20during%20tertiary%20continental%20extension%20in%20the%20northern%20colorado%20river%20region.PDF

The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region, U.S.A.

Expression of active tectonics in erosional landscapes

http://www.igg.cas.cn/xwzx/yjcg/200910/W020091020357735122250.pdf

Zircon U-Pb geochronology and Hf isotopic constraints on petrogenesis of the Gangdese batholith, southern Tibet

Structural, thermobarometric, and thermochronologic investigations of the Kangmar Dome, southern Tibet, suggest that both extensional and contractional deformational histories are preserved within the dome. The dome is cored by an orthogneiss which is mantled by staurolite + kyanite zone metasedimentary rocks; metamorphic grade dies out up section and is defined by a series of concentric kyanite-in, staurolite-in, garnet-in, and chloritoid-in isograds. Three major deformational events, two older penetrative events and a younger doming event, are preserved. The oldest event, D1, resulted in approximately E-W trending tight to isoclinal folds of bedding with an associated moderately to steeply north dipping axial planar foliation, S1. The second event, D2, resulted in a high strain mylonitic foliation, S2, which defines the domal structure, and an associated approximately N-S trending stretching and mineral alignment lineation. Shear sense during formation of S2 varied from dominantly top S shear on the south dipping flank of the dome to top N shear on the north dipping flank. The central part of the dome exhibits either opposing shear sense indicators or symmetric fabrics. Microtextural relations indicate that peak metamorphism occurred post-D1 and pre- to early D2 deformation. Quantitative thermobarometry yields peak metamorphic conditions of ∼445°C and 370 MPa in garnet zone rocks, increasing to 625°C and 860 MPa in staurolite + kyanite zone rocks. Pressures and temperatures increase with depth and northward within a single structural horizon across the dome and the apparent gradient in pressure is ∼20% of the expected gradient, suggesting that the rocks were subvertically shortened after the pressure gradient was frozen in. Mica 40Ar/39Ar thermochronology yields 15.24 ± 0.05 to 10.94 ± 0.30 Ma cooling ages that increase with depth and young northward within a single structural horizon across the dome. Diffusion modeling of potassium feldspar 40Ar/39Ar spectra yield rapid cooling rates (∼10–30°C/Myr) between ∼11.5 and 10 Ma and apatite fission track ages range from 7.9 ± 3.0 to 4.1 ± 1.9 Ma, with a mean age of ∼5.5 Ma. Both data sets show symmetric cooling across the dome between ∼11 and 5.5 Ma. The S2 mylonitic foliation, peak metamorphic isobars and isotherms, and mica 40Ar/39Ar isochrons are domed, whereas potassium feldspar 40Ar/39Ar and apatite fission track isochrons are not, suggesting that doming occurred at ∼11 Ma. Our data do not support simple, end-member metamorphic core complex-type extension, diapirism, or duplex models for gneiss dome formation. Rather, we suggest that the formation of extensional fabrics occurred within a zone of coaxial strain in the root zone of the Southern Tibetan Detachment System (STDS), implying that normal slip along the STDS and extensional fabrics within the Kangmar Dome were the result of gravitational collapse of overthickened crust. Subsequent doming during the middle Miocene is attributed to thrusting upward and southward over a north dipping ramp above cold Tethyan sediments. Middle Miocene thrust faulting in the Kangmar Dome region is synchronous with continued normal slip along the STDS and thrust motion along the Renbu Zedong thrust fault, suggesting that extension and contraction was occurring simultaneously within southern Tibet.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999TC001147

Evolution of the Kangmar Dome, southern Tibet: Structural, petrologic, and thermochronologic constraints

We use new space geodetic data from very long baseline interferometry and satellite laser ranging combined with other geodetic and geologic data to study contemporary deformation in the Basin and Range province of the western United States. Northwest motion of the central Sierra Nevada block relative to stable North America, a measure of integrated Basin and Range deformation, is 12.1±1.2 mm/yr oriented N38°W±5° (one standard error), in agreement with previous geological estimates within uncertainties. This velocity reflects both east-west extension concentrated in the eastern Basin and Range and north-northwest directed right lateral shear concentrated in the western Basin and Range. Ely, Nevada is moving west at 4.9±1.3 mm/yr relative to stable North America, consistent with dip-slip motion on the north striking Wasatch fault and other north striking normal faults. Comparison with ground-based geodetic data suggests that most of this motion is accommodated within ∼50 km of the Wasatch fault zone. Paleoseismic data for the Wasatch fault zone and slip rates based on seismic energy release in the region both suggest much lower slip rates. The discrepancy may be explained by some combination of additional deformation away from the Wasatch fault itself, aseismic slip, or a seismic rate that is anomalously low with respect to longer time averages. Deformation in the western Basin and Range province is also largely confined to a relatively narrow boundary zone and in our study area is partitioned into the eastern California shear zone, accommodating 10.7±1.6 mm/yr of north-northwest directed right-lateral shear, and a small component (∼1 mm/yr) of west-southwest - east-northeast extension. A slip rate budget for major strike-slip faults in our study area based on a combination of local geodetic or late Quaternary geologic data and the regional space geodetic data suggests the following rates of right-lateral slip: Owens Valley fault zone, 3.9±1.1 mm/yr; Death Valley-Furnace Creek fault zone, 3.3±2.2 mm/yr; White Mountains fault zone in northern Owens Valley, 3.4±1.2 mm/yr; Fish Lake Valley fault zone, 6.2±2.3 mm/yr. In the last few million years the locus of right-lateral shear in the region has shifted west and become more north trending as slip on the northwest striking Death Valley-Furnace Creek fault zone has decreased and is increasingly accommodated on the north-northwest striking Owens Valley fault zone.

/pdf/constraints-on-present-day-basin-and-range-deformation-from-4ixa3q8a2p.pdf

Constraints on present‐day Basin and Range deformation from space geodesy

New ion microprobe U/Pb dates from zircon in deformed orthogneiss and migmatite and an undeformed granite in Mabja Dome are the first to constrain the timing of peak metamorphism, and onset and duration of mid-crustal ductile extension, in southern Tibet at 35.0 ± 0.8 Ma and ∼12–19 million years. The structural, metamorphic, and intrusive his tories in mid-crustal rocks exposed in these north Himalayan gneiss domes are similar to those recorded in the Greater Himalayan sequence, suggesting that middle crust was continuous from beneath southern Tibet southward to the high Himalaya. Strain compatibility indicates that 35 Ma ductile extension in mid-crustal rocks of southern Tibet was accommodated to the south at shallow crustal levels via normal slip along the southern Tibetan detachment system, the oldest age estimate for slip along this normal fault zone, and extrusion of its footwall. If gravitational collapse is an additional important process driving extension, then southernmost Tibet may have been at or near maximum elevation by late Eocene–early Oligocene time.

/pdf/onset-of-mid-crustal-extensional-flow-in-southern-tibet-2xhq7suuk7.pdf

Onset of mid-crustal extensional flow in southern Tibet: Evidence from U/Pb zircon ages

https://www.geology.cwu.edu/facstaff/lee/publications/pdf.papers/MD.JSG04.pdf

Evolution of North Himalayan gneiss domes: structural and metamorphic studies in Mabja Dome, southern Tibet

Active microplate boundaries in ocean-continent subduction zones may induce deformation of the overlying plate and spatial or geochemical variations in the volcanic arc. We discuss two modern cases. The first is the South Gorda-Juan de Fuca plate boundary in the Cascadia subduction zone, where there is little or no effect on the overriding plate and the oceanic plate takes up much of the deformation. The second case is the Cocos-Rivera plate boundary in the Middle America trench, where the overlying Colima graben contains substantial deformation in a zone extending from the trench to the volcanic arc and the sub-duction-related volcanism is spatially and geochemically complex. We apply these observations to boundaries of the Arguello, Monterey, Guadalupe, and Magdalena microplates, which existed in the subduction zone west of Baja California at various times from 20 to 12.5 Ma. The past positions of these boundaries relative to Baja California are constrained by global plate reconstructions, closure of the Gulf of California, and an estimate of extension in the Mexican Basin and Range province. Existing regional mapping and our additional reconnaissance mapping show that Paleocene to Eocene fluvial and marine sedimentary rocks south of Ensenada along the western Baja California peninsula and eastward to the mid-Miocene volcanic arc are undeformed. Limited available data reveal no major spatial or geochemical variations in the mid-Miocene volcanic arc that might correlate with the past positions of the microplate boundaries. Thus these microplate boundaries had little to no effect on the overriding continental plate. The nature of Guadalupe and Magdalena interactions with North America may have been closer to the South Gorda-Juan de Fuca example, with possible internal deformation of the microplates. The Monterey and Arguello microplates may have behaved like the modern Explorer plate, with largely strike-slip motion relative to North America during their last stages of existence. Tectonic patterns similar to these examples may be expected from other plate boundaries where a plate is fragmenting as it enters a subduction zone (e.g., the Aluk plate in the trench beneath West Antarctica in early Tertiary time). Whether these microplates subsequently become attached to the overriding continental plate or to a larger oceanic plate and whether this causes deformation in the region of the former subduction zone may depend on the velocities of the nearby major plates and the relative orientations of the microplate boundaries.

/pdf/do-microplates-in-subduction-zones-leave-a-geological-record-1xklsc0b2o.pdf

Jeffrey Lee

Papers

Evolution of the Kangmar Dome, southern Tibet: Structural, petrologic, and thermochronologic constraints

Constraints on present‐day Basin and Range deformation from space geodesy

Onset of mid-crustal extensional flow in southern Tibet: Evidence from U/Pb zircon ages

Evolution of North Himalayan gneiss domes: structural and metamorphic studies in Mabja Dome, southern Tibet

Do microplates in subduction zones leave a geological record