A review of the geologic history of the Himalayan-Tibetan orogen suggests that at least 1400 km of north-south shortening has been absorbed by the orogen since the onset of the Indo-Asian collision at about 70 Ma. Significant crustal shortening, which leads to eventual construction of the Cenozoic Tibetan plateau, began more or less synchronously in the Eocene (50–40 Ma) in the Tethyan Himalaya in the south, and in the Kunlun Shan and the Qilian Shan some 1000–1400 km in the north. The Paleozoic and Mesozoic tectonic histories in the Himalayan-Tibetan orogen exerted a strong control over the Cenozoic strain history and strain distribution. The presence of widespread Triassic flysch complex in the Songpan-Ganzi-Hoh Xil and the Qiangtang terranes can be spatially correlated with Cenozoic volcanism and thrusting in central Tibet. The marked difference in seismic properties of the crust and the upper mantle between southern and central Tibet is a manifestation of both Mesozoic and Cenozoic tectonics. The form...

Geologic Evolution of the Himalayan-Tibetan Orogen

Seismic techniques provide the highest-resolution measurements of the structure of the crust and have been conducted on a worldwide basis. We summarize the structure of the continental crust based on the results of seismic refraction profiles and infer crustal composition as a function of depth by comparing these results with high-pressure laboratory measurements of seismic velocity for a wide range of rocks that are commonly found in the crust. The thickness and velocity structure of the crust are well correlated with tectonic province, with extended crust showing an average thickness of 30.5 km and orogens an average of 46.3 km. Shields and platforms have an average crustal thickness nearly equal to the global average. We have corrected for the nonuniform geographical distribution of seismic refraction profiles by estimating the global area of each major crustal type. The weighted average crustal thickness based on these values is 41.1 km. This value is 10% to 20% greater than previous estimates which underrepresented shields, platforms, and orogens. The average compressional wave velocity of the crust is 6.45 km/s, and the average velocity of the uppermost mantle (Pn velocity) is 8.09 km/s. We summarize the velocity structure of the crust at 5-km depth intervals, both in the form of histograms and as an average velocity-depth curve, and compare these determinations with new measurements of compressional wave velocities and densities of over 3000 igneous and metamorphic rock cores made to confining pressures of 1 GPa. On the basis of petrographic studies and chemical analyses, the rocks have been classified into 29 groups. Average velocities, densities, and standard deviations are presented for each group at 5-km depth intervals to crustal depths of 50 km along three different geotherms. This allows us to develop a model for the composition of the continental crust. Velocities in the upper continental crust are matched by velocities of a large number of lithologies, including many low-grade metamorphic rocks and relatively silicic gneisses of amphibolite facies grade. In midcrustal regions, velocity gradients appear to originate from an increase in metamorphic grade, as well as a decrease in silica content. Tonalitic gneiss, granitic gneiss, and amphibolite are abundant midcrustal lithologies. Anisotropy due to preferred mineral orientation is likely to be significant in upper and midcrustal regions. The bulk of the lower continental crust is chemically equivalent to gabbro, with velocities in agreement with laboratory measurements of mafic granulite. Garnet becomes increasingly abundant with depth, and mafic garnet granulite is the dominant rock type immediately above the Mohorovicic discontinuity. Average compressional wave velocities of common crustal rock types show excellent correlations with density. The mean crustal density calculated from our model is 2830 kg/m3, and the average SiO2 content is 61.8%.

Seismic velocity structure and composition of the continental crust: A global view

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

Delamination and delamination magmatism

International Deep Profiling of Tibet and the Himalaya (INDEPTH) deep seismic reflection profiles show that the Indian lithosphere is underthrusting the central Himalaya along a gently north dipping decollement that is traceable northward beneath the Tethyan belt to ∼28.6°N and to a depth of about 45 km. The decollement carries in its hanging wall a near-crustal-thickness slice of internally deformed Indian continental basement and cover represented in outcrop by the Greater Himalayan belt and structurally overlying Tethyan belt. Geometric relationships suggest that the decollement probably ramps downward beneath the northern Tethyan belt to near the base of the crust and that the hanging wall crustal slice was detached from Indian mantle lithosphere that presently underlies southern Tibet to the north. The North Himalayan anticlinorium (Kangmar dome) appears to be a large duplex ramp anticline in the hanging wall of the decollement. The South Tibetan Detachment appears to have been imaged in two areas. In the Wagye La area it appears to follow the Tethyan belt/Greater Himalayan belt contact northward in the subsurface and to have been folded over the Kangmar dome. In the Zherger La area it appears to have been active relatively recently as a ∼30° north dipping normal fault that cuts deeply into the Greater Himalayan belt allochthon. Palinspastic reconstruction indicates a minimum of about 326 km of shortening of the Indian basement across the Himalaya since the initiation of the Main Central Thrust. Hence Indian continental lithosphere, largely stripped of its overlying crust, can extend northward beneath the Tibetan plateau to at least 32°N, where earthquake seismological observations show that the properties of the upper mantle change markedly [e.g., McNamara et al., 1995]. Taken together, the seismic reflection and earthquake seismological observations lend support for the view that the Indian mantle lid mechanically underplates roughly the southern half of the Tibetan plateau. The INDEPTH reflection data do not show that Indian crust attached to this lithosphere extends north of southernmost Tibet (Kangmar dome), nor do they negate this possibility. Similarly, the reflection data do not yield direct evidence for or against lower-crustal subduction beneath the Himalaya.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98TC01314

Crustal structure of the Himalayan orogen at ∼90° east longitude from Project INDEPTH deep reflection profiles

Comparison of Moho geometry and reflection character of active collision zones, like the Alps, with those of fossil collision zones, like the Appalachians, lends support for the view that lithospheric delamination is both a common and fundamentally important component of collisional orogeny. In particular, the relative crustal thinning commonly observed beneath the internides of old collisional orogens is an expected consequence of delamination. Neither precollisional crustal thickness variations nor an unrelated postcollision extensional event is required to produce this feature. Acceptance of the delamination hypothesis, in turn, fundamentally alters one's view of the tectonic evolution of old collisional orogens. In the case of the Appalachians, delamination has direct implications for the interpretation of Alleghanian granites, the direction of subduction leading to final closure between Laurentia and Gondwana, and the thermal state of the lithosphere at the onset of Atlantic rifting. A corollary of the delamination hypothesis is that lateral reflectivity variations cannot be used to define paleogeographic terranes in the deep crust of old orogens. Conversely, the observation that delamination leaves a distinctive geophysical imprint on the crust itself provides an important additional clue for deciphering the tectonic evolution of such regions.

Are crustal thickness variations in old mountain belts like the Appalachians a consequence of lithospheric delamination

The COCORP 40°N Transect of the Cordillera of the western United States crosses tectonic features ranging in age from Proterozoic to Recent and provides an acoustic cross-section of a complex orogen affected by extension, compression, magmatism, and terrane accretion. The key features of the transect, centered on the Basin and Range Province, include (1) asymmetric seismic fabrics in the Basin and Range, including west-dipping reflections in the eastern part of the province and predominantly subhorizontal ones in the west; (2) a pronounced reflection Moho at 30 ± 2 km and locally as deep as 34 km in the Basin and Range with no clear sub-Mono reflections; and (3) complex-dipping reflections and diffractions locally as deep as 48 km in the Colorado Plateau and Sierra Nevada. The eastern part of the transect, shot above known and inferred Precambrian crystalline basement, probably records features related to the entire history of the orogen, locally perhaps as old as 1800 Ma. In this region, major paleotectonic features probably controlled subsequent structural development. In title western half of the transect, however, most reflectors are probably no older than Mesozoic. Within the Basin and Range Province, there appears to be a strong Cenozoic overprint that is characterized by asymmetric half-grabens, low-angle normal faults, and a pervasive subhorizontal system of reflections in the lower crust; no one model of intracontinental extension is universally applicable. Processes that produce or are accompanied by thermal anomalies (magmatism, enhanced ductility, and extension) appear to be essential in developing a highly layered lower crust.

Overview of the COCORP 40°N Transect, western United States: The fabric of an orogenic belt

SUMMARY

An attempt is made to integrate recent continental seismic reflection, refraction, and geologic observations into a unified scheme of craton evolution.



1
Nascent continental crust of basaltic bulk composition is produced in island arcs. As with oceanic crust, Moho in this setting lies within the magmatic edifice and corresponds to a downward transition from dominantly mafic cumulate rocks above to dominantly ultramafic cumulates below. The petrologic crust/mantle boundary, lying ∼25 per cent deeper in the lithosphere, has no seismic expression, and corresponds to a depositional/magmatic contact between cumulate ultramafic rocks above and residual mantle (ultramafic tectonite) below. Both contacts are presumably extensively disrupted by magmatic injection.

2
Island arcs are amalgamated into continents at collision zones. Delamination of the lower mafic/ultramafic portion of the crust along with the mantle lithosphere during ‘hard’ collisions acts as a mechanical refining process that pushes the bulk crust toward intermediate composition.

3
Subsequent extensional collapse of the overthickened crust together with thermal re-equilibration of the lithosphere results in ‘youthful’ stable continental crust, which is ∼30 km thick with its top surface awash at sea level and has the bulk composition of andesite. This crust characteristically exhibits extensional features (half grabens) in the upper portion, and is prominently laminated in the lower portion due to the injection of modest amounts of basaltic magma in the form of sills during delamination. The Moho and petrologic base of the crust coincide in this setting and can correspond to either a ductile high-strain zone or intrusive contact separating mafic/intermediate composition material above from fertile mantle (lherzolite) below.

4
Subsequent ‘cratonization’ is the cumulative effect of episodic injection of the crust by mafic magma at intervals of hundreds of Myr (as manifest by the ubiquitous overlapping dike swarms of the Precambrian shields). This results in net long-term thickening of the crust by underplating, shifts the bulk crust back toward mafic composition, periodically produces a new deeper Moho and petrologic crust/mantle boundary that do not coincide, and through repeated dike injection disrupts pre-existing laminated reflectors. The first magmatic injection event ‘sweats out’ the light melting fraction in the pre-existing andesitic crust, producing voluminous granitic magmatism typified by the great Proterozoic anorogenic granite/rhyolite provinces. Subsequent injection events into refractory crust produce uplift with attendant erosional denudation of supracrustal sequences. The cumulative result is shield-type crust in which intermediate crustal levels are exposed over wide areas, crustal thickness commonly exceeds 40 km, average crustal velocity is somewhat higher than in younger stable crust, and the crust is commonly complexly reflective/diffractive throughout.

A unified view of craton evolution motivated by recent deep seismic reflection and refraction results

New COCORP profiling in the southeastern United States has revealed a broad zone of dipping reflections that extends downward through the crust beneath the coastal plain in western Georgia. The zone is over 50 km wide, and most of the reflections dip moderately steeply toward the south. Regional considerations suggest that this feature marks the late Paleozoic suture between North America and Africa. Where crossed by the COCORP survey, the suture occurs beneath the north flank of the Triassic-Early Jurassic south Georgia basin. The main depocenter of the south Georgia basin occurs about 90 km to the south and is formed by a large half graben containing more than 5 km of rift basin fill. Farther south, the Paleozoic Suwannee basin sequence beneath northern Florida is poorly imaged on the COCORP profiles. However, weak reflections suggest that these strata (including basal felsic volcanics) may have an aggregate thickness of about 6 km in north-central Florida. At the northwest end of the COCORP traverse, a prominent horizon imaged in the upper crust beneath the inner Piedmont probably marks the southern Appalachian detachment. The detachment appears to be cut off by the Towaliga fault, implying that the Towaliga fault is in part a down-to-the-north normal fault. Intermittent Moho reflections occur at 11–12-s two-way time along the length of the COCORP survey, indicating that the crust in the region has a roughly uniform thickness of about 33–36 km (assuming an average crustal velocity of 6 km/s).

K. D. Nelson

Papers

Crustal structure of the Himalayan orogen at ∼90° east longitude from Project INDEPTH deep reflection profiles

Are crustal thickness variations in old mountain belts like the Appalachians a consequence of lithospheric delamination

Overview of the COCORP 40°N Transect, western United States: The fabric of an orogenic belt

A unified view of craton evolution motivated by recent deep seismic reflection and refraction results

New COCORP profiling in the southeastern United States. Part I: Late Paleozoic suture and Mesozoic rift basin