Crustal flow in Tibet: geophysical evidence for the physical state of Tibetan lithosphere, and inferred patterns of active flow

作者：KLEMPERER

文章类型

• 综述总结

摘要

Crustal weak zones & Channel Flow

Lateral strain variations, and vertical strain and strength profiles in Tibet

• 总结：

  Deformation & Strength &Flow

• 要点：

  1. Two end-member model for HORIZONTAL deformation: (1) discrete & undeformed block & expelled between faults / (2) continuous deformation

     1. (1) is less plausible from GPS observations

  2. VERTICAL deformation: cannot be directly measured, contentious

     1. Possibility 1: vertical coherent deformation

     2. Possibility 2: dominated by a more rapid ductile flow in the middle/lower crust

  3. Lithosphere strength model:

     1. Jelly-Sandwich: Stronger-(upper crust & mantle); Weaker-(lower crust); from earthquake depth analysis

     2. Stronger lower crust (than upper mantle): from re-evaluation of the focal depth

  4. Flow:

     1. strength minumum

     2. time scale of flow → inversely proportional to the cube with → flow layer thickness

     3. transient flow ← introduction of fluids

     4. lower-crustal flow conjuction with uppwe-mantle flow

Modes of crustal flow and channel flow

• 总结：

  Types of flow models. Flow models & previous studies in Tibet (eastern plateau margin; NW syntaxis; Greater Himalayan Sequence)

• 要点：

  1. Definition of flow: Any viscous or plastic deformation may be regarded as flow

  2. Definition of channel flow (this paper): Any flow in which a viscosity minimum at some depth strongly localizes horizontal material flow and partially or totally decouples flow at different depth

  3. End-member models for flow: Poiseuille flow (pipe flow); Couette flow

  4. Examples about previous applications of flow models to Tibet

  5. *deudation to exhume the channel (flow)

• 图片：

  

Geophysical data bearing on crustal flow in Tibet

• 总结：

  Methods to constrain partial melts or rheology, and review of past studies.

• 要点：

  1. Introduce to 3 important surveys (Sino-French; Sino-US PASSCAL; INDEPTH): Types of data; Utilization and potential

  2. Relief Data

     1. wavelength → viscosity;

     2. elastic thickness & Indian mantle suture;

     3. weak lower crust in the eastern margin of the plateau

  3. Geothermal measurements (recent magmatism):

     1. Puga geothermal exploration: large variation over <25km → shallow heat source

     2. Moho temperature

  4. Seismicity cut-off at depth locates brittle-ductile and ductile-brittle transitions

     1. 95% earthquakes above 20km

     2. crustal seismicity cut-off: brittle-ductile transition at 250-450℃

     3. sub-Moho earthquakes at southern Tibet and Himalaya; temperature no more than 600-800℃

  5. Seismic velocity and crustal thickness

     1. Velocity: lithology, temperature and fluid distribution: SiO2↑ velocity↓; temperature↑ velocity↓; aqueous / magmatic fluids ↑ velocity↓

     2. warmer mantle in the northern Tibet east of 85°: Sn attenuation & Rayleigh velocity

        1. velocity contrast from south to north: 200-300℃ difference

        2. SS-S differential traveltimes: variability

     3. Lithospheric thickness: from S receiver function

     4. Pnl modelling: laterally averaged crustal velocities (hundreds of kilometers)

        1. Low average Vp at all depth, comparing with global average

     5. Receicer function / P-waveform modelling: high-velocity layer at the base of crust

        1. Greater Indian lower crust??

     6. Receiver function

        1. Low velocity zone below 20km at Lhasa Terrane?

        2. High or normal Vp/Vs in Qiangtang?

     7. Surface wave tomography

        1. Low Vs compared with global values

        2. Low Vs channel (mid-to-lower crustal) at Lhasa terrane; Less pronounced in Qiangtang as overall Vs here is low; Restricted south to IYS

        3. Strongest low Vs & bright spots; Not restricted to the extensional grabens

     8. Possible Processes: partial melts; partial eclogitization (High-Himalaya; Lhasa; northern margin);

     9. Abrupt change in crustal thickness? (northern margin: Tarim and Qaidam basins) Comparing with Te (elastic thickness)?

     10. Seismic Attenuation: evidence for high temperatures and partial melts

         1. Northern Tibet: strong Sn attenuation in the mantle & strong attenuation in the crust (Lg, Pnl)

         2. Yangbajin: strongest attenuation

         3. IYZ: Q ~100; High Himalaya: Q ~300

            1. Comparison: 650 for stable central and eastern North America; 200 for tectonically active western North America

  6. Conductivity, evidence for fluids and crustal weakening

     1. High conductivity (Yangbajin, Yamdrok Tso): + high heat flow → magma

     2. INDEPTH profiles: High crustal conductance; High conductivity below 15km; continuity of conductivity along Himalayan arc

     3. Interconnected fluid along the grain boundaries of the rock?

     4. Southern TP: The relationship bwtween inferred partial melt & north-south grabens? Southward flow?

     5. Northern TP: Pressure dependence of dihedral angles; Fluids interconnection

  7. Seismic Anisotropy, evidence for past or present strain of mantle and crust

     1. require flow or recrystallization under non-uniform stress; cannot determine whether an ongoing flow

     2. Observations: SKS splitting; multilayer anisotropic receiver function inversion

        1. SKS splitting:

           1. arising from lithosphere mantle (mostly), because olivine in the mantle allows easy creation of strain-induced lattice prefferred orientation.

           2. Cannot resolve delay times <5s due to long periods of SKS waves

           3. southern Lhasa terrane / India craton: small or negligible SKS splitting

           4. Qiangtang Songpan-Ganzi: large split times

        2. INDEPTH-3: Jiangli fault, strong variation in crustal anisotropy

        3. correlation distance

        4. considerate anisotropy in the crust; time-integrated strain history;

           1. brittle crust: open cracks or old metamorphic fabrics

           2. mid / lower crust: active deformation superimposed on older strain fabrics

     3. Observation: surface waves, large scale behavior of the crust

        1. vertical slower than horizontal: near-horizontal reorientation of anisotropic minerals (micas?) during thinning of anisotropic layer

        2. flattening flow

  8. Seismic reflectivity, evidence for fluids and flow

     1. bright-spots: unusually strong amplitudes; strong P-to-S cibversuibs; variation of amplitude with incidence angle of reflections (fluid > 15% porosity?)

        1. Yangbajin: very likely to be hot enough to cause granitic melt, unless imaged very recent intrusion prior to cooling (unlikely); aqueous fluid, volumes too large to be easily maintained (unless deep fluid source exists)

        2. will be exposed in the furture at the surface in a return channel flow??

     2. pronounced lower-crustal reflectivity

        1. tectonically imposed layering (shear zones, horizontal planar mafic intrusions)

           1. mafic intrusion, less likely?

           2. active ductile deformation and flattening foliation, extensional flow (likely)?

  9. Indirect Methods:

     1. Satellite Magnetic Low: Curie isotherm (550℃) ?

     2. alpha-beta quartz transition → seismic reflector → 700~800℃ ?

     3. Xenoliths

     4. Pn tomography → Uppermost mantle temperature (more sensitive to lateral difference than absolute temperature)

     5. volcanic events, adajutes

     6. geodynamic modelling: kinematic or thermal-mechanical models

• 图片：

  

  

  

Physical properties inferred from geophysical observations

• 总结：

  Flow models in southern or northern Tibet.

  Evidences supports or disapproves flow models.

• 要点：

  1. Inferred vertical strength profiles

     1. southern Tibet: channel, ductile process

        1. Tethyan Himalaya: middle crust strength minimum

        2. Helium emission, thin zone of warmer Asian mantle

     2. northern Tibet: less clear-cut

        1. crust, weak; upper mantle, low Vs, strong attenuation

        2. Removal of lithosphere?

        3. Strong anisotropy; Impedance decrease

  2. Inferred vertical velocity profiles

     1. southern Tibet

        1. GPS: surface moving NNE, with respect to Eurasia

        2. Underthrusting: lower lithosphere move at similar direction

        3. Similar focal mechanisums in south Tibetan upper crust and upper mantle

        4. erosion: moving many marterials from Himalaya

           1. Presume: the mountain front is not rapidly migrating northwards due to erosional retreat → needs replenishment (channel flow, thrust wedging) → advecting material southwards from Tibet to Himalayan font

           2. southernmost Tibet: reduced nothward velocity in the middle crust, relative to upper crust or upper mantle (channel flow, Poiseuille form, Couette component)

           3. Jiali Fault?

     2. North of Jiali fault:

        1. GPS: eastward velocity, continuity → more-distributed east-west extension

        2. strain field: correlation of surface strain field with the mantle strain field → similar boundary conditions? coupling?

        3. Couette-type component driven by the strong upper crust?

        4. Northern boundary: mirror image of the Himalayan thrust front, low erosion rates prevents the emergence of a channel flow

     3. Evaluation of existing channel flow models: southern Tibet:

        1. mid-to-lower crust; south-directed to compensate for orographic exhumation?

        2. Works from isotopic

        3. Continuing coeval motion of MCT and STD; whether STD merging with the MCT at depth

     4. Evaluation of existing channel-flow models: northern and eastern Tibet

        1. modelling; geohistory

        2. upper crust stronger

        3. decoupling

• 图片：

  

  

Summary

• 总结：

  Liquid weaken the middle and lower crust

  Southern Tibet: Indian underthrusting, south-directed flow of the middle crust (Poiseuille-type)

  Northern and eastern Tibet: flow patterns less certain, weakly coupled flow, both Poiseuiller- and Couette-type flow.

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