GEO- DEEP9300 Lithosphere and Asthenosphere: Composi;on and Evolu;on Kaiming Wang

Transcription

1 Con$nental lithosphere and evolu$on GEO- DEEP9300 Lithosphere and Asthenosphere: Composi;on and Evolu;on Kaiming Wang

2 1. Con$nental lithosphere Con$nental lithosphere houses the oldest and thickest regions of the Earth's surface. The earliest con$nental material preserved in the geological record predates the transi$on to plate tectonics and could serve as a witness to whichever mode of tectonic ac$vity was present Much of that history has been dominated by the forces of plate tectonics which has repeatedly assembled super con$nents together and torn them apart the Wilson Cycle.

3 2. Lithosphere defini$on lithosphere :an imprecise concept Different disciplines defining the layer of strength in a variety of ways. what is the primary control on the strength of Earth materials temperature? Composi$on? pressure? the $mescale of loading and response plays an important role in interpre$ng strength

4 Thermal boundary layer (1) treat lithosphere as the thermal boundary layer Strength is due to temperature as mantle's viscosity is temperature dependent In the past Earth's mantle was warmer, thermal boundary layer was thinner. As the Earth cools, average thickness of thermal boundary layer increases use surface heat flux measurements. The surface heat flux is propor$onal to the near surface temperature gradient

5 Thermal boundary layer (Copper et al, 2017)

6 Chemical boundary layer. (2) treat lithosphere as the chemical boundary layer. The lithosphere can be described as a composi$onally dis$nct region. the layer is chemically dis$nct from the convec$ng mantle. The strength of the lithosphere can be explained by increased buoyancy and viscosity driven by composi$onal differences. Cratons are neutrally buoyant despite their cold and old interiors This combina$on of the posi$ve chemical buoyancy and increased viscosity are responsible for lithospheric strength and stability

7 Seismological lithosphere (3)treat lithosphere as seismologically mul$ple different ways, using different techniques. Generally, high- velocity outer layer of the Earth is underlain by a low- velocity layer and a dis$nct change in observed seismic anisotropy. whether this high- velocity layer coincides with the thermal or chemical boundary layer or, perhaps, even some other physical proper$es.

8 Coincide with each other? The base of the chemical boundary layer and the thermal boundary layer need not coincide chemical boundary layer give thinner values for lithospheric thickness than the seismic observa$ons seismic observa$ons may coincide with the base of the thermal boundary layer

9 2. Lithosphere defini$on several other approaches seismic lid (Priestley and Tilmann, 2009) tectosphere (Jordan, 1978) electric lithosphere (Jones et al., 2001)

10 3. Seismological observa$ons Con$nental lithosphere structure provide constraints on seismic velocity crustal and lithospheric thicknesses rheological contrasts presence of anisotropic layers internal structures crustal- mantle boundary(moho), mid- lithospheric discon$nui$es (MLD), lithosphere- asthenosphere boundary (LAB).

11 3. Seismological observa$ons Body wave tomography studies robustly image long wavelength lateral features in the upper (and lower) mantle with travel$mes Highlight the contrast in veloci$es and a^enua$on not only beneath the oceans, but between the ac$ve tectonic regions of the con$nent and their stable cores (cratons)

12 3. Seismological observa$ons Surface wave tomography infer the absolute shear- wave velocity structure of the uppermantle and through analysis of various periods of horizontally propaga$ng surface waves. not as much ver$cal smearing of the velocity structure near- horizontal boundary structures can be difficult to image

13 3. Seismological observa$ons Sca^ering and discon$nuity imaging Receiver func$on mapping sub- horizontal velocity discon$nui$es in the crust and mantle. P receiver func$ons (PRFs), (P- to- s conversions), are commonly used to map the crust- mantle boundary (Moho) and internal crustal structure S receiver func$ons (SRFs), (S- to- p conversions), are a powerful tool for imaging deeper sub- horizontal structures such as the (LAB) or (MLDs)

14 3. Seismological observa$ons anisotropy within the con$nental lithosphere (1)receiver func$ons For an isotropic, flat- layered Earth structure, tangen$al component receiver func$ons should be zero, and the radial component receiver func$on should have signals that are independent of back azimuth In prac$ce, coherent and azimuth- dependent signals are observed on both the radial and tangen$al component receiver func$ons, which can be used to infer anisotropic structures

15 3. Seismological observa$ons anisotropy within the con$nental lithosphere (2)Shear wave splicng analyses in par$cular splicng of core phases (SK(K)S), are based upon the proper$es of birefringence of nearly ver$cally propaga$ng shear waves beneath seismic sta$ons. The lateral measurements of splicng of SKS phases allow for interpreta$on of the back- azimuthal varia$on of delay $mes and fast direc$ons to infer the presence dipping structures in the mantle

16 3. Seismological observa$ons anisotropy within the con$nental lithosphere (3)Surface wave methodologies. Two types of seismic anisotropy, azimuthal (horizontal) and radial (ver$cal) anisotropy Radial anisotropy in the upper mantle has been detected with the discrepancy between Rayleigh and Love wave propaga$on azimuthal anisotropy is determined by changes in direc$onal dependence of the propaga$on of Rayleigh waves

17 3. Seismological observa$ons Con$nents lithosphere idea: The oldest cores of the con$nents have thicker, fast velocity structure; The younger regions of the con$nents have much thinner lithosphere Thick cratonic mantle lithosphere has high velocity in comparison to the same depths beneath the Phanerozoic lithosphere. subduc$on- like processes were fundamental in assembling the con$nental lithosphere.

18 3. Seismological observa$ons Con$nents lithosphere idea: The stable interiors of the con$nents are generally, the base of the lithosphere is not observed at a uniform depth The varia$on in thickness and structure has been interpreted seismic studies to be a signature of Archean- Paleoproterozoic tectonics The con$nental suggests that the internal structure, including mul$ple layers, variable depth to LAB, and dipping structures, is indica$ve of the long- term evolu$on and forma$on of both the youngest and oldest part of the con$nents.

19 4. Dynamic interpreta$on geodynamic modeling can guide our interpreta$on and determine whether modern and ancient processes produce similar or different lithospheric structure. Two hypotheses for cratonic lithosphere forma$on groups: (1)those that invoke processes unique to the pre- plate tectonic Earth (2) those that call upon modern day plate tectonic processes

20 4. Dynamic interpreta$on (Copper et al, 2017)

21 4.1 A pre- plate tectonic regime The majority of the pre- plate tectonic ideas centralize around primarily ver$cal mo$on (1) mel$ng events caused by large mantle plumes in the early Earth, the ho^er mantle temperatures allowed for incipient mel$ng to occur at greater depths within a plume

22 4.1. A pre- plate tectonic regime (2) density inversion the cratonic lithosphere was formed through diapirism and ver$cal tectonics where dense volcanic material is moved ver$cally deeper into the mantle in response to a density inversion the successive ver$cal accumula$on of the residuumg generated from mel$ng the mafic volcanics that moved downward during the keeling process or from mel$ng driven from delamina$on events

23 4.1. A pre- plate tectonic regime (3) heat pipe regime For the early Earth, the heat pipe regime, allows for the rapid removal of heat through pervasive, localized volcanism as well as the development of thick lithosphere via ver$cal transport. During a heat pipe mode of convec$on, the en$re surface acts as a single plate and rapid, localized volcanic erup$ons connect, as pipes, the hot, convec$ng mantle to the surface, effec$vely removing heat.

24 4.1. A pre- plate tectonic regime All of these hypotheses are capable of forming a strong, thick chemical boundary layer residing within the thermal boundary layer. However, these hypotheses fail to describe the more complex nature, men$oned above, of some of the MLDs observed within the cratonic regions that show con$nuous dipping surfaces mul$ple MLDs in a single sugges$ng a differing origin than mel$ng events alone, at least in those areas.

25 4.2. Early development of plate tectonics In the framework of plate tectonics, cratonic lithosphere was formed by thickening pre- exis$ng lithosphere in a subduc$on zone secng. forming stable lithosphere (Craton) (1) through thrust stacking of buoyant oceanic lithosphere (2) through accre$on/accumula$on of arc lithosphere

26 4.2. Early development of plate tectonics Two scenarios arose: the lithospheric material thickening (1)via localized deforma$on (akin to thrust stacking) (2)viscous deforma$on (more indica$ve of arc amalgama$on) Both of these condi$ons depend on (1) material proper$es of the lithosphere (2) dynamic secng of the convec$ng mantle.

27 4.2. Early development of plate tectonics thrust stacking /arc accumula;on /con;nental collision thickening of the lithosphere as driven by lateral mo$on introduces complex structure at depth This provides an explana$on for the varied nature of lithospheric structure in differing cratons regions; thickening driven by lateral mo$on can produce a range of features depending on the rheological condi$ons.