scholarly journals The Role of Lower Crustal Rheology in Lithospheric Delamination During Orogeny

2021 ◽  
Vol 9 ◽  
Author(s):  
Lin Chen
Keyword(s):  
Lower Crust ◽  
Numerical Models ◽  
The Tibetan Plateau ◽  
Mantle Lithosphere ◽  
Laboratory Studies ◽  
Surface Uplift ◽  
Continental Lower Crust ◽  
Lower Crustal ◽  
Flow Laws

The continental lower crust is an important composition- and strength-jump layer in the lithosphere. Laboratory studies show its strength varies greatly due to a wide variety of composition. How the lower crust rheology influences the collisional orogeny remains poorly understood. Here I investigate the role of the lower crust rheology in the evolution of an orogen subject to horizontal shortening using 2D numerical models. A range of lower crustal flow laws from laboratory studies are tested to examine their effects on the styles of the accommodation of convergence. Three distinct styles are observed: 1) downwelling and subsequent delamination of orogen lithosphere mantle as a coherent slab; 2) localized thickening of orogen lithosphere; and 3) underthrusting of peripheral strong lithospheres below the orogen. Delamination occurs only if the orogen lower crust rheology is represented by the weak end-member of flow laws. The delamination is followed by partial melting of the lower crust and punctuated surface uplift confined to the orogen central region. For a moderately or extremely strong orogen lower crust, topography highs only develop on both sides of the orogen. In the Tibetan plateau, the crust has been doubly thickened but the underlying mantle lithosphere is highly heterogeneous. I suggest that the subvertical high-velocity mantle structures, as observed in southern and western Tibet, may exemplify localized delamination of the mantle lithosphere due to rheological weakening of the Tibetan lower crust.

Lower crustal recycling: Reconciling Petrological and Numerical Constraints from the Pamir

2020 ◽  
Author(s):  
Ryan Stoner ◽  
Mark Behn ◽  
Bradley Hacker
Keyword(s):  
Positive Feedback ◽  
Lower Crust ◽  
Numerical Study ◽  
Numerical Models ◽  
Return Flow ◽  
Dense Layer ◽  
Mantle Lithosphere ◽  
Slab Breakoff ◽  
Lower Crustal ◽  
Temperature Time

<p>Geochronological and thermobarometric data from a lower crustal xenolith suite in the Pamir offer a unique record of the transport of lower crust to mantle depths after an episode of slab breakoff. We compare petrologically constrained pressure-temperature-time paths from the xenoliths to pressure-temperature-time (P-T-t) paths of tracked markers in 2-D numerical geodynamic models of density foundering with thermodynamically calculated densities. We investigate whether gravitational “drip” instabilities or the peeling back of a dense layer—delamination—can reproduce the P-T-t paths seen in the xenoliths, with the ancillary goal of capturing the positive feedback between mechanical thickening and densification of the lower crust. Key thermobarometric observations from the xenoliths we try to match in our numerical study are: (1) initial heating at near-constant pressure followed by (2) a sharp increase in pressure with continued heating. We find that thick crustal sections develop P-T-t paths in numerical models of delamination that match the observations from xenoliths: the lower crust initially heats due to return flow from upwelling asthenosphere, and then foundering mantle lithosphere and crust show a marked increase in pressure with additional heating. Initial gravitational drip instabilities founder with relatively little heating yet may thin the mantle lithosphere sufficiently to allow for subsequent delamination or asymmetric drips to nucleate in the region of hotter, thinner mantle lithosphere. Such subsequent asymmetric drips or delamination entrain crust that closely follows the P-T-t path from xenoliths. This suggests that the xenoliths were not derived from an initial drip instability, but instead from later instabilities or delamination enabled by thinning of the lithosphere. In all models where density foundering occurs, the positive feedback between contraction and densification of the lower crust leads to the loss of initially positively buoyant lower crust. The combination of geological and numerical methods constrains the geometry and triggers of lower crustal foundering during collision. Contraction alone does not match the record of foundering; the lithosphere must have also been asymmetrically thinned.</p><p> </p>

scholarly journals Subhorizontal fabric in exhumed continental lower crust and implications for lower crustal flow: Athabasca granulite terrane, western Canadian Shield

Tectonics ◽  
2010 ◽  
Vol 29 (2) ◽  
pp. n/a-n/a ◽  
Author(s):  
Gregory Dumond ◽  
Philippe Goncalves ◽  
Michael L. Williams ◽  
Michael J. Jercinovic
Keyword(s):  
Lower Crust ◽  
Canadian Shield ◽  
Continental Lower Crust ◽  
Lower Crustal Flow ◽  
Lower Crustal

The Mechanism and Dynamics of N-S trends normal faults in Tibetan Plateau: Insight From Thermochronology, Magnetotellurics, Magmatism and GPS Measurements

2020 ◽  
Author(s):  
Han-Ao Li ◽  
in-Gen Dai ◽  
Le-Tian Zhang ◽  
Ya-Lin Li ◽  
Guang-Hao Ha ◽  
...  
Keyword(s):  
Tibetan Plateau ◽  
Lower Crust ◽  
Normal Faults ◽  
The Tibetan Plateau ◽  
Gps Measurements ◽  
Southern Tibet ◽  
Magnetotelluric Data ◽  
Deep Process ◽  
Lower Crustal Flow ◽  
Lower Crustal

<p>The N-S trends normal faults are widespread through the whole Tibetan Plateau. It records key information for the growth and uplift of the Tibetan Plateau. Numerous models are provided to explain the causes of rifting in the Tibetan Plateau based on the low-temperature thermochronology<sup>1</sup>. With the developments of the geophysical and magmatic geochemistry methods and its applications on the Tibetan Plateau, we could gain more profound understanding on the sphere structure of the Tibetan Plateau. This would give us more clues on how the deep process affect the formation and evolution of the shallow normal faults. However, few researchers pay attention on this and the relationship between the surface evolution and deep process of these faults. In order to solve these puzzles, we collected the published thermochronology data, magnetotelluric data, faults-related ultrapotassic, potassic and the adakitic rocks ages and present-day GPS measurements. We find that the distribution of the N-S trends normal faults are closely related to the weak zones in the middle to lower crust (15-50 km) revealed by the magmatism and magnetotelluric data<sup>2</sup>. Besides, the present-day GPS data show that the E-W extension rates match well with the eastward movements speeds interior Tibetan Plateau<sup>3</sup>. Combined with the thermochronology data (25-4 Ma), we concluded that 1.The weak zone in the middle to lower crust influence the developments and evolution of the N-S trends normal faults. 2. The material eastward flow enhance the N-S normal faults developments. 3. The timing of the middle to lower crustal flow may begin in the Miocene.</p><p><strong>Key words:</strong> N-S trends normal faults; Thermochronology; Magnetotellurics; Magmatism; GPS Measurements; middle to lower crustal flow</p><p><strong>References:</strong></p><p><sup>1</sup>Lee, J., Hager, C., Wallis, S.R., Stockli, D.F., Whitehouse, M.J., Aoya, M. and Wang, Y., 2011. Middle to Late Miocene Extremely Rapid Exhumation and Thermal Reequilibration in the Kung Co Rift, Southern Tibet. Tectonics, 30(2).</p><p><sup>2</sup>Pang, Y., Zhang, H., Gerya, T.V., Liao, J., Cheng, H. and Shi, Y., 2018. The Mechanism and Dynamics of N-S Rifting in Southern Tibet: Insight from 3-D Thermomechanical Modeling. Journal of Geophysical Research: Solid Earth.</p><p><sup>3</sup>Zhang, P.-Z., Shen, Z., Wang, M., Gan, W., Bürgmann, R., Molnar, P., Wang, Q., Niu, Z., Sun, J., Wu, J., Hanrong, S. and Xinzhao, Y., 2004. Continuous Deformation of the Tibetan Plateau from Global Positioning System Data. Geology, 32(9).</p><p><strong>Acknowledgements:</strong></p><p>We thank Shi-Ying Xu, Xu Han, Bo-Rong Liu for collecting data. Special thanks are given to Dr. Guang-Hao Ha and Professors Jin-Gen Dai, Le-Tian Zhang，Ya-Lin Li and Cheng-Shan Wang for many critical and constructive comments.</p>

scholarly journals Long-lived Paleoproterozoic eclogitic lower crust

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Sebastian Buntin ◽  
Irina M. Artemieva ◽  
Alireza Malehmir ◽  
Hans Thybo ◽  
Michal Malinowski ◽  
...  
Keyword(s):  
High Velocity ◽  
Lower Crust ◽  
Crustal Layer ◽  
Crustal Rocks ◽  
Long Term Stability ◽  
Eclogite Facies ◽  
Conventional Models ◽  
Lower Crustal

AbstractThe nature of the lower crust and the crust-mantle transition is fundamental to Earth sciences. Transformation of lower crustal rocks into eclogite facies is usually expected to result in lower crustal delamination. Here we provide compelling evidence for long-lasting presence of lower crustal eclogite below the seismic Moho. Our new wide-angle seismic data from the Paleoproterozoic Fennoscandian Shield identify a 6–8 km thick body with extremely high velocity (Vp ~ 8.5–8.6 km/s) and high density (>3.4 g/cm3) immediately beneath equally thinned high-velocity (Vp ~ 7.3–7.4 km/s) lowermost crust, which extends over >350 km distance. We relate this observed structure to partial (50–70%) transformation of part of the mafic lowermost crustal layer into eclogite facies during Paleoproterozoic orogeny without later delamination. Our findings challenge conventional models for the role of lower crustal eclogitization and delamination in lithosphere evolution and for the long-term stability of cratonic crust.

Geochemical and isotopic data of Zheduo-Gongga granitic intrusive complex, eastern margin of the Tibetan Plateau: no evidence for middle-lower crustal flow

2020 ◽  
Author(s):  
Fangyang Hu ◽  
Fuyuan Wu ◽  
Mihai Ducea ◽  
James Chapman
Keyword(s):  
Tibetan Plateau ◽  
Partial Melting ◽  
Lower Crust ◽  
Biotite Granite ◽  
The Tibetan Plateau ◽  
Eastern Margin ◽  
Granitoid Rocks ◽  
Lower Crustal Flow ◽  
Group 2 ◽  
Lower Crustal

<p>Geophysical studies have shown that middle-lower crustal flow started from central Tibetan Plateau may exist in the eastern margin of the Tibetan Plateau, which controls the mountain building, crustal thickening and deformation (Schoenbohm et al., 2006; Bai et al., 2010; Bao et al., 2015; Zhu et al., 2017). However, no geological and petrological evidence have been presented. We carried out detailed studies on the geochemical and isotopic compositions of the Mesozoic-Cenozoic Zheduo-Gongga granitic intrusive complex on the eastern margin of the Tibet Plateau. Geochronology studies show that these granitoid rocks are formed during Mesozoic to Cenozoic, including ~220-200 Ma Gongga granodiorite to biotite granite with mafic enclaves, ~40 Ma Zheduo gneissic granite, ~28 Ma Zheduo monzogranite, and ~20-4 Ma Zheduo biotite granite and monzogranite. Two groups of geochemical features are obtained: Group 1 (gnessic granite, granodiorite, monzogranite, and leucogranite) has relatively low K2O, Th/La, La/Yb and Rb/Sr ratios, but high Sr/Y ratio with no Eu negative anomalies; Group 2 (biotite granite) has relatively high K2O, Th/La, La/Yb and Rb/Sr ratios, but low Sr/Y with strong negative Eu anomalies. The Sr-Nd-Hf-O isotopic studies on plagioclase, apatite and zircon show that their sources are primarily the basement of the western margin of Yangtze Craton and Songpan-Ganzi sediments. These features indicate that they have different petrogenesis processes. Group 1 is mainly derived from partial melting of mafic rocks in the lower crust, whereas the Group 2 is primarily derived from partial melting of metasedimentary rocks experiencing fractionation of plagioclase. Magma derived from different sources mixing with each other are observed as well. Therefore, from geochemical aspects, no exotic materials are involved in the formation of granitoid rocks during Mesozoic to present. The flow of crustal material in the middle-lower crust may be not existed. The low velocity and high conductivity layer in the middle-lower crust may represent a regional partial melting zone, which could be related to the upwelling of asthenosphere. Both crustal deformation and upwelling of asthenosphere may contribute to the crustal thicknening and uplift.</p>

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