GEO-DEEP9300 Lithosphere and Asthenosphere: Composition and Evolution

Transcription

1 GEO-DEEP9300 Lithosphere and Asthenosphere: Composition and Evolution Summary Presentation The Structural Evolution of the Deep Continental Lithosphere Focused on the Junction of Arabian, Eurasian and Indian Plate By: Shabeer Ahmed (Ph.D. Candidate) Supervising Professor: Prof. Dr. Sarfraz Hussain Solangi (Meritorious Professor) University of Sindh, Jamshoro, Pakistan Contact Cellphone No: GEO-DEEP9300

2 Introduction 2 Arguably: Continental lithosphere is less understood than the oceanic lithosphere. Reason? Less understood of first order features of The Oceanic floor. As these are directly attributed to the sea floor Age through plate tectonics. Features of Oceanic Floor Thickness Heat Flux Bathymetry The valid approach: Seismic Observations Note: Same theory fails to provides an equivalent context for continents.

3 Internal Gaze: Connection of new observation and examples from other datasets? 3 Geological and geophysical constraints (Laske et al., 2013) Cratons Regionalization based on clustering in the top 350 km of global upper-mantle, seismic tomography model (Lekic & Ramanovicz, 2011) Broad Scale: It serves to highlight the vertical coherence of the lithospheric structure. Output: Clustering suggest (regional scale) that deep and shallow lithospheric evolution are linked and deep structure can be related to the long term evolution of the lithosphere. Cluster 3 and 4: Stable continental interior from the tectonically mobile regions Deforming orogens

4 Crustal thickness in the past: Appears to be less sensitive 4 Derived from the layer thickness maps of litho1.0 (Pasyanos et al., 2014)

5 Dichotomy: Relative thickness of crust to that of the lithosphere overall 5 Global Indication: The thickness of CC only may not indicate deep time events but variations in the sub-crustal continental may be potentially providing history information about the deformation in the earth. CC = Continental Crust Derived from the layer thickness maps of litho1.0 (Pasyanos et al., 2014)

6 The Complex Dynamic Earth: What are the factors to uncover? 6 The present is the key to the past (Lyell, 1837) Bridging up geological observation on large spans of time. Uniformitarian approach has proven useful to uncover how the processes were slow/ fast but persistent nature of changes. Should we expect to modern plate tectonics as an outcome? (Dewey, 2007) Conveyer belt style of subduction is thought to have become during the Archean. But Shirley and Richardson (2011) argued that it occurred at 3.0 Ga (Ga=one billion years) based on the appearance of eclogitic diamonds which are indicating modern style of subduction. Recently Tang et al., (2016) demonstrated a change in the bulk upper crust composition further supporting the onset of plate tectonic around this time. Geodynamic point of view: The ability of modern style subduction is unclear. Reason? Higer modern temperatures in the past resulted thicker crust as well the oceanic lithosphere may have been too buoyant to engage in subduction.

7 The Layer of Strength: Lithosphere 7 What is the primary control on the strength of earth material (temperature, composition, pressure) [Watts et al., 201]? Copper et al., (2017) treated lithopehere as thermal-boundary layer or the region encompassing the greatest temperature near the surface of earth? If it is temperature? Yes, then what to do? Approach of heat flux measurements: The surface heat flux is proportional to the near surface temperature gradient (Fourier law of heat conduction) Reliability for the composition of crust as well as for the upper mantle. This approach can indicate variations in the thickness of the thermal boundary, age, absolute age (OL) or time of the last tectono-thermal event (Muller et al., 2008)

8 Discussion and Conclusions on Continental Lithosphere 8 McKenzie et al., (2015) observed that when global lithospheric thickness maps were reconstructed to the configuration of Pangea that the thicker regions aligned in a continuous manner. Suggested: Todays regions of thicker lithosphere (minus cratonic regions) are holding on to the evidence of the last continental collision.

9 Consideration: Integration of Regional Surface Observations with Seismic Tomography and Anisotropy 9 Regions under consideration : 1. Himalayas orogenic regions 2. Failed Rift Regions Signatures of surface geology and may confirmation and correlation with biostratigraphy of samples from deep wells. Regions under consideration : Himalayas orogenic regions and failed rift regions Chattarjee et al., 2013)

10 Failed Rift 10

11 TECTONIC EPISODES: INDIAN PLATE 11 Himalayas Failed rift

12 PRESENT DAY VELOCITY: INDO-PAKISTAN PLATE 12

13 Seismic Tomography Approach 13 What is the primary control on the strength of earth material (temperature, composition, pressure) [Watts et al., 201]? Seismic tomography: Imaging the subsurface with seismic waves produced by earthquakes or explosions. P-, S-, and surface waves can be used for tomographic models of different resolutions based on: a. seismic wavelength, b. wave source distance, and c. seismograph array coverage (Nolet, G. (1987). Utilization: Geoscientists use these images to better understand core, mantle, and plate tectonic processes. NASA tomographic image of the subducted Farallon Plate in the mantle beneath eastern North America (Liu et al., (2008).

14 Reconstructing Farallon Plate Subduction Beneath North America Back to the Late Cretaceous 14 Inverse mantle convection model Reconstruction of Farallon plate subduction back to 100 million years ago. Models consistent with stratigraphy constrain the depth dependence of mantle viscosity and buoyancy, requiring that the Farallon slab was flat lying in the Late Cretaceous, consistent with geological reconstructions. The simulation predicts that an extensive zone of shallow-dipping subduction extended beyond the flat-lying slab farther east and north by up to 1000 kilometers. The limited region of flat subduction is consistent with the notion that subduction of an oceanic plateau caused the slab to flatten. The results imply that seismic images of the current mantle provide more constraints on past tectonic events than previously recognized. Reconstructing Farallon Plate Subduction Beneath North America Back to the Late Cretaceous (Liu, L., et al., 2008)

15 Discussion and Conclusions on Seismic Tomography Approach 15 Still there are many unanswered questions? So what to do? Most of the geodynamic simulations during the early earth are limited to two-dimensional geometric framework. Further work must be advanced by exploring three-dimensional studies. We can start to uncover many of the confusions and unanswered questions with significant deformation (need of regional studies) for both cratonic and accretion settings by integrating 3D seismic imaging of oldest events Gravity and Magnetotelluric Studies. But I would like to prefer: Seismic tomography, anisotropy approach as significant. Integrated Considerations: 1. Integration of Python programing and A.I to get upto date information about Indian Plate 2. Seismic Tomography (Liu et al., 2008) 3. Seismic Anisotropy (Silver 1996; Royden et al., 1997; Ozacar and Zandt, 2004; Lev et al., (2006); Soto et al., 2012; Wu et al., (2015)

16 16 Thank You for Listening Any Questions / Suggestions?