Gruppo Nazionale di Geofisica della Terra Solida 27° Convegno Nazionale

LithoFLEX

NEW INSIGHTS INTO THE BASEMENT STRUCTURE OF THE WEST SIBERIAN

BASIN FROM FORWARD AND INVERSE MODELLING OF GRACE SATELLITE

GRAVITY DATA

Gruppo Nazionale di Geofisica della Terra Solida27° Convegno Nazionale

6 - 8 Ottobre 2008, Trieste

Carla BraitenbergDepartment of Earth Sciences, Trieste University

Jörg EbbingGeological Survey of Norway, NGU, Trondheim

Department for Petroleum Technology and Applied Geophysics, NTNU Trondheim

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Overview

• Goals

• Starting point and data

• Gravity field modelling and inversion

• Results and conclusions

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Goals

• Identification of terranes of WSB

• Identify rift structure and relation to underlying crust WSB

• Thickness of basalt layer

• Interpretation of the gravity field in WSB

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Starting point

• Gravity anomaly and Geoid undulations

• Regional seismic profiles

• Sediment thickness (pre- and post volcanic)

• Moho model

• Uncertain basalt layer extent

• Hypothesized rifts in basin•Vyssotski et al., 2006

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Topo

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Global gravity fields

• Grace satellite– given in spherical harmonic development. – GFZ-Potsdam (Förste et al., 2008): EIGEN-

GL04C– Up to degree and order 120: satellite only.– Up to degree and order 360: integration with

terrestrial data.

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Gravity anomaly

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Bouguer field

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Gravity Modeling

• Gravity effect of known masses:– Crustal thickness variation

• Fixed density contrast

– Sediments• Compaction: density increase with depth• Use velocity increase with depth

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Moho

(Kovylin, 1985)

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Moho gravity

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Base pre-volcanic sediments

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Converting TWT to velocity-depth

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Gravity of sediments

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Residual gravity

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Gravity Inversion

• Residual Gravity– after correction of known masses

• Gravity inversion of residual– Superficial mass

• 5 km reference depth, fixed density contrast

– Lower crust • Density variation in layer 20 km above Moho

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Residual gravity- low pass filtered (150km)

mgal

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Residual gravity high-pass filtered

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Lower crustal density variation

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Surface layer inversion

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Crustal model gravity

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Isostatic anomaly

Loads: topography and sediments

mgal

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Interpretation

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Results 1/4

• Rifts: positive gravity anomaly– Basalt fill necessary: up to 3 km thick.

• Rift-graben structures identified– Koltogor- Urengoy – Khudottey -> 100 km west– Khudosey– North and South Khatanga– Yenisey– Ob

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Results 2/4• Deviation from classical relation

Topography- Bouguer anomaly

• Density increase needed in lower crust to explain gravity field– Arch-like structure along eastern border to

Siberian craton– NE-SW trending mid-basin high density

anomaly• Extending from Pur Taz South-westwards

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Conclusions 1/2• Rift structures:

– Yenisei rift (1500 km length) underlain by high density arch

– rifts in mid basin: underlain by moderately increased density

• Compares well to results from QUARTZ (Morozova et al., 1999) deep seismic profile

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Conclusions 2/2• Crustal blocks:

– South-, Mid-, North- and Northwest- segments of basin

– Smooth transition to Kara Sea and Yenisey-Khatanga trough

• Eastern Arch

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Thank you for your attention!

Reference: the work is summarized in the paper: Braitenberg C. and Ebbing J. (2009) New insights into the basement structure of the West Siberian basin from forward and inverse modelling of GRACE satellite gravity data, in editorial process Journal of Geophysical Research.