Potential Fields SIG: En echelon magnetic anomalies in the Western Indian Ocean: a foundation for ... - Mar 22nd

Sep 15th-Potential Fields SIG: Evaluating the Compatibility of Plate Kinematic Models with ...
Complete Title: En echelon magnetic anomalies in the Western Indian Ocean: a foundation for potential fields interpretation of the geological structure of the East African continental margin.

Meeting Location:

HESS Club (Houston Engr. & Science Society)
5430 Westheimer
Houston, TX  77056

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Speaker: Andrew Long, Subterrane Ltd.

Utilizing two complimentary satellite gravity/magnetic datasets -  Sandwell’s latest Free Air gravity and the Enhanced Magnetic Model (2015 edition), we present a qualitative interpretation of the East African continental margin. The data are residualized using conventional systematic corrections, yielding signatures respectively of shallow crustal density variation and deep crustal magnetization variation. By making correlations to seismic reflection profiles, onshore outcrop geology,  wells and seismicity it is possible to determine structures from gravity and magnetics indicating strike slip tectonics have played a dominant role in the break-up of Gondwana. The earlier Permo-Triassic Karoo extensional rifting across East Africa propagated almost orthogonally to the Jurassic break-up margin of Madagascar and the Lamu Basin, Kenya indicating a major change in the regional stress regime. 

We present an overview gravity and magnetics review of the East African margin that highlights present and past structures, commencing onshore within the Anza Basin, and finishing offshore southern Madagascar. Along the tour, evidence is offered to suggest distinct changes in strike slip structural zones along the margin that are clearly imaged in the residualized gravity and magnetic record, and have been well documented and researched in structural geology. Underpinning the interpretation are the presence of four magnetic anomaly highs arranged in an echelon configuration offshore Northern Mozambique and Tanzania. Together with the residual gravity signature and seismic correlations, an elegant present-day duplex system is identified. By extending the curved fault solutions, the duplex system is seen to be bound to the north by transpressional structures, and to the south by a narrow transtensional rifted corridor.   

Recent and historical seismicity data correlate with many of the mapped fault traces suggest seismicity is controlled by strike slip movement along old faults, and not the offshore East African Rift System (E.A.R.S.) which appears locked by older transtensional structure. Our results indicate the continental margin extends much further eastward.  The extent of Somalia Basin oceanic spreading is less than previously thought, and restricted in the north by offshore Lamu oblique rifting. It follows there are important consequences for petroleum exploration with regard the extent and presence of older source rocks and their maturation in the present day deepwater Somali Basin. The influence of offshore E.A.R.S. dissipates south along the margin. The influence of the Davie Ridge offshore western Madagascar conversely dissipates towards the northern transtensional zone, and resembles a fold axis plunging to the north, north, west in a transpressional zone of the strike slip margin.

This interpretation is still in development to extend across onshore East Africa towards the western arm of the E.A.R.S. and outboard to the Western Indian oceanic spreading centre, and south to the offshore Angoche Basin, southern Mozambique and the Beira High. 

Speaker Biography: Andrew Long, Subterrane Ltd.
Andrew Long is a geologist and owner of Subterrane Ltd., graduating from Imperial College, London with BSc Geology with Geophysics and MSc Petroleum Geophysics. Andrew started his career with PGS Reservoir as a geophysicist and specialized in potential fields methods with Geosoft. Andrew is a Fellow of the Geological Society of London and an active member of the S.E.G. Andrew has research interests driven by projects in the East African Continental Margin, West African offshore Mauritania-Senegal-Gambia-Guinea-Bissau-Guinea Basin, and the Pacific and Caribbean Margins.
5:30  Registration / Cash Bar
6:15  Dinner Served
7:00  Presentation Begins
8:00  Adjourn

Price List:
Pre-Registered Late/Walk-Up
 $35  $45
 Non-Member  $40  $50
 Student Member
 $15  $25

*HESS Club requires 3 days notice for a vegetarian dinner order. Please make request below, thank you.


3/22/2018 5:30 PM - 8:00 PM

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