Results from the Wide Angle SeismicExperiment in the Southern Cape

Jacek Stankiewicz

SeismicSeismic Experiment in Experiment in thethe Southern CapeSouthern Cape

SeismicSeismic Experiment in Experiment in thethe Southern CapeSouthern Cape

On-Shore:

240 km

13 shots

48 receivers

Off-Shore

400 km

20 OBSs

9 Air-guns

at ~ 150m

MarginsMargins of Southern of Southern AfricaAfrica

Data example: First (direct P-wave) arrival.

Data example: Air-gun shot registered by a land receiver

What is Travel-Time Tomography?

A method for obtaining crustal and upper-mantle

velocity structure using arrival times of seismic waves.

In this study a two-dimensional structure (i.e. cross section) is computed beneath the linear profile of

seismic receivers and controlled sources.

Provide an initial velocity „model“

Trace a ray from source to receiver and compare

the calculated travel time to the observed one.

Adjust the model to satisfy observed travel time.

This becomes harder with more available travel times.

Reflected phases can also be included in the model.

Problem becomes very complicated very fast.

Inverse problem: use computer software to solve.

Karoo – Agulhas Transect

21,831 Rays

RMS = 0.135 s

χ2 = 1.75

Karoo – Agulhas Transect

Ray Coverage Density

Karoo – Agulhas Transect

Moho Moho DepthDepth SummarySummary

� Depth of ~40 km benath the Karoo Basin

� Slight increase in depth beneath the Cape Fold Belt

� Rapid decrease to ~30 km at the present coast

� Very gradual decrease southwards in the Agulhas Bank (to ~ 20 km over 200 km)

� Decrease to 11-12 km over ~ 50 km at the AFFZ

Moho geometry almost identical to the Western Margin of Southern Africa,

which is a typical Volcanic Rifted Margin.

LowerLower Crustal Crustal PP--wavewave velocityvelocity SummarySummary

High (up to 7.4 km/s) velocities beneath the on-shore section of the

profile, and in the first 20-30 km of the off-shore part.

Much lower (6.5 – 6.8 km/s) velocities in the thinned off-shore crust.

These are typical of sheared margins.

Western margin has high (7.2 – 7.6 km/s) lower crust velocities throughout.

These are typical for Volcanic Rifted Margins.

ConclusionsConclusions

� AFFZ marks the boundary between continental and oceanic crust.

� Our velocity model is consistent with a non-volcanic mode of Gondwana breakup

at Africa’s southern margin due to shear along the Agulhas-Falkland Transform Fault.

� Thick, high-velocity lower crustal zone inland from the present coast is interpreted

as mafic magma underplating, which suggests the early extension was accompanied by

volcanism, most likely from the 180 Ma Karoo – Chon Aike event, which pre-dates the

opening of the South Atlantic by ~ 50 Ma.

ThankThank YouYou

Geophysical Instrument Pool, Potsdam

Captain and Crew of RV Sonne

Trond Ryberg, Ali Schulze, Michael Weber, Bob Trumbull (GFZ)

Nicole Parsiegla, Karsten Gohl (AWI)

Maarten de Wit (AEON)