This part 4 section of the AusIMM webinar shows the detailed structural geology of the South American continent from 40 – 200km depth for the first time.
South America Seismic tomography 40 km depth – Mid lower crust.
Going down only 13 km to 40 km depth the lower crust looks entirely different. There is a large enclave of dark cratonic material surrounded by lighter cratonic material.
There is a large ring structure in the centre-right composed of three concentric rings around which mines are located. The diamond mines appear to form where this ring structure has been destroyed by later, smaller ring structures. The WNW and NNE linear structures are still visible but a lot weaker.
South America Seismic tomography 67 km depth ~ Moho depth.
This figure shows the tomography at 67 km depth, about the depth of the Moho. The Moho is a 10 to 30 km thick unconformity between the lithosphere and the crust. In seismic sections it shows up is very bland layer and that’s what it looks like in the plan seismic tomography, it has a very subdued relief.
The rings are also much more subdued however the surface mines do follow these rings. Especially in the centre of the plan, Bolivian ring, the diamond mines follow either side of this ring structure in the NW.
One thing that is noticeable is that the texture of the Moho in South America is exactly the same as in the Pacific Ocean.
This is very different from the texture in the Atlantic Ocean at this depth. The Atlantic Ocean shows highly textured, cratonic lithospheric material which suggests that this area is part of the cratonic South American continent.
South America Seismic tomography 170 km depth – Middle Lithosphere.
We now descend to the tomography at 170 m kilometres depth. The inset at the top-left is the original data which is just the seismic tomographic data with no detail.
The lower left-hand inset shows large rings in the top-left corner. This ring shows up in the top left corner in the right-hand plan. It actually forms the outline of the coastline of the top third of South America. This major ring has mines all around it. This ring can be easily seen on the next image.
The tomography at 170 km depth has been specially enhanced to show large ring structures. The inset at the top-left is the mildly enhanced data seen in the previous figure.
The lower left-hand more enhanced inset shows large rings in the top left corner. The top left corner (in the right-hand plan) with the mines all around it following the coastline of America is actually the outline of median ring of the major ring.
If this large ring is similar to the Late Heavy Bombardment rings on the moon then the centre will be full of lava. There’s probably 100 km thickness of lava in this huge ring structure which would form a rigid block of lithosphere. When Plate Tectonics started, probably 2.7 billion years ago, (McCall 2010) these large Impact structures would be the last to be broken up.
From the evidence of the active edge of the rings in Australia and South America, South America probably traveled to the NE during the Hadean to Permian and is currently being rammed into by the Pacific Ocean Plate. However, the northern half of the continent retains a circular shape. It can be seen that the diamond mines in particular form on this concentric ring structure. In the Andes and NE Guyana, the mines are located on the outside of this major, 5,000 km diameter ring.
Comparison of South American ring structures with the Lunar Mares.
This figure shows the South American ring structures on the left and the Lunar Mare ring structures on the right. The South American rings are about three times larger than the lunars mares. This would only be a scale affect because the largest South American impact is larger than the moon’s diameter. However, they correlate in number, density and morphology (Grieve and Pilkington 1996, French 1998, Frenchand Koeberl 2010 ).
The Lunar Mares are known to have formed during the Late Heavy Bombardment from 4.1 to 3.8 billion years ago (Hiesinger et al., 2000 and 2010) and it is 100% certain that the South American ring structures formed at the same time and by the same method.
South America Seismic tomography 200 km depth – Middle Lithosphere.
This plan is of the deepest seismic tomography I could get of South America at only 200 km depth.
In Africa, China and North America seismic tomography down to 400 km depth was available. South America is nowhere near as well as studied as these other continents. This plan was optimised for lithology but many rings and linears are seen.
Focusing on the diamond mines it can be seen they follow the rings in the centre and edges of the large dark ring structure at the eastern corner of South America. Just to the south of this large ring structure the diamond mines lie in a small dark ring structure and to the centre-west the diamond mines lie on the north rim of a 1000 km diameter ring structure.
So, it’s obvious that the diamond mines are associated with ring and crosscutting linear structures. There are a couple of diamond mines located on the north and east of a south-central 2000 km diameter ring. This ring would be a good diamond target.
In the next section we will be looking at different methods of diamond targeting.
If you found this research useful and interesting share it with your colleagues.
If you disagree with the findings of the research contact me in the comments below or on LinkedIn.
Kind regards
Bob
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