Working at the southern Central Andes we have found some interesting features. In the hinge zone of a broad (ca. 6 km) basement-cored anticline there are sub-vertical clastic dikes of breccia cutting the sequence. The basement is composed by Paleozoic metamorphic rocks and granites and is cropping out several km away from the dikes outcrop. It is covered by Jurassic volcanics of the syn-rift stage of the Neuquén basin. The dikes have between 1 and 3 meters wide and are not oriented in none particular strike. The breccia is composed by basement and syn-rift volcanics angular clasts and boulders ranging from 1 cm up to 50 cm. We are working with the structural data obtained from field work and are interested in any reference we can use to better understand their genesis. The timing of the intrusion is not clearly related with folding, it could be more related with a prior extensional stage in the region...
Do I understand it right that your dikes consist of clasts that are derived from different exposure levels (i.e. basement and overlying cover?). We had a similar situation Australia, although it was more of a conglomerate than a breccia. I attach the paper about it, just in case it could have any bearing on the situation you are facing.
If I understand correctly, your dikes could be very similar to those beautifully outcropping in the Dolomite, Latemar Massif, vertically cutting mid-triassic dolomites and interpreted as triassic diatremes.
Try to look for references on Latemar, Dolomites, Italy
Thanks for the responses and references! The matrix of the dykes is clastic, not volcanic allowing us to discard the diatreme option. The breccia is composed by clasts of basement rocks (metamorphic and granites) and volcanic rocks similar to those they are intruding but there are none clast from the uppermost clastic post-rift sequence. Because of this they can not be collapse breccia...
Do not discard too fast the diatreme option, it can be very elusive in some cases. The volcanic material can be in very small amount and of very small size. If the lava is confined into an aquifer, very few volcanic material goes up but you have a lot of fluids/volatiles generated so you can create massive injections. I attach a paper I wrote on a quite different setting but you might find it interesting. The system you study seems fascinating!
What is the nature of the matrix? we have a breccias relatively similar in the first stage of a freatomagmatic volcanic episode. This first stage has not any volcanic evidence in the matrix, nor in the clast. We have a submitted paper if you interested I could send you.
Thanks Julien for the paper! Indeed we have a lot of work to do analysing the samples in detail. We do not have yet the thin sections. The diatreme option could be the more logical answer to the problem giving the volcanic environment where the dykes are intruded....And yes, it is a really fascinating issue!!!
Yes Mario! Please, could you send me a copy of your paper? It seems that the diatreme option could be the best answer for our problem...
Mr V.H.Garcia, I am fascinated to see the Clastic Dyke, Once it has no directional orientation, and magnitude is only 1 to 3 meter wide, look for its root. And link it with Seismically originated dyke (Palaeo seismic Event may be of very high energy ) further if it has no intrusive wall contact effect then you may confirm to be of seismic dyke. However size of 50 Cm. boulder may call explanation. In (tectonic-ally dynamic) Quaternary terrain such clastic dykes are seen in NE India.
Dear Dr. Mazumdar, have you references about the clastic dykes of NE India. It would be great to compare them with ours!
I also just remembered of this Clastic Detritus blog:
http://clasticdetritus.com/2007/08/01/conglomeratic-intrusions/
No idea if it has been published a bit more
Although on a much smaller scale, here is an old (2000) paper on the relationships between clastic dykes and the hinge zones of anticlines in Precambrian rocks.
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