mountain chain, a north of barrie, drive through gneiss rocks that are very deeply
buried. Granite has a high content of silica (very acidic opposite of basalt) melt large
area of gneiss produce granite, may get what we call rhyolite produced, highly
explosive volcanism, but by and large you aren’t melting crust on a large scale
because there’s no subduction. We do not see volcanoes in obduction zones, no
volcanoes in the Himalayas. On either side of the Himalayas we get fold and thrust
belts, this is where the cover rocks sitting on top of the continental rocks
(dominantly sedimentary) have been shoved, they’ve been put in a huge vice and
squeezed. There will be a lot of oil and gas in these belts, because you’ve got the right
structures to trap hydro carbon. This process welds to formally separate pieces of
continental crust together. Just beyond this weld you’ll see ophiolite (remnants of
oceanic crust; green rocks result from metamorphism of basalts) basalt that has
erupted under water pillows (pillow basalts).
•Slide 9: People can actually build pieces of equipment and model what happens
during obduction, you can see the initial indent and the chinese southeast Asian
area trying to escape from that. Eathquakes in china are the result of India moving
•Slide 10: shows complete Alp Himalayan orogeny, some people argue that it
continues down through the arc down to new guinea. Orogeny describes the process
of mountain building. Orogen is the name given to the rocks that result, this
diagram shows the orogen. Alpine-himalayan orogeny.
•Slide 11: closure of the Tethys ocean as Pangea breaks up, this is basically the end of
Pangea, it had a crudely C-shaped mass, southern continents called gondwana.
Right in the heart of the C is the Tethys ocean, there is a long history before this are
blocks rifting off of Australia being carried north and then colliding with laurasia.
India and Antarctica are separated by a mid ocean ridge, pushes india was to the
north, then Australia also moves away and the ocean is destroyed.
•Slide 12: fig 229: subduction on the far right (top figure) this is the simarian
orogeny (sp?) it’s subduction because we’re subducting the ocean floor, ocean crust
subducts below continents (more dense). And then top left india finally collides and
we get the obduction phase. Foreland basin: when we have high mountains it’s a
considerable load on the softer rocks that we call the asthenosphere (means weak
sphere in greek) it’s the softer plastic rocks over which the plates move, without this
we would be fixed. When we have these huge mountains (think of it as a giant
iceberg) it depresses more than india and we get these broad foreland basins, this is
important when we look at the Appalachians or rocky mountains (next week) very
important because we have shallow sea waves sometimes invade, we get deltas down