About Me

My photo
I'm a consulting geologist for a small company in the Denver area. I study problems related to active tectonics, using geomorphology, structural geology and remote sensing.

Monday, March 13, 2006

lesson time

hey all...

so i'm back from the field, safe and sound, despite sitting on the back of a scooter and riding all over the peikang-hsi valley and puli basin. found some cool outcrops and decided that when i finally draft a cross section through this area, the areas where the shui-chang-liu and pai-leng formations are deformed by intense folding will just be shown as squiggley lines... the real pattern of deformation is totally confusing to me. the one thing that is very clear to me is that these rocks have really been through the wringer. actually, an old style laundry wringer is a fairly effective analog for what happens to the deeper rocks of the chinese continental margin as they are accreted into the critical wedge of the taiwanese orogen. sorry for all the lingo, if you don't understand it, google it, that's my best suggestion. in so many words, the westward motion of the pacific-sea-plate is cramming the entire island of taiwan up onto the chinese continental margin. as these rocks are fed into the orogen (read: mountain range) they are over-ridden by the weight of the entire island. the rocks get crunched and rolled and smooshed, sort of like feeding a flat towel into a wringer, and letting it just pile up on the other side. once enough of the overlying rocks have been stripped away by the amazing erosive power of typhoon's rains, the "munched" rock finally reaches the surface. this 'exhumation' of material occurs because surface material is stripped away and more rock has been placed underneath the island... essentially cycling material from deep underground up towards the surface. picture putting the same towel through the same wringer, but then wring out another towel, and let it pile up underneath the previous one. repeat for a while until you have a really big pile of towels and you start taking the dryest ones off the top, though you continue to add towels to the bottom of the pile... eventually you'll see your favorite beach towel you wrung out half an hour ago because you're working your way down through the pile. the cool thing is that the pile never gets any bigger once you start taking the towels off the top... this is referred to as 'steady-state tectonics' (well, not when you're talking about towels, but you get the idea).

another process that happens, on a smaller scale, is 'flexural slip faulting'. this is specific to folds (especially cylindrical ones), so i'll try to keep the analog simple. as i mentioned before, rocks fold. it seems odd at first, but under the right circumstances, rocks will fold just like cotton towels. a better analog however, is a book. folding is most easily understood when talking about sedimentary rocks. you can fold any kind of rock, but only sedimentary rocks start out in thin flat layers, like the pages of a book... parallel planes resting against each other. quickly, a fault is basically when rock breaks. flexural slip faulting is specifically the kind of fracture that happens parallel to bedding (the pages) when folding occurs. all of you have picked up a phone book and bent it in half... or at least rolled up a magazine. it's all the same thing, what you notice when you do this is that the open edge of the book sort of splays out... the edges of the page are no longer even with each other. the ones on the inside of the fold stick out further than the outside cover... pages (or beds) which are all the same width are being forced to conform to different radii... and as this happens, the pages rub against each other. if you glued every page in a phone book together (which is a classic prank, by the way) you essentially end up with a big piece of wood, which bends about as easily (interestingly enough) as a big piece of wood. when you fold rocks, the beds can break along their interfaces, and slide along each other... this is the easiest way to accomodate the shearing created by folding. some pics are here to help illustrate this... the fold: this is what folded rocks can look like. this is a pretty tight fold for beds this thick... thin beds are easier to fold and thick ones tend to have a larger fold radius... think about the difference between folding a magazine and one of those nearly-indestructible cardboard baby's books. the book: the two covers are the same size. when the book is bent, the inside cover has a tighter radius, and sticks out further. look at the triangle. the short leg is the thickness of the bed. the long leg is the total bed-parallel slip of the unit (book). the angle between the hypotenuse and the short leg is the shear-angle. the blue plane is the axial plane. the rock wall: this is a bed. the exposed surface is in fact a flexural slip fault plane. it is practically polished from the sliding, and exhibits excellent "slickensides"... kinematic indicators that show the direction of sliding. these can be used to reconstruct the stress-field responsible for creating the fold and can also record multiple episodes of slip in different directions. btw, the two people in the pic are ling-ho and po-no (thanks guys!), two of the students who help me with field work here. the close up of a slickenside: just that... the best slicks often form in recrystalized mineral deposits left by thermal fluids that flow along the fracture plane of the fault.

ok.... enough lesson for now... geez that was kind of long. anyway, i hope that satisfies the technical info request i got a little while ago... maybe i'll post something about the actual geologic history of the at some point too...



Michael Turton said...

great stuff, Tarka.

Sooner or later spammers are going to find your blog and fill it with garbage. You should consider enabling COMMENT VERIFICATION which blocks 99% of spam (you have to read those funky letters at the bottom of the comment; computers can't do that). It's in the COMMENTS submenu of your SETTINGS menu.


Tarka said...

thanks for the tip michael!