TERRANES C: WRANGELLIA

                                    WRANGELLIA, NORTH

THE WRANGELLIA PALEOMAGNETIC CONUNDRUM

In the earliest days of terrane excitement the name most prominently discussed was “Wrangellia”.  As Nick has explained, Wrangellia is an accreted terrane outboard of most of the current version of the western North American collage of terranes.  Rocks comprising Wrangellia show it to be latest Paleozoic and earliest Mesozoic in age.  Wrangellian-type rocks occur in bits and pieces from Alaska to – possibly – northeastern Oregon.  A big chunk is exposed on Vancouver Island.  The largest, as you might expect is located in the Wrangell Mountains of southern Alaska.

Lucky for paleomagnetists,, Wrangellia contains some volcanic rocks that have survived with their magnetic signal mostly intact.  These have been investigated in several places, notably the Wrangell Mountains proper (Talkeetna Fm; Jack Hillhouse & USGS crew) and Vancouver Island  (Karmutsen Fm Ted Irving & Co).  I am going to drastically over-simplify what these two studies found.

The rocks in question are roughly 230 Ma on age.  Their direction of magnetization is - very approximately -   shallowly upward, to the north.  This gives rise to the conundrum:

As you certainly know, the geomagnetic field has two steady states (polarities); we call them normal (N), and reverse (R).  In an N field, a rock magnetic direction of northward and up indicates origin in the southern (geographic) hemisphere.  However, given an R field the upward magnetic direction indicates origin in the northern (geographic) hemisphere, but with the magnetic vector pointing SOUTH.  

Thus, the paleomag data, which are nearly  impeccable, leave us with two choices:  either Wrangellia was in the southern hemisphere 230 Ma ago, hence has been transported many, many  thousands of km northward, OR it was in the northern hemisphere at that time and subsequently has moved much less further northward -  but has rotated 180 degrees in the process!  One of these is almost certainly correct.  What to do?*

Well, in the 80s it was common to yield to ones innate fixist bias and opt for whichever scenario required the least relative displacement, so the second alternative tended to be favored.  At that time however, I was a wild-eyed mobilist (still am), so I favored the southern hemisphere alternative (still do).

What do you think?

*See diagram in Jones et al, 1980. p 78

 

 

S. WARREN CAREY, PART 2

                     VISCO ELASTICITY MODEL

Did you ever wonder why it was so hard for the geological community of the early and middle parts of the 19^th century to accept the concept of continental drift?  Probably not, but now’s your chance.  After all, Wegener, du Toit and others had presented a varied body of evidence that it HAD occurred which, nowadays, appears totally conclusive.  So, why did so many prominent geologists disagree?

Well, human cussedness obviously played a part.  If you grew up instinctively harboring  fixist ideas, and those ideas had served you well throughout your career, damned if you were going to let a bunch of  Johnny-come-latelies turn everything upside down!

However, if you rejected drift you needed some scientific argument to back you up.  And such, you thought, existed – from the field of seismology!  Continental drift couldn’t be true because it was impossible!

Early drifters had at best a bunch of crude ideas about HOW drift occurred, and all of them required some degree of mobility in, at least. the upper mantle.  Nay-sayers could point to seismic evidence that indicated that the mantle, far from being the least bit soupy, was more rigid than steel!  Hence, no drift.  It was all an illusion.  Here is how that works. 

It transpires that some physical properties of the interior of the earth can be deduced by what happens to earthquake waves as they pass along their paths.  In particular, how rigid a material happens to be is reflected in how rapidly the energy contained in a seismic wave is dissipated – converted to heat.  Precise seismic measurements showed conclusively that the mantle was extremely rigid.  Voila!  The mantle had the properties of hard, cold steel – and continental drift, Q.E.D,  was out of the question.

Well, S. Warren Carey blew that particular ship out of the water with an important paper that nobody reads any more:

Carey, S. W., 1953, The rheid concept in geotectonics, , Journal  of the Geological Society of Australia, Volume 1, Issues 1, 2.

In this paper, the old Tasmanian Devil points out the fact – that should have been obvious – that the response of a material to an applied force depends in large part on for how long the force is applied.  Look at the model at the top of the page.  If a force is applied for only an instant,  the spring will show significant (elastic) displacement but the piston will not have time to respond very much.  Hence the material will appear to be totally elastic.  However, apply the force for a geological significant time and the “dashpot” (think a cylinder filled with goop) will deform so much that the elastic contribution can be totally ignored.  In other words, on a tectonic time scale, the mantle behaves very much like a soupy, viscous fluid!  Among other things, this makes thermal convection not only possible, but likely.

If you have trouble with this concept, try a thought experiment.  Mentally strap on your crampons, grab an ice ax, and find a nice steep glacier to climb.  On the time scale of your ascent you can treat the glacial ice as a fine, trustworthy solid – even though, as a good geologist, you know that, on a time scale of weeks or even days, it is flowing steadily  downhill!

 

S WARREN CAREY; PART 1

                           S. Warren Carey and an expanding earth

S. Warren Carey, the Tasmanian  Devil, Part 1

I am going to describe the work of one of the most important geotectonic thinkers of the early and middle years of the 20^th century.  S (Sam) Warren Carey was a professor at the University of Tasmania; although he had visiting appointments at prestigious universities elsewhere.  He is sometimes described as an early advocate of continental drift – but that, while true, requires an asterisk, and a big one.  Carey certainly believed that the sialic continents of the earth had once been together and have separated subsequently, creating ocean basins in between.  However, Carey knew nothing (or cared not much) of mantle convection.  Instead, he proposed that the earth once had a much smaller diameter – about half as big as now, if you must know – and had a complete sialic shell!  Then, as the earth expanded, the sialic shell broke into pieces and moved apart, creating relatively young oceanic crust in between.

This was not as wild an idea in 1930, say, as it seems today.  After all, the astrophysics people had shown that the universe itself is expanding.  So, people mused, what if Newton’s “universal constant, K – as featured in his equation F = K(m1Xm2)/r² was actually a variable)?  If gravitational attraction decreases, do objects get farther apart?  Would the earth expand as a consequence?

Well, as it happens, probably not – but Carey had good reason to think otherwise, before Vine and Mathews and the plate tectonics scientific revolution.  It can be shown (has been shown) that Carey’s specific model doesn’t work - but that is a topic in itself, which I may get into someday.

So if Carey was so wrong, why do I maintain that he was an important figure in geoscience?  That will be argued extensively in my next blog.  However, note that terms defined by Carey on the basis of his expanding earth hypothesis are in common use today.  Who has not heard of “oroclines” – bent linear crustal features – first defined by the Tasmanian Devil?  Less commonly used are “rhombochasm” (the Atlantic Ocean basin could be so described}, or “sphenochasm” – the Arica depression in the western edge of South America could be one of these – although I doubt it.

Here is Dr. Carey, pleading his own case:

https://www.youtube.com/watch?v=Othb0xsvZb4&list=PLRA8S2qijlkE9bJe69VlglZuU-ppoxVAU&index=2&t=0s&app=desktop 

TERRANES B

                                                    Lopez Island

EARLY EVOLUTION OF THE “TERRANE” CONCEPT

Early on, when I first began to believe that large chunks of crust were allochthonous* with respect to interior North America I simply referred to them as “displaced crustal blocks”.  Some others interested in this line of work called them “micro-continents”, but that particular awkward terminology  quickly disappeared, although it left the term “microplate tectonics” behind as a memento.

The “official” terrane definition arose out of a GSA Penrose Conference held on Lopez Island, Washington, in the summer of 1980.  A report on that conference was published in the GSA journal later that same year:

            Beck. Cox and Jones, Mesozoic and Cenozoic microplate tectonics of western North America, Geology, v. 8, pp. 454-456, 1980.

The order of authorship is alphabetical; Davey wrote most of it.

Anyway, this “official” definition arose from three fun-filled days of argument, field trips and liquid refreshment.  Here it is::

A tectonic (or tectonostratigraphic) terrane is a fault-bounded geologic entity characterized by a distinctive stratigraphic sequence  and/or structural history differing markedly from its adjoining neighbors ….

Later, of course, new terms arose:

            Suspect terrane:  A chunk of crust that may fit the definition given above, but not for sure.

            Exotic terrane: A tectonostratigraphic terrane that didn’t originate as part of North America.

*Two useful words that geologists toss around:

            Autochthonous:  Formed right where it is now.

            Allochthonous:  Formed somewhere else than where          it is now.

Thus, the B.C. terranes Nick is describing are allochthonous, whereas the young cover rocks that partially hide them are autochthonous.