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