Bits and Pieces of Siletzia
Listening to
Nick’s lectures on Siletzia and Farallon-Kula-North America tectonic
interaction has chipped some rust off of my iinitial “understanding” of
a half-century ago. Maybe some of you
would enjoy a few vignettes from that time.
As usual, these are MY recollections, and may not be wholly correct,
First, the
name… Siletzia surely is named for the
Siletz Volcanics of northwestern Oregon.
It so happens that these rocks were some of the first ever studied
paleomagnetically in North America – by Allan Cox, as part of his Ph.D.
dissertation, I believe. He published
the result in Nature, sometime in
1957. He used to tell amusing stories of
that field work; he operated out of an ancient reconditioned hearse, which had
the habit of breaking down at inopportune times – as, for instance, just in
time to prevent the unloading of a car ferry!
I wish I could do justice to Allan’s tales of woeful field work!
The Siletz
Volcanics are only one of several bits of volcanic Siletzia protruding from
beneath the sedimentary covering. Unless
I am grossly mistaken, study of these essentially coeval volcanic groups gave
rise to the notion of localized tectonic rotation. To explain why requires a lengthy detour.
In the 60s I was working for the Regional Geophysics Branch of the USGS, under Isidore
Zietz and alongside Randolph (Bill) Bromery.
Bill was the resident expert on aeromagnetic surveying; I was his
side-kick and “expert” on the magnetic properties of rocks. To digress even further:
Aeromagnetic
surveying consists of measuring the strength of the earth’s magnetic field
using a device (magnetometer) attached to an airplane. This is then compared to a reference value, calculated
by some complex mathematics I won’t attempt to describe. The difference between the measured and “expected”
intensities, if consistent over a substantial stretch of real estate, is
regarded as a geologically significant “magnetic anomaly”, and may receive
further geological investigation. The
USGS, under Izzy, published many useful aeromagnetic maps/
And
continuing to digress: Bill was in
charge of surveying coastal Oregon. When
his crew flew their instrument over exposed volcanics, Bill expected to record
a positive anomaly just south of the body, and a smaller negative anomaly just
to its north. A line drawn from peak
high to peak low would point due magnetic north. (To explain why this is so would require
several illustrations. (maybe Nick will oblige someday. Anyway, much to Bill’s surprise, his maps
showed the expected high-low pattern, but oriented in such a way as to suggest
that the total magnetic moment of the rock bodies was roughly NE-SW. Clearly, these Tertiary rock units retained a
permanent (remanent) magnetization that largely overwhelmed the induced magnetic
moment Bill had expected to find. How to
explain this?
Well, there were two possible solutions. One was that continental drift had been such as to produce a NE-trending direction of remanent magnetism in these early Tertiary rocks. This was known (from studies made elsewhere) to be partially true, but the amount of such rotation was wholly inadequate. Something else was required. Namely, localized rotation. How had it come about?
Well, I immediately invoked the effect of a coast-parallel dextral shear zone (now known as the ball-bearing model). Cox’s group, on the other hand, preferred a big-block rotation model, resulting from differential extension in the Basin and Range province.
I am pretty sure that, after decades of work by other people, both mechanisms must have been at work.
