MY SISTER, SUSANNAH

 One of the more disheartening aspects of aging is the inevitable necessity of watching your contemporaries perish.  My sister, Susannah, has just died, and of course I feel rotten about it.  Of course, now that it is too late, I strongly regret how little attention I paid to her during her 83 years of life.  As a child she was a bundle of joy to my parents but, I regret to say, an annoyance to me.  I was four years her senior and teased her mercilessly - she once threw a hammer at me, certainly for good reason.  She was not an excellent student, but by dint of hard work and dedication she mastered a difficult program at U. Arizona and then functioned successfully asa health care professional.  She was married twice – her first husband Harry, a prince of a guy, died tragically at an early age. With Harry, Susannah had two children, subsequently two grandchildren and, just before her death, a great grandchild.

Susannah was not a particularly gregarious person, but she always seemed to be involved in things.  She was a birder on and off, delighted in taking her little dog to visit nursing homes, at various times taught a fitness class, went on group hikes, and so forth.  I had the impression that she needed little “down time”.  I envy her that.

Well, Hell!  Susannah, what can I say?  I will always regret not making more of an effort to keep our lives entwined.  Now all I can say, after the Navajo – Go in peace, dear sister.  We always will love you.

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Okay, I give up!

Another of the more annoying aspects of advancing age is the increasing inability to organize/store/retrieve things, ranging from favorite coffee cups to invaluable computer files.  I have spent many fruitless and frustrating hours searching for pictures of, and writings by, my sister.  That is why this blog has no introductory picture – and also lacks a wonderful attachment.  Family members should have a CD with remembrances of our days at the Lake City cabin (late 40s and most of the 50s).  True to form, I have hopelessly misplaced my copy.  If any of you can find it, please share it, along with any pictures you might retain.  Susannah’s contribution is absolutely priceless.

ANOTHER COMPLICATION

             NOTE THE ANGLE OF SUBDUCTION

Nick has amply demonstrated the fascinatingly complicated nature of tectonic activity along the western margin of North America during late Mesozoic and Tertiary time. Zentnerds will of course know exactly what I am talking about; all others Google “Nick Zentner geology lectures” and go from there.  This little blog is meant to add a bit more complication, as if any more were needed.

If western North American tectonic history were as simple as the original Dickinsonian model a lot of present-day academic geologists (including, probably, me) would have spent their lives earning an honest living as bricklayers, plumbers, and other useful people.  Fortunately, it wasn't..  

To slightly oversimplify, the early model called for monotonic eastward subduction of the Farallon plate beneath North America, resulting in formation of the “California triad”; from east to west: magmatic arc, forearc basin, sedimentary subduction complex.  The earlier model also called for creation of the San Andreas Fault a few tens of million years ago, as the Pacific plate impinged on North America.  Zentnerds will know this stuff backwards and forwards; the rest of you should study up.  Anyway, this model – which works well for California proper -  is owing to vital early geophysical work by Tanya Atwater and others, and rendered into geological dogma by Bill Dickinson and his followers.  As I said earlier, it works pretty well for the southern portion of the Cordilleran tectonic belt – if, of course, you ignore Baja B.C.

However, north of Cape Mendocino things get very  much  more complicated.  Nick has done a masterful job of describing what might be termed “accretionary tectonics”; that is, growth of the continental margin by adding large scraps of foreign lithosphere to the craton.  Again,  Zentnerds will know exactly what I mean; anyone else still with me will have even more to study.

So, did you follow up on the bit I wrote about triple junctions?  If you did, you will know that some of them are mobile, and can go sliding along the continental margin.  That introduces some secondary complications, particularly with timing.  For instance, as the Mendocino TJ tracked northward from southern California it should have altered the margin tectonic setting systematically, from subduction to transform.   This ought to show up ,in the geologic record.  Does it?  Damned if I know.

Well, finally, my additional complication.  Magmatic arcs appear above the point at which certain pressure-temperature conditions are encountered.  This, obviously, depends mainly on the angle of subduction.  Most models depict that angle as about 45 degrees, but in the real world it can range from nearly vertical to nearly horizontal.  For the most part the actual subduction angle seems to be determined by the age of the subducting lithosphere.  Old, cold lithosphere is “negatively buoyant”, hence tends to sink at a steep angle, pulling its trailing plate along with it.  This kind of subduction is a positive plate-driving force.  Conversely, young lithosphere is hot, positively buoyant, and has to be forcibly shoved under to be subducted.  Such subduction zones retard plate motion, and can produce super-large earthquakes, like those found in Chile – and, unfortunately, our own Cascade forearc!

Finally, what determines the sink/not sink properties of a slice of oceanic lithosphere?  Well, pretty obviously: how far it is from the ridge that gave it birth, and how fast spreading at that ridge is taking place.  (Cooling rates seem to be fairly constant).   So, the additional complication I threatened earlier is this; potentially, the location of subduction-related magmatism depends largely on the location and activity of an offshore ridge – and these  can, and do, change.  So, poentially, the location of the magmatic arc likewise can skip around.  Hasit?  Again, damned if I know.

ISOSTASY: Another great cocktail-party word

                        Regional  isostatic compensation

Upsalla, the medieval capital of Sweden, was once a seaport but now is landlocked, many meters above the level of the sea.   From the point of view of that ancient university town, the sea has retreated markedly in just a few short centuries.  So, a conundrum: where did all the water go?

Another conundrum:  One frequently finds ancient sedimentary basins now filled with far more detritus than it would have taken to fill the original depression; say, 3000 meters of sedimentary layers in a hole known, on other geological observations, to originally have been no more than about 1000 meters deep.  How can such a thing come about?

Well, many of you geology enthusiasts will know the answer: isostatic compensation. 

Decades of measurements of gravity as a function of elevation have proven conclusively that the crust if the earth is for the most part in a kind of floating equilibrium.  The name “isostasy”, in fact, means “equal pressure”; at a suitable depth in the mantle all pressures are equal, no matter the surface topography; mountain range, ocean basin, etc.  Departures from this condition are known as isostatic anomalies and mostly occur in places where something interesting geologically is going on.  Take the case of Upsalla, for instance.  Prior to the foundation of the city the local region (the Baltic) was covered by a thick ice cap.  The weight of the glacial mound depressed the crust regionally.  When the glacier disappeared the land surface began to “rebound”; that is, gradually rise up.  To Upsallaers it must have appeared that the sea was receding but, of course, the converse was true; they were rising. 

Note that, because of the quasi viscous nature of the upper mantle*the uplift, still going on, is gradual.  In fact, it follows a sort of logarithmic law, assuring that as it nears final equilibrium the rebound rate gets smaller and smaller.  Thus, Stockholm may be safe as a seaport, but poor old Upsalla will be forever land-bound; baring, of course, another ice age!

As for the over-stuffed sedimentary basin, the answer is obvious: as stuff is washed in, the bottom sank until isostatic equilibrium was obtained.

Isostasy can introduce some minor complications into the task of working out the local geological history.  For instance, look at the diagram at the top of the page.  Because the crust has some elastic properties, when a load depresses an area partially compensating upward bulges will be generated.  Then, if the load (a glacier, say) is removed both up and down crustal displacements are generated, complicating the life of the Quaternary geological community no end!

I am tired of this subject, although I have just scratched its surface.  All good Zentnerds not familiar with this concept should go immediately to Google and read up on regional vs local compensation, as well as the models proposed by Airy and Pratt.  But: no exam, I ptomise.

·        https://frivilousessays.blogspot.com/2020/10/s-warren-ncarey-part-2.html