That Old Tectonic Feeling…

This is the story of one of those innocent questions that came up from a student. In asking about plate tectonics, they wondered about the origin of the term “tectonics”. This led to a merry chase…

The place you expect to start is the Oxford English Dictionary. And there indeed is an entry for tectonic as a word that went back to the mid-1600s. But then it wasn’t a geological term; it referred to construction or building practices. Now geology is one of the more thieving branches of science (we steal words and make them do something else…or even make off with seeming misspellings or incorrect usages, as in remanent and terrane), so it makes sense that somebody at some point decided that building mountains was an act of earthy construction and hence tectonism and so stole the term.

But the OED claims the earliest geological usage was in 1894. This felt quite late; lots of geological work was already in existence. So GG went to the History, Philosophy and Geoheritage Division of GSA and asked if anybody there knew. Christopher McCauley took up the challenge and noted that etmonline pointed to an 1887 origin. He also noted the Greek roots and so concluded that the ‘geological “tectonics” = literally “the building or fabrication of the Earth’s crust (or its structures)”.’

Aja Tolman then carried it farther back, noting that Eduard Seuss’s Die Enstehung der Alpen used the term in 1857, though he felt the more geologic usage came after an 1873 quake.

Ken Taylor then pointed to Gabriel Gohau’s Histoire de la tectonique (Paris: Vuibert / Adapt-Snes, 2010). There, Gohau finds that “the beginnings of modern tectonics are situated mainly in Élie de Beaumont, 1829.”

So we’ve got some heavy hitters in Suess and de Beaumont. But Mott Greene whose book on 19th century geology reflects his expertise, noted in an email to Jody Bourgeois that a prominent 1888 glossary of terms did not include tectonic. He contends it is not in Suess’s The Face of the Earth in either German or English and that the first use would appear to be Karl von Zittel in his History of Geology and Paleontology (1899). That section uses the term so casually, though, that you’d think it was in circulation before then.

Walter Szeliga then got ahold of this and started clawing through the old papers. It turned out that Suess had used geoteckonisch and tektonisch in his 1868 paper Über die Eruptivgesteine des Smrekouz-Gebirges 

Rasoul Sorkhabi then chimed in with an 1850 volume where geotektonik was used: Karl Friedrich Naumann’s Lehrbuch der Geognoise (A Textbook of Geognosy). Alerted to that usage, Szeliga then went to a couple older contributions where the term might plausibly have been present (Dana (1840’s) and Rogers and Rogers (1820’s), both addressing the Appalachians) and found that the term was not present there.

And so that is where the trail was finally left. Gohau alluded to Beaumont having presented the beginnings of modern tectonics back in 1829, but that phrasing doesn’t necessarily mean that the term tectonic was used then. It was out there and employed by Suess in the middle 19th century, though it seems American geologists were not quick to hop onto this term (Gilbert didn’t use it in his 1890 Lake Bonneville masterpiece, for instance, though he did latch onto the more recently coined “isostasy”). It would seem that tectonic seeped into the literature in Europe and lingered a long time before its heroic turn as a key part of plate tectonics.

The Curious Case of the Sierra Foothills

GG is having to come back to grips with this for a field guide, so will expose some ignorance and bias, no doubt, in reviewing the problem.

First, we’re talking about the foothills on the west side of the range between roughly the San Joaquin River and the Tule River. Along this stretch, basically there are no exposures of Tertiary rock between the granitoids and metamorphic rocks of the Sierra and the Quaternary sediments of the San Joaquin Valley. Knobs of such basement rocks rise up, surrounded by gentle topography usually under a cover of some sediments. The absence of Tertiary sedimentary rock led Saleeby and Foster to suggest that this segment of the foothills was drowned, having been pulled down by descending “arclogite” (dense, arc-related lower crust; mostly garnet pyroxenite) into the mantle. This predicted that paleocanyons at the lower end of the Tule, Kaweah, and Kings rivers should be pretty deep. This also explained why there were no exposures of Cenozoic sedimentary rocks at the edge of the range.

Drowning old river canyons, however, is not the case. Sousa et al. (2017) note that water wells in the area hit bedrock less than 75m down going out as much as 10 km from the Sierra front. The Kaweah River traverses a lengthy stretch where the basement is less than 10m down–hardly a buried canyon–and indications are that the Tule and Kings also lack a buried paleocanyon. Sousa et al. term these flat parts peneplanes and incorporate those into a broader umbrella of a pediment, which also has knobs rising abruptly from the flats. Weathering of the basement rocks is unusual: in general there is a later paleosol, and where this cuts across ultramafic rocks, unusual nickel rich minerals are found. Using low-temperature geochronology and this geology, the authors infer a phase of rapid cooling prior to ~80 Ma followed by slow cooling to pretty low temperatures by about 40 Ma, when these rocks were close enough to the surface to develop this paleosol. They infer that this was subsequently buried under as much as a few hundred meters before being re-exhumed in more recent times.

Original caption from Sousa et al. 2017: Figure 2. Geologic map of the southern Sierra Nevada foothills (central California, USA) from ~36°N to ~37°N, modified after Saleeby (2011). Each sample site is labeled with white on black circles. Bedrock pedi- ment is shown in a light red shade. The first four rows of the legend under Bed- rock Pediment each show two columns representing the map unit of the underlying lithology and the right-hand column indicates the shade that appears on the map where the bedrock pediment overlaps the lithology at left. State highways 180, 198, and 190 are drawn as thin black lines, and labeled with the corresponding highway number. SSNP—southern Sierra Nevada pediment; SJ River—San Joaquin River.

Deeper to the west, the Cenozoic section is present and documented through drill holes and seismic profiling. Phillips has used this to estimate a tilting rate along the margin of the Sierra. He estimates about 4° of tilt since the pediment of Sousa et al. was created. A topographic profile across the Tule segment of pediment loses 170m over about 20 km distance…about a half a degree slope. On the Kaweah, over 25 km out to the 75m depth contour, the pediment loses a total of about 180m of elevation…which is about 0.4°.

Houston, we have a problem.

85 Ma Trainwreck: Igneous Rabbit Hole to Wonderland

OK, that might seem a bit off-center, but as GG has finished a paper on the Mojave Waistland and wanted to share some…well, GG hopes are insights but maybe qualify as delusions. This approach is a somewhat different direction than the paper takes and so is a complementary means of approaching this material.

There is so much that is screwy about the Mojave. The one hook that seems to unlock things for GG is the igneous history of the Mojave. So let’s start with a quasi-modern map where the Mojave distribution of igneous rocks is the modern basemap (stuff west of the San Andreas has been moved back). So this is GG’s update to Jacobson et al.’s 2011 map:

Notice that on the Peninsular Ranges to the south and in the Sierra Nevada to the north, there is a nice, compact (roughly 50 km wide) swath of 100-135 Ma plutons just west of another similar swath from 85 to 100 Ma. This pattern has long been recognized. The Mojave? There appears to barely be any 100-135 Ma plutons. The 85-100 Ma stuff, while seemingly limited to near the Garlock, is either a 50 km wide swath now rotated 90° or represents a really wide swath of plutonism. The younger stuff seems to cover the whole Mojave, and this is true even if you focus on smaller subsets of time evident in the individual ages marked with symbols.

What does this mean? Maybe, you say, this is the result of Cenozoic deformation and things were fine before all this ugly extensional and trans current faulting came into play.

Weighing Gravity

OK, GG has found something scientific to grump about instead of the increasing “Perils of Pauline” stuff going on politically. And it is, just how do you use gravity to understand the support of mountains? (This is going to get long…you’ve been warned). So look for the terms used, the history of gravity in the Sierra, what isostatic gravity is trying to tell you (and what it isn’t) and what a purely gravity-focused study might look like.

Terms (quickly). There are three main gravity anomalies out there. Free air is great in some places, but not mountains, so let’s not worry about it. Bouguer anomaly (and we’ll use that as meaning the complete Bouguer anomaly) removes effects like distance from the center of the Earth and the attraction of rock above sea level. The isostatic anomaly is a less standardized but often assumes a reference crust 30 km thick with a density of 2.67 g/cc above sea level that is in turn above a Moho with a 0.35 g/cc density contrast. This means the crust should be 30 km + (2.67/0.35)*elevation. The gravity from that root is then subtracted from the Bouguer anomaly.

History. The case under study is, to nobody’s surprise, the Sierra Nevada. For many years, the story there was that the Sierra were supported by an Airy crustal root and this was supported by gravity measurements. To be most charitable, sort of true. When Andrew Lawson wrote his 1936 paper on the “Sierra Nevada in the Light of Isostasy“, there was precisely one measurement of gravity in the range. So his hypothesis was lacking any real geophysical support. Which led Lawson to get his colleague Perry Byerly involved. Byerly was a seismologist and so he sought seismological evidence…which he found in late P-wave arrivals at stations on the east side of the Sierra. This, he inferred, meant there was a very thick crust–an Airy-type root–under the high part of the Sierra. By 1961, some seven seismological studies had confirmed that there should be a thick crust under the Sierra. This was the situation when Howard Oliver and colleagues tried to interpret a new dataset of >1000 gravity measurements, far above the 20 stations available prior to this work.

The Broken Plate fallacy

OK, this is kind of classic grumpy-ism. When people go to model deformation at scales of 10s to 100-200 km, they will often use elastic plate theory. This gives us things like foredeeps and forebulges and the topography of oceanic trenches. It has its issues (as a purely elastic solution, it should recover when the loading is removed…which isn’t what we see a lot of the time), but it is pretty helpful.

Well, until somebody says “oh, there is a big fault zone, so that must be breaking the elastic plate, so let’s use a broken plate model!” And, you know, many times this is utter silliness and reflects ignorance of just what the assumptions of a “broken plate” really amount to.

What is the assumption? It is that there is no moment transmitted at the broken end of the plate. But when you dig in, you find this is actually saying that no in-plate normal stresses beyond ambient pressure can be transmitted, which is quite a different issue.

The missing Sierran model

There are two facets to tectonics: observations that constrain models, and the models that provide a framework for gathering more observations. So, for instance, in the early days of trying to understand the Sierra Nevada, Andrew Lawson developed a model where the crust kept getting thicker as the range rose, which led to seeking an observation on the thickness of the crust.

Now we’ve been having a bit of a tug-of-war between the old high Sierra and the young high Sierra camps. Most of this has centered on observations and their interpretation. Was that deposit tilted or was that slope its primary depositional geometry? How accurate is the relationship between del-O-18 and elevation? Is that incision from climate or from tilting? But where we aren’t seeing real back-and-forth is in the framework around the two hypotheses.

Now the young Sierra crowd has been busily at this really since the late 1970s when wholesale modification of the crust seemed off the table. The range could pop up once the subducting slab was removed, it could heat up and rise for a similar reason, you could remove mantle lithosphere. The current favorite is some flavor of foundering lithosphere, where a very dense lower crust and upper mantle was removed in some fashion.

How about the old Sierra crowd?

Old Fogey or Inbred?

Another year has passed and so the Grumpy Geophysicist risks toddling more and more into the realm of the Old Fogey. So with the new year, a post that stalled in limbo is completed…

One of the curious aspects of science is that dueling camps develop that seem to ossify. Often the two sides keep firing the same kind of observations at each other with little impact. Examples include the pro-plume and anti-plume camps (the latter most energetically defended by Gillian Foulger and the late Don Anderson), the Baja-British Columbia vs. small offset groups addressing the Cretaceous evolution of western Canada (mostly paleomag purveyors versus structural geologists). In many ways, an earlier example was the fixist versus mobilist debates of the middle twentieth century. So how much of these kinds of stagnant debates is simply resistance to change, how much is a result of echo chamber action, how much is genuine ambiguity, and how are these stalemates broken? And how do you avoid being an Old Fogey?

Outsider/Insider

GG is polishing off a paper on a place where he has gathered precisely zero new observations. Does that make him an evil parachuting scientist, leaping in to interpret others’ data before zooming off to some other locale?

Um, er, maybe?

Eroding Confidence

When a student, GG felt that erosion was such a dominant force in earth science. You’d go to the Grand Canyon and be told, all this created in six million years (about 0.1% of Earth’s whole history). You might hear of the boo-boo that threatened to fill the Imperial Valley (and did create the Salton Sea); the redirected Colorado River started incising in soft valley fill, producing a waterfall that migrated upstream nearly a mile a day. Lava flows that dammed the Colorado in the Grand Canyon for tens of kilometers were wiped away in fairly short order.

But then you look at incision rates of the Colorado in the same places as the lava dams and it seems surprisingly low. Then there is low-temperature thermochronology interpreted to mean that the lower canyon has been sitting there for tens of millions of years with little change. Measurements of erosion rates on high Sierran surfaces are minuscule (~0.01 mm/yr, or 10m/Ma), similar to other bedrock erosion rates on mountain ranges, suggests that erosion is pretty feeble over large areas.

So let’s go on an expedition into erosion…with a few numbers and a lot of speculation.

Reading A Section

or, “you point to this section a lot, but I don’t think it means what you say it means.”

The section in question today is a north-south profile drawn by Jason Saleeby in his paper arguing that the Shatsky conjugate came through the Mojave Desert…

Caption: North-south profile across the Tehachapi Mountains (at right) into the southern Sierra Nevada (at left) leveraging an interpretation of a CalCrust profile (source of the reflection fabrics, Malin et al., 1995). Pink is the Sierra Nevada batholith (SNB) while the green is inferred extent of Rand Schist.

What does this show? Saleeby wrote that “geophysical data indicate that the base of the batholithic crust and its underlying mantle lithosphere have been tectonically removed and replaced by the Rand schist.” Many people have given GG the impression that this diagram shows that the lower crust has been removed tectonically and therefore requires the collision of an oceanic plateau.

It shows no such thing.