Differential Motions of Continents, Mesozoic and Cenozoic Eras

Posted on January 25, 2018 by gymnosperm

The total volume of ocean floor production becomes increasingly difficult to constrain as we work backwards in time. Ocean floor is increasingly lost to subduction, and efforts to replicate the missing parts become speculation.

The continents, by contrast, have grown steadily  through time. Stable parts of continents exist that have seen little or no deformation at Phanerozoic scale. The uncertainties of continental movement derive from remagnetization, inclination, and true polar wander; which can confound Euler rotations.

These uncertainties should not be ignored. Nevertheless, there is general agreement since the beginning of the Mesozoic era on the positions of the major cratons. The positions of continents abutting the Atlantic ridge system are well constrained by existing ocean floor.

The positions and velocities of continents can be shown as vectors which represent the net effects of the forces exerted on them. These forces could be pushing from well-connected adjacent ocean floor, pushing from less connected adjacent subduction, shear, collision with other continental masses, and possibly from unknown forces acting on the deep roots of cratons that extend below the Mojo discontinuity.

In 2014 we obtained the ArcMap data from Christopher Scotese’s Paleomap Project. Using the Point Tracker utility, we projected the southwest and northeast corners of the State of Colorado back to 250mya. We used these and other anchor points to project existing western US surface geology back as well.

 

G160

The view above at 160mya shows how this worked, and provided a wander path for North America.

In the current study we extend the wander paths of stable points on the major cratons of Eurasia, South America, Africa, Antarctica, and Australia in the hope that these paths can help constrain the forces exerted on these continents.

The image above shows the points chosen to begin with at 250mya. Also shown are projected rasters from Ron Blakey showing substantial agreement.

Above are the evolved wander paths on today’s globe. Quite a spectacle. It can be seen from point clustering that there are variations in spreading rate with time. Using the coordinates, individual continental motion rates can be calculated.

As can be seen above, the individual motions are quite a mess. There is general agreement on a spike in motion ~230mya, a slowing in the 170 to 120 time frame, an increase during the Cretaceous superchron time frame, and (with the notable exception of Australia) a decrease to the present.

The above shows the total measured movements over the period.

Using GPlates software and some different assumptions based on true polar wander and geological interpretations, Domeier and Torsvik (2014) produced a graphic of the motion of Africa from the late Paleozoic, the time focus of their paper. The points used to derive their vectors were not specified.

The blue and green curves above are the true polar wander corrected and not corrected rates after Domeier and Torsvik, respectively, for the period we consider here. The red curve is northern Chad from this study.

It is worth noting that according to this study, Africa’s motion has been principally meridional.

Africa’s motion was very rapid NNW from 240 to 200mya, followed by a hundred million years of hesitation and slow backtracking; before assuming an arcuate NNE path at moderate speed to its present location. This trajectory is unique in several respects, perhaps making Africa not the best choice for a general plate motion proxy.

South America was ostensibly welded to Africa until its separation. It therefore shares similar paths with Africa through the “great hesitation”, but South America hesitated somewhat later, and rather than backtrack, it began separating to the SW.

Above are the motion rates for Africa and South America. The two continents are geographically “keyed” together such that differential motion that does not result in separation would result in shear and compression for which there is no evidence. We conclude that the early more rapid northward movement of Africa resulted in a separation zone, perhaps a rift valley, that accommodated subsequent differential motion long before the Atlantic ridge began their official separation.

In the next post, we will examine another austral continental pair: Australia and Antarctica.

 

 

Posted in Continental Wander Path, Paleogeography | 2 Comments

The Pacific Triangle, Take III

Posted on December 28, 2017 by gymnosperm

Essentially, the Pacific Triangle IS the Pacific Plate. Unless you wish to consider it a sinkhole, a point subduction zone, all that we know as the Pacific Plate has grown out of this spot in the last 175 million years.

Seafloor isochrons are two dimensional space-time maps. Every point is also a clock, an increment of time when mid ocean ridge basalt (MORB) crystallized. Leaving aside questions of how the triangular form emerged, every isochron marks the position of the spreading ridge at that particular time. All points older than the isochron are hardened as part of the Pacific Plate, and all points younger do not yet exist as part of the plate.

Christopher Scotese has developed an impressive series of maps as part of the Paleomap Project. These maps, and a Point Tracker utility make it possible to track any point back in time based on the assumptions of Euler rotations.

We decided it would be interesting to track the center of the Pacific Triangle back to the time of it’s origin.

Wow, that beginning is 40 degrees south near current New Zealand. Uncertainties being large in these sorts of things, we thought it wise to check internal consistency.  Visible in the southwest corner of the Pacific Triangle today, we plotted another point and traced its relationship. At 180mya, the southwest corner and the center of the Pacific Triangle are in the same place. At 170mya, the southwest corner moves away to the same distance as today, and tracks the center at the same separating distance thereafter. Test passed.

The Mendocino Fracture Zone is arguably the longest single feature in the ocean floor. It currently extends from the edge of the continental shelf off Pt. Mendocino, California, some 4400 miles to at least the 165mya isochron near the current center of the Pacific Triangle. It manifests as a topographic feature, a gravity low, and a progressive offset in the isochrons.

The “M” points along the top of the image are prominent corners in the isochrons along Mendocino fracture zone where they have been offset. We then ran paleo locations for the isochron age at the corner. These can be seen as red M’s in the lower part of the image. The distances between the paleo centers of the Triangle and the paleo locations of the corners are always the same as today. Second test passed.

Here is a zoomed out view on a gravity map.

Scotese has incorporated this well on his Paleomaps.

At the bottom left of this view, the position of the only slightly offset fracture zone corner can be seen. The view also shows the motion vectors of two corners of Colorado since 250mya, the current position of the US, and a raster of an alternate reconstruction by Ron Blakey for comparison.

A more offset fracture zone corner can be seen in this view at 60mya.

Was the Mendocino Fracture Zone generated in place by differential growth along the ridge, or was it a feature of the prior ocean floor that has vanished? Interesting in this regard, the Sierra Batholith was largely emplaced in the 120 to 100mya time frame, and on the modern map the Sierra batholith and the Great Central Valley appear to be bounded by the Mendocino and Murray fracture zones. The unusual feature of the Transverse Ranges aligns with the Murray fracture on the modern map.

If Scotese is correct, it was actually the Mendocino fracture that aligned with the transverse ranges 30mya. You have to try to imagine the upper Mendocino fracture and the heavy 30mya isochron slid down to the position of the red balloon.

The red line got cut off, but it is the translation at 30mya. From top to bottom, the fractures are Mendocino, Murray, Molokai, and Clarion. The zone between them appears to define the strike slip/extensional regime, with subduction above and below. My intuition is that these fracture zones are prior features, and not merely generated in place during the growth of the Pacific Triangle.

Well, Scotese has a consistent model, but is it right? The triangular form for the origin of the Pacific plate makes no tectonic sense, and the wander path is astonishing. There is another triangle in the Beaufort Sea, which according to Point Tracker has not moved at all in the last 150 million years.

We are not done with the Pacific Triangle yet…

Posted in Continental Wander Path, Geological Evolution of the Western United States, Seafloor Isochrons | 1 Comment

The Cretaceous Superchron, Beaufort Isochrons, and the Motion of North America

Posted on December 19, 2017 by gymnosperm

In the last post we explored a very peculiar set of seafloor isochrons in the Beaufort Sea and Arctic ocean. We found that a perpendicular shift in seafloor spreading defined the Cretaceous Normal Superchron, the longest known period in earth history without a magnetic pole reversal.

We had always been curious what the seafloor isochrons looked like worldwide during the Superchron, and figured out a way to highlight these using Christopher Scotese’s Paleomap Project software. What we did was turn the color off on the modern globe for all  the isochrons except the 80-120 mya period.

We left the paleo coordinates and motion vectors for the SW and NE corners of Colorado in for kicks. There is nothing particularly surprising except the astonishing spreading rate in the western Pacific 120-115 mya. This is also known as the Ontong Java Plateau/Large Igneous Province. We typically think of LIP’s as spreading out and covering existing crust, but if the isochrons are correct, this was actual seafloor spreading.

A South Polar view puts the Ontong Java isochrons in different perspective.

A North Polar view. We became interested in Scotese’s treatment of the Beaufort Sea isochrons.

Scotese has treated the Beaufort Sea very differently, which we will explore. What is interesting in this view is the motion vectors for Colorado. The red coordinate markers are turned on only for the Superchron. During the Superchron Colorado moved initially southwest, but turned northeast. At the end of the Superchron, Colorado resumed the southwest vector that continues to the present. This can be seen more dramatically in the first image. Colorado’s change of direction at the end of the Superchron seems to correspond with the beginning of the modern spreading ridge perpendicular to the isochrons in the Beaufort Sea.

When we zoom in to the Arctic, we see that Scotese has the Superchron Beaufort/Arctic isochrons simplified to two periods. The darker green is 120mya and the lighter green 100. When compared with the Nachon isochrons below, it can be seen that Scotese ascribes a bilateral 120mya to an area left blank, and picks up a 100mya area on the continental shelf of Siberia.

Overlain for comparison, we see very significant differences. The general impression is that Scotese is trying to show Cretaceous extensional forces parallel to the modern ridge. The Nachon isochrons show Cretaceous extension perpendicular to the modern ridge  in line with the Pacific ridge.

Nobody really knows what the North Pacific ridge system looked like 120mya. If the Nachon isochrons are correct, the Beaufort Sea may be a relict of what we now know as the Juan de Fuca ridge. The motion of North America can be explained by extension from this system as well, with waning spreading allowing a more northerly trajectory before the modern ridge from the Atlantic system pushed North America strongly southwest at the end of the Superchron.

Food for thought. Scotese Paleomap for 120mya at the beginning of the Superchron.

 

 

Posted in Continental Wander Path, Cretaceous normal superchron, Geography, Magnetic Reversals, Paleogeography | Leave a comment

Idosyncratic Isochrons, and the Cretaceous Normal Superchron

Posted on December 15, 2017 by gymnosperm

The magnetic lineations in the ocean floor were first discovered using magnetometers designed for submarine detection. These isochrons are tenets of our notion of plate tectonics, yet many isochron patterns are inexplicable by our current tectonic model.

We have previously explored The Pacific Triangle, Charybdis and the Oldest Ocean Floor, and The Ring Around Antarctica. I had occasion to be perusing the Beaufort Sea Floor recently and discovered yet another astonishing set of discordant isochrons:

The geographic North Pole is just below and to the right of the only IODP drill site at the top of the image.

The shapes of isochrons are determined by the patterns of new ocean crust formation and crystallization. The sizes of isochrons depend on both the rate of magma extrusion and the time interval between geomagnetic reversals. Realistically, the magnetic data is messy a fair amount of interpolation goes into the even spacing of isochrons. For example, it is well known that between 83 and 121 million years ago, the longest known interval without a geomagnetic reversal took place. This is known as the Cretaceous normal superchron. If you look at this interval around the globe, you will see an even spacing of isochrons. The isochrons do not represent reversals.

What we see on the floor of the Beaufort Sea is two different systems of extrusion that took place immediately before the Cretaceous superchron. To the north is a small piece of typical seafloor extrusion between transform faults. This was very slow spreading compared with the Pacific ocean floor during this period. If we zoom in and add color we see something very strange going on.

North Beqaufort Isochrons

The lightest color is 120mya ocean floor, the next lightest blue is 125mya, and so on in 5mya increments. There were multiple parallel extrusion fins, each its own miniature mid ocean ridge spreading both directions. This pattern held for 120mya, 125mya, 130mya, and less predominantly 135mya; before the transition to more rapid extrusion 140mya.

While this was going on in the North Beaufort Sea and Arctic Ocean, the South Beaufort shows its own weird extrusions roughly parallel to the North Beaufort.

In the center part of a system with more “normal” progression, there is a series of fins even more tightly packed than to the north. They are less linear, and more like individual plutons. Note the tiny sliver of 120mya to the right against the continental shelf.

There are no trenches up here to relieve the pressure.

Zooming out to where the modern spreading ridge ends unceremoniously in the Laptev Sea, we can see that this perpendicular system is discontinuous in a line of seamounts to the left that must have been a trench. This view is basically looking straight down on the North Pole.

The green to blue color contrast basically separates extrusion before and after the Cretaceous Superchron. Very little, if any, extrusion took place at the North Pole during this interval.

 

Posted in Cretaceous normal superchron, Geography | 1 Comment

Radiation vs the Adiabat

Posted on December 10, 2017 by gymnosperm

Amongst those who have come to realize that climate models based on radiative transfer are running too hot, there is disagreement as to whether the hot bias results from emergent complexity the radiative transfer equations have yet to include; or the irrelevance of the radiative model.

Several people, including Ned Nikolov, have impressively modeled the atmospheric profiles of the earth and other planets using only atmospheric pressure and completely ignoring radiative effects.

As discussed before, atmospheric pressure effect warming is called adiabatic warming. It literally means “without the devil”. For our purposes here, the “devil” may be thought of as radiation.

Adiabatic warming is well known in meteorology. It includes the increased surface temperatures when atmospheric pressure is high, and the warming of “Foehn” winds as they fall from higher elevation plateaus. Such winds are the root of the disastrous fire season this year in California.

Ultimately, the adiabatic effect is due to gravity.We had not found an analysis from physical first principles so we offer the following:

Note: Gordon Dressler was kind enough to point out that the diminution of gravity begins at the center of the earth rather than the surface. Accordingly, the original graphic has been replaced by the corrected one above.

 

It can be seen that gravity approximates the density and pressure profiles of the atmosphere, but that there is significant divergence between about 3 and 20 kilometers above 5 kilometers altitude. This divergence is greatest at the tropopause, 11 kilometers in the US Standard Atmosphere. about 25 kilometers and converges somewhat thereafter.

We wondered about entropy. Noting the density curve above, we find the following:

Entropy by Density

The entropy of the atmosphere (the number of ways the molecules can be rearranged without changing properties) correlates very well with atmospheric density, and very poorly with gravity.

We conclude that gravity may well be the predominant factor in the atmospheric profile, but that other factor(s) must be in play. Very likely radiation is one of these factors. Seemingly, the devil remains ensconced in the details.

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Is the Saturation of CO2 Logarithmic?

Posted on December 3, 2017 by gymnosperm

It is generally agreed by both the red and blue teams in the climate debate that the incremental effects of increasing atmospheric CO2 diminish with increasing concentration. It is often said that the reduction of effect with increasing concentration is logarithmic.

If this is the case, where we fall on the curve at ~400ppm is critical information. If we are on the steep part of the curve, additional CO2 is far more important than if we are on the long tail.

MODTRAN offers a way to evaluate the questions of whether the diminishing effect is actually logarithmic, and where on the curve we fall at 400ppm.

Logarithmic

What we did here is scale and invert MODTRAN W/m2 looking up (for downwelling) at one meter and looking down (for upwelling) from 70 km. A simple base 10 log of ppm is added for comparison. Clearly, the logarithm is a very good approximation of the energy reaching 70 km as a function of ppm. The downwelling at one meter falls off far, far more rapidly.

The data was parsed tediously by noting the values for tropical atmosphere “full bore” (with all the other variables at default values) and varying only CO2. The one meter downwelling varies in “steps” of increasing length, resulting in the increasing sparseness of the data points. Seventy kilometer upwelling shows much more subtlety with every ppm having a different value, at least up to 20 ppm, where we switched to 10ppm increments to save time.

So, is the saturation of CO2 logarithmic or not?

Yes, and no…

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The Fire Came in the Night

Posted on October 29, 2017 by gymnosperm

The deadly fires come in the night, with the swoosh of the wind. Fingers of fire, almost liquid, reaching forward just above the ground; fireballs of mistletoe sailing high in the sky; sparks flying everywhere between.

Entropy unleashed. Potential energy gone wild. Or…simply the dragon’s breath.

Who are we to comprehend this, let alone prevent it?

A hunter’s cigarette, bad hot tub wiring, a windblown tree crashing through power lines. Errors of humans, acts of God, does it matter? The same errors or acts, absent the blow torch wind, have no issue.

That wind, and the issue have visited before.

Credit Mike Hargreaves. Only details changed since 1964. Ignition near Calistoga in Napa County, transit across the Mayacamas Mountains to Santa Rosa…if a wild assed fire chief had not stolen a bulldozer and cut broad fire breaks through what would have been houses this year, the hospital would have burned in 1964. He was lucky. The blow torch wind stopped.

The New York Times did an interesting timeline map:

NYTimesMap

By 3:AM fire trucks from Berkeley and San Francisco arrived to slow the fire’s progress at the bottom of the map…and the blow torch wind slowed. Almost on cue.

Fifty-three years is over half a human lifetime, a very long time for modern cultural memory to hold.

We built plywood houses in the dragon’s breath.

 

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River of Time

Posted on October 11, 2017 by gymnosperm

I’ve been rowing the Grand Canyon for about a month. Sixteen of us were on the river for 20 days. It takes a couple days to drive there and a couple more to drive back. It takes a day to rig and much of another to unrig.

I used to write poems more often when I was younger. Much younger.

This is what I wrote on the trip:

River of Time

When I close my eyes I see the water,
Pulsing, swirling,
Chaotic to me,

When I look up between the canyon walls,
I see the sky,
Pulsing, swirling,
Water as white clouds,
Wind that presses,

The water in the sky and in the river yearn for the ocean,
From whence they came,

We have sprung from the water,
And share the yearning,
We dance the pulses and swirls in bright artifacts,

We feel the pull of the final destination,
We know the water will reclaim us,
But we have learned that the river is the arrow of time,

We must take our temporary leave,
To dance another day.

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Socrates Died for our Sins

Posted on July 24, 2017 by gymnosperm

Was Socrates a troll? Maybe. He was not a diplomat. He sought no consensus.

Still, hemlock?

Given the chance to recant or drink, he drank.

Human superstition is boundless. We are not enlightened beings.

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Barefoot Gophernating

Posted on July 3, 2017 by gymnosperm

Barefoot gophernating is one of those senseless acts of beauty.

First of all, it requires an illegal fuel-air bomb assembly a la Caddyshack. Actually, it is not illegal as a welding rig. Mine uses a standard torch handle and propane rather than the traditional acetylene as the solvent gas. Many ironworkers use propane for cutting as well. It is cheaper and has more BTU’s.

Whenever the neighbors complain, I point to my welding rig and explain that sometimes it makes a pop.

If you have ever watched a tomato plant disappear from your garden, pulled to the subterranean netherworld by these nefarious creatures, you will understand.

This sort of terrorism demands revenge.

Enter the illegal by various Torts of nuisance gophernator.  You can blow fifty feet of gopher tunnel ten feet in the air. Even if you miss the gopher, it is enormously satisfying.

It is dangerous, like many satisfying things in life. The barefoot thing is extra spice. The directions admonish you to wear sturdy shoes. It is like brazenly standing on the second step of a three step ladder that says, “This is not a step”.

Have a great Fourth of July. My fireworks will be underground.

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