Differential Motions of the Continents, Mesozoic and Cenozoic Eras V: The 80mya Inflection

In the last post on constraints we decided that the east Pacific and Juan de Fuca ridges were useless in constraining the motions of the Americas. We decided that the Atlantic, Greenland, and Lomonosov ridges were acceptable explanations for the motions of the Americas; and that we simply have no explanation for the motions of Antarctica and Australia, as they are out of scale with the adjacent spreading.

There was clearly a disturbance of the forces acting to move the continents about 80 million years ago.

On the 250mya base map above, Eurasia can be seen to very nearly reverse field, and set a course that rotates  continuously to its current position.

North America made about an 80 degree change of course to the west.

South America was the least affected, but changed course modestly. Africa ceased drifting eastward and headed north.

 

 

Antarctica came to a flying stop and reversed field slightly. This projection exaggerates the retrograde motion. Australia slowed and drifted north.

Above is the 80mya base map with the 80 to 70mya ocean floor extrusion. Even at 80mya one begins to appreciate the difficulty caused by missing ocean floor. There really does not seem to be anything extraordinary going on here.

The south polar 80mya view tells a different story. The map has been simplified to mere suggestions of the continental shelves to focus on the relation between the ocean spreading and the relevant  points. At this scale the blue 70mya Antarctica points are barely visible below and to the right of the red and green 80mya marker. Australia drifted somewhat more down and to the right 80 to 70mya as the separation of the two continents begins.

There is a lot of southern hemisphere and South Polar spreading. India launched about 120mya and its astonishing trajectory is aided by nearly 600 kilometers of spreading between India and Antarctica.

Ridge jumping is a murky concept in tectonics as the discipline has been fixated on the notion of plates. Ridges define plates, and it is very inconvenient when they jump, since real estate formerly on one plate gets transferred to another; or entirely new plates are created. We have zoomed in above and it is easier to see the 70mya Antarctica points behind the 80mya marker. The green is the 90 to 80mya seafloor production, and the red is the 80 to 70.

In a sense, the Pacific ridge has been “jumping” continuously since its inception about 175mya as it moved away from the Pacific Triangle. It continued this habit from 80 to 70mya, except at the very bottom of the image we can see a secondary jump to the other side of the 90 to 80mya ocean floor. We believe this “double” extrusion is the hallmark of the 80mya inflection.

Well behaved ridges extrude into and separate symmetrical bands of older ocean floor. On the 80mya globe this did not happen much. Ridges either jumped to offset or doubled to enclose older ocean floor.

In the next post, we will explore the need to get beyond plates. Ridges must be the focus, and a coherent system of naming the ridges must be developed.

 

 

 

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Differential Motions of the Continents, Mesozoic and Cenozoic Eras IV: Constraints

We know the position and velocity of stable points on our six cratons. A reasonable question to ask is, “Can we account for this motion?”

Above is a map of the ocean floor created in the last 10 million years. We will start at the present and work backwards, as 100% of the ocean floor is available to aid our constraints.

The present ocean floor is 361 million square kilometers. This map shows 27 million square kilometers, or ~7.5% of the current ocean floor extruded in the last 10 million years.

Of the last 10my extrusion, nearly 12 million square kilometers, or ~44%, was extruded by the East Pacific Ridge, shown in magenta. The Nazca, shown in rose, extending toward Chile, and the Juan de Fuca shown in lighter magenta off the Pacific Northwest add another ~1.4 million square kilometers.

A careful examination of the isochrons around the prominent “arm” of the Pacific ridge pointing towards Panama, and the Nazca arm as well, shows that even these exerted spreading force according with the predominant east-west axis of the ridge.

Very nearly 50% of the ocean floor extruded in the last 10 million years has been in the Pacific Ocean basin. The Americas are poorly connected to the Pacific basin and have continued moving westward even as the spreading ridge has moved eastward. The Pacific basin has been subducted beneath the advancing Americas. Whatever braking effect this subduction may have had, half of the ocean spreading in the last 10 million years cannot be used to constrain the motion of the Americas.

 

There is currently little or no subduction going on around Antarctica. The Australian-Antarctic ridge in rose and the Pacific ridge join in a peculiar fashion. In a  2013 post we discussed the oddity that Antarctica is circumscribed with ridges, yet has no trenches. Antarctica has moved about 135 kilometers and rotated about 3 degrees as shown. This Antarctic motion is about 50% more that the spreading of the Atlantic and Indian ridges apparently pushing the right direction, and is less than half of the antagonistic spreading of the Pacific and Austral-Antarctic ridges.

In order for this motion and growth to be accomplished without expanding the earth, the entire system must have moved with Antarctica. The motion of Australia is ~28% greater than the combined spreading of the ridge and motion of Antarctica.

Africa is also caught between spreading ridges with little or no current subduction. It has moved only about 40km west to east at its 58 degree bearing. The motion of South America is in reasonable agreement with the spreading of the Atlantic ridge.

It would be truly astonishing, and call into question our most fundamental concepts of plate tectonics, if the motions of North America and South America could not be accounted for by the spreading of the Atlantic ridge. The pieces fit together like a puzzle, existing magnetic lineations cover the entire interval since separation, and the existing ridge tracks the shapes.

We can see that the motions do generally conform to the scale of Atlantic spreading over the last 10 million years. What remains to be explained is why Africa and Eurasia remained relatively fixed in longitude while the ridge and the Americas moved to the west.

There are two areas in the mantle with the chewy name of LLSVP’s where shear waves are attenuated. I call these Low Velocity Provinces doughboys. They appear to be extrusions from the core. One is under Africa, and the other out in the Pacific. Their footprints at the core-mantle boundary (the bottom of the box) are at the 1% anomaly contour and all sorts of plumes and kimberlite pipes seem to originate there as suggested in the upper panel and red dots at the surface.

Above we have plotted the 1% footprints of the doughboys (after Torsvik). These footprints are thought to have remained fixed in relation to the core. Their highest reaches flatten out at the 660km discontinuity, and the crust has moved around above them. It seems possible that Africa is somehow grounded in its doughboy, and being substantially connected to Eurasia for the last 10 million years, has restrained this entire mass while the Americas moved to the west.

We have the extrusions of the Greenland Ridge, seemingly an extension of the Atlantic ridge; and the Lomonosov ridge, nearly crossing the North Pole, to complete our 0-10mya analysis. These ridges seem to be in accordance with the very modest SE motion of Eurasia and the bearing of North America ~15 degrees south of due west. Once again, with no trenches, the Arctic spreading ridges must have moved away from Eurasia their spreading distance less the 49km opposite motion of Eurasia.

The answer to the question at the beginning of this post, “Can we account for this motion?”, is ambiguous. We can account for the motions of the Americas, Eurasia, and Africa; with the caveat that the explanation for the relative fixation of Africa and Eurasia is hypothetical. The motions of the Austral pair, Antarctica and Australia, are wildly out of scale with adjacent spreading. As the ridges must necessarily have moved with Antarctica, we must conclude that the motions of Antarctica and Australia remain unconstrained.

It seems far too tedious at this point to step back 25 times through our 10my increments. In the next posts, we will focus on the inflection points.

 

 

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Differential Motion of the Continents, Mesozoic and Cenozoic Eras III: North America and Eurasia

In the first post in this series we set the goal of using motion vectors of the continents to analyze the forces exerted on them, and described the motions of Africa and South America. In the second post we described the odd circuit travelled by Antarctica and Australia.

Eurasia travelled in a tightening circular path until 100mya. It then took a sudden nearly reversing 20 million year excursion to the southeast, before resuming its seemingly ordained tight arc to the present position.

The points chosen were plate vertices on the Asian side of the suture zone that unified Europe and Asia. This zone was active in the Paleozoic, but the unification was complete before 250mya. The dates of some major inflection points are noted.

At 250mya, Eurasia was oriented substantially north-south, and the differences in the paths of the currently more northern and southern points, respectively, reflect the rotation to the current east-west orientation.

It can be seen above that Eurasia hasn’t ventured much since 250mya and most of its motion is a net rotation of about 90 degrees clockwise. This rotation must have caused shear Eurasia and North America/Greenland. Indeed, Roberts and Lippard (2005), Redfield et al 2004, and Andersen et al (1999) find evidence of strike faulting in the Caledonian corridor.

North America has always shown a wanderlust. It is the oldest Craton. We are skeptical of reconstructions deeper in time than this study, except in the broadest sense, but many reconstructions show North America as the first craton to leave the fold the late Precambrian supercontinent of Rodinia.

Perhaps we should not be surprised that North America was the first continent to leave Pangea. Its wander path was unique, following neither the north then south about face of Africa and South America before their split, nor the circuit and rotation of Antarctica and Australia, nor the extreme rotation of Eurasia.

After an excursion to the southwest at 240mya (consistent with a pattern of anomalous 250-240mya motions), North America meandered northwest with significant inflection points at 210 and 180mya. Like Eurasia, North America took an excursion from 120 to 100mya before seemingly resuming its prior meander. At 80mya, the meandering years ended. From 80 to 70mya North America headed nearly due west from its former northerly meander, and since 70mya North America set a WSW beeline to its current location.

We have described the wander paths of our six cratons. In the next post, we will bear down on constraints.

 

 

 

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Differential Motions of Continents, Mesozoic and Cenozoic Eras II: Antarctica and Australia

In the first post of this series we explored the rationale for extending precise analysis of continental motion only 250 million years. The ultimate goal of this exercise is to develop rigorous constraints on the forces creating the different motions, but first we will finish the descriptions.

Pangea was like a pile of flotsam. It was buffeted from various directions, and rather than responding as a unit, different units moved differently as the pile evolved and eventually broke up. Of the six continents we are working with, Africa and South America were keyed together for a while, North America and Eurasia were always dancing to different drummers than the rest, and Antarctica and Australia stayed together for a long time.

It is interesting to view above the motions from the starting point 250mya.

Antarctica and Australia can be better seen from a polar view above. Since the paths cross, we turned Antarctica blue for clarity. It can be seen that the two were very closely bonded for 150 million years. Between 110 and 100mya, their wander paths very nearly completed a circuit. The Australia point at the mouth of the Roper River and the Bay of Carpentaria passed within 17 kilometers of its starting point 150 million years earlier.

After the separation, Antarctica moved west to straddle the South Pole, and Australia arced mostly north. Since the points are stable, differences between pairs of points result from rotation centered closer to one point than the other.

It is worth noting that at 200mya, the points corresponding to current east Antarctica and north Australia were rotated to positions at 37 degrees latitude, the current positions of San Francisco and Melbourne, Australia. This should be borne in mind when marveling at “alligators and palm trees” in Antarctica. It is also notable that the 200mya northern apex of the circuit and the reversal correspond to one of the great extinction events in earth history.

The circular gambit of this pair of continents before their separation seems bizarre, but we will see that Eurasia has a spiral track when we examine the remaining two individualistic continents in the next post.

 

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Differential Motions of Continents, Mesozoic and Cenozoic Eras

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.

 

 

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The Pacific Triangle, Take III

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 isobars 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…

 

 

 

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

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.

 

 

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