Slab pull versus subduction suction

The dominant forces that drive plate tectonics are those created by subduction.  Slab pull is transmitted throughout a plate system when subducted oceanic lithosphere remains mechanically attached to its parent plate.  However, detached slabs that descend into the mantle, excite viscous flow that might exert traction on the base of the lithosphere, thereby sucking plates along.  Attached slabs also create suction.  The relative influence of the two forces is an important input to global dynamics that prevail today.  Slab pull operates to draw subducting plates towards destructive margins, whereas subduction suction should act on both the under- and over-riding plates to drive them towards subduction zones.

Using plate motions, estimated from Mesozoic to Recent magnetic stripes, and subduction history at nine destructive margins, Clinton Conrad and Caroline Lithgow-Bertelloni of the University of Michigan compared them with motions predicted from slab-pull and subduction suction (Conrad, C.P. & Lithgow-Bertelloni, C. 2002.  How mantle slabs drive plate tectonics.  Science, v. 298, p. 207-209).  Much simplified, their findings suggest that slab-pull forces account for around half of the driving force of plate tectonics, with a nearly equal contribution from subduction suction induced by subducting slabs.  However, both attached and detached portions of lithosphere that descend beneath the 660 km deep mantle transition zone probably do not transmit stresses into higher-level slabs, and only their suction effect adds to plate motions.

Continental insulation at the Precambrian-Cambrian boundary

Shortly before the Neoproterozoic ended with the Cambrian Explosion of animals with hard parts, much continental lithosphere clumped together in a Vendian supercontinent, called Pannotia by some geologists.  If the idea described in Empirical geochemistry points to continents’ role in mantle dynamics (earlier) is realistic, that surely would have created “pressure-cooker” conditions in the mantle beneath it.  Possibly piled with as much as 2 km of ice sheet during a “Snowball Earth” episode, this assembly of cratons would also have been somewhat depressed.  From about 650 to 500 Ma Pannotia experienced generally outward extensional forces.  The Pan-African and Braziliano orogens, formed slightly earlier, underwent widespread magmatism unrelated to any crustal thickening and deposition in many sedimentary basins.  Spanish and Moroccan geologists have tried to explain this evolution in terms of the blanketing effects of Pannotia (Doblas, M. et al. 202.  Mantle insulation beneath the West African craton during the Precambrian-Cambrian transition.  Geology, v. 30, p. 839-842).  Pan African and Braziliano orogens surround the West African craton, and the authors opinion is that their anorogenic magmatism stemmed from a build-up of heat resulting from insulation by thick continental lithosphere.  More controversially, they see this as an escape mechanism from Snowball Earth conditions, through the associated magmatic release of CO2.  In turn, they see this addition leading to increased flux of calcium to the oceans, toxic stress from this spurring an evolutionary response by soft-bodied metazoans in the form of carbonate secretion by their cells; hence continental clustering leads to the Cambrian Explosion!

The lost world of the Galápagos hotspot track

The Galápagos islands straddle both a hotspot and the active spreading centre that generates the Cocos and Nazca Plates in the Easter Pacific.  Consequently, both those plates have topography owed to former activity at the Galápagos hotspot, the Cocos Ridge and associated seamount chain, and a set of ocean-floor uplands that resulted from complex evolution of the Nazca Plate.  Both plate vectors drive this topography towards subduction zones beneath Central America and the Andes.  Unsurprisingly, this gives rise to a kind of inverse tectonic constipation, as both subduction zones attempt to consume awkward knobbles on top of the downgoing slabs.  Detailed seismic profiles have revealed the current state of affairs, which has been going on for around 71 Ma.  Some of the seamount and aseismic ridge materials parted company with the downgoing slab, to be obducted onto the Central American arc.  These ophiolites represent the lost history of the Gala Galápagos hotspot, from about 71 to 16 Ma ago, and information from them has allowed a team of German and Cost Rican geoscientists to piece together an enthralling tale that feeds into the evolution of the Central American land bridge (Hoernle, K. et al. 2002.  Missing history (16-71 Ma) of the Galápagos hotspot: Implications for the tectonic and biological evolution of the Americas.  Geology, v. 30, p. 795-798).  Central America not only formed a land bridge that allowed the Late Tertiary mingling of faunas from South and North America, but by disconnecting the Atlantic and Pacific Oceans it transformed low-latitude ocean currents, and probably set in motion the climatic cooling towards the Great Ice Age.  However, the story now seems considerably more complex.

Galápagos-related igneous rocks bear strong geochemical similarities to those of the 90 Ma Caribbean Large Igneous Province (CLIP), now to the east of Central America.  This supports a long-held view that the CLIP formed during the initial evolution of the Galápagos hotspot, and was driven eastwards by spreading from the predecessor of the East Pacific Rise.  Being a huge, low-density patch of ocean floor, it failed to subduct, but passed between North and South America when it encountered  the volcanic arc of the Greater Antilles, channelled by two large fracture zones.  Subduction flipped beneath the Antilles, to consume Atlantic lithosphere westwards, while Pacific subduction restarted in the “lee” of the CLIP and began to generate the Central American arc.  This Late Cretaceous to Palaeogene transformation formed the first land bridge connecting both continents, allowing terrestrial fauna and flora to mingle, including late dinosaurs.  Emplacement of the CLIP in its present position removed the land bridge of the Antilles Arc, again separating both continents for most of the Tertiary.  Assorted sloths, armadillos, elephants, ferocious cats and the like, eagerly awaited the next chance to rampage, evolving awhile.  By the Early Pliocene, the growing Central American arc slid in to fill the gap, and biotic pandemonium ensued.  This signal event of recent geological times was itself encouraged by the continued magmatic productivity of the Galápagos hotspot, and the failure of its low-density products to return to the mantle.

Cunning means of estimating uplift

Rises and falls of the continental surface have frustrated geologists trying to assess their timing and rates, largely because the available methods are tiresome.  Fission-track, Ar-Ar and U-Th/He measurements, used to work out when rocks became sufficiently cool either to retain scarring tracks of high-energy particles or to allow radiogenic isotopes to accumulate in specific minerals, are notorious stumbling blocks to research.  So it is extremely encouraging to learn that there is possibly another way.  Bubbles (vesicles) that form in lavas, when dissolved gases escape from erupting magmas are sensitive to atmospheric pressure; the lower the pressure, the larger they become.  Bubbles at the top of a flow form under atmospheric pressure, whereas those at the base emerge under the extra pressure of the overlying load of lava in the flow.  Comparing top and base vesicle sizes, and applying the known thickness of a flow seems to be a means of calculating ancient atmospheric pressure.  This lateral thinking has been applied by Dork Sahegian, Alex Proussevitch and William Carlson of the Universities of New Hampshire and Texas (Austin) to the uplift of the Colorado Plateau (Sahagian, D. et al. 2002.  Timing of Colorado Plateau uplift: initial constraints from vesicular basalt-derived paleoelevations.  Geology, v. 30, p. 807-810).  They first calibrated the vesicle palaeobarometer using nine samples of recent Hawaiian lavas from widely different elevations, finding that their method matched actual elevation with a statistical precision of ±410 m.

Plotting the difference between modern and ancient elevations in the Colorado Plateau against the lavas’ Ar-Ar age reveals a history of uplift that tallies well with known geomorphological  evolution.  The authors have been able to show that uplift began at least 20 Ma ago, at a rate of 40 mm per year, which accelerated to 220 mm per year over the last 5 Ma.  This has resulted in a total uplift of almost 2 km in the two phases.

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