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It ’s meter to redraw the mapping of the reality during the reign of the dinosaurs , two scientists say .
Picture the U.S. West Coast as a excruciate architectonic boundary , similar to Australia and Southeast Asia today . Erase thegiant subduction zoneresearchers have long snuggle against western North America . discharge a vast archipelago into the ancient Panthalassa Ocean , usually drawn as an empty void , the kind on which medieval mapmakers would have depicted fantastical animal .
Under the west coast of North America, seafloor from the Pacific Basin sinks back into the earth’s mantle. The subducted seafloor remains visible to seismic tomography, a geophysical imaging method that uses earthquakes as signal sources. This 3-D image renders the mountainous topography of the western U.S., and the ancient oceanic plate from the surface down to 1500 km depth (colour changes in depth increments of 200 km).
" Now it fits together , " said Karin Sigloch , a seismologist at Ludwig - Maximilians - University in Munich , and lead subject area author . " We ’ve come up with a passably different resolution that I think will sustain up . "
With a stunningly clear look at the remnants of ancient subduction zones under North America , Sigloch and her colleague Mitch Mihalynuk have revised 200 million year of geological history . The termination of their study are publish today ( April 3 ) in the journal Nature . [ Have There Always Been continent ? ]
The old Benjamin West
Under the west coast of North America, seafloor from the Pacific Basin sinks back into the earth’s mantle. The subducted seafloor remains visible to seismic tomography, a geophysical imaging method that uses earthquakes as signal sources. This 3-D image renders the mountainous topography of the western U.S., and the ancient oceanic plate from the surface down to 1500 km depth (colour changes in depth increments of 200 km).
The level set about 200 million years ago , just before the Atlantic Ocean basin first open up . North America was crammed into thePangaea supercontinentand the vast Panthalassa Ocean covered the rest of the planet .
Panthalassa ’s sea floor has almost entirely disappear under North America as Earth ’s tectonic plates have dislodge . , so its story is moderately kabbalistic . A few geologic clues led scientists to the prevailing model , contested by the new cogitation . The puzzle pieces let in the few remaining mo of Panthalassic ocean floor , rocks scattered along western North America , and remnants of the old pelagic crust seen under the continent , where the plateful go away into the blanket , thelayer of Earthbeneath the incrustation .
After North America started shifting to the west 185 million years ago , forth from Africa and Europe , the prevailing good example aim a subduction geographical zone along the West Coast that consumed Panthalassa ’s huge oceanictectonic plate , the Farallon plate . Imagine a background like today ’s Andes in South America . Like a elephantine conveyor belt knock , the subduction zone deliver exotic terranes that build up out the western continent .
The breakup of the Pangaea supercontinent.
But for some scientist , the puzzler pieces just did n’t fit this ikon . Sigloch and Mihalynuk think they know why .
What lies beneath
Sigloch lookeddeep into the mantle , at the remnants of oceanic insolence that remain beneath North America . In a subduction geographical zone , two of Earth ’s tectonic scale collide and one slue into the mantle . The plates are dusty and denser than the surrounding mantle stone . Seismic waves commute their pep pill as they hand through the plates , uncover their location . No one had seen these sometime pieces of crust in this point before because it requires powerful computers and very impenetrable array of seismal monitoring equipment , both of which came about in the last decade .
A cartoon illustrating how a vertical slab wall could form under North America after Pangaea breaks up.
Sigloch find a upright pile ofoceanic crustsegments stack on top of one another alike bricks . The massive wall run from northwest Canada to Central America , and is 250 to 375 miles wide ( 400 to 600 kilometer ) and extends from between 500 to 1,200 miles ( 800 to 2,000 km ) beneath the surface .
These former slab , as geologist call subducted freshness , secure the locating of Panthalassa ’s ancient subduction zone in time and space , the study evince . The slab drop vertically about 0.4 column inch ( 1 centimeter ) per year and have not change over in space . Because geologists know the travels of North America in contingent , thanks tomagnetic stripes in the Atlantic Ocean , the researchers can pull back the continent and compare it to the location of subducted cheekiness .
" What this does is provide us with a time machine . The slab are say us the site of past sea trenches and the locality of island electric arc , which are the construction blocking of continents , " said Mihalynuk , a geologist with the British Columbia Geological Survey in Canada .
The geologic history of western North America since the Jurassic, based on subducted oceanic crust. The left shows time slices of subducted oceanic crust, and the right shows an intepretation of the position of geologic features based on the position of the subducted slabs.
Two new plates
What the mantle evidence show is that as Pangaea break up , out in the Panthalassa Ocean , a Brobdingnagian archipelago sat far offshore of North America . It was shaped like an arrowhead , 6,200 miles ( 10,000 kilometre ) long , with its tip pointing to the proto - Pacific Northwest . Two sea basins ride between the archipelago and North America , called the Angayucham and the Mezcalera . On the other side of the archipelago , further to the Dame Rebecca West , was the Farallon plate .
Instead of an Andes - style subduction zone , with the Farallon shell slide to the east under North America , there was a westward - dip subduction zone , with North American crust sliding beneath the Angauychum and Mezcalera plates .
A cross-section of a portion of the Cascadia subduction zone.
" It make perfect sense , " said Robert Hildebrand , a geologist at the University of California , Davis , who was not involved in the study . " If there was eastward subduction , as most people would suggest , it would leave behind a bread crumb trail strewn out across the blanket . That ’s not what you see . "
As North America deal westward , with its gross profit sinking the two plates , it ram into the arrow point about 150 million years ago . This impact was responsible for raising the initialRocky Mountains , Mihalynuk say .
Further west of the archipelago , the Farallon ocean plate was descending into its own subduction zone , the mantle evidence indicate .
Violent architectonics
In fact , the entire western Panthalassic was more like the Southwest Pacific , where northeast Australia collides with islands , microcontinents evaporate into the Earth , and there are summerset - flop subduction zones , the study author state .
" The ocean washstand was not a vast abyssal plain , " Mihalynuk told OurAmazingPlanet . " The position is much more like the Southwest Pacific than seaward South America . " [ World ’s Biggest Oceans and Seas ]
Over time , collisions with the unlike subduction zones smeared exotic terranes along the continent ’s westerly margin , a rock track record that has long suggest at North America ’s red tectonic history . The history revealed by the cape slab avail explain some thwart geology , such as rock transmit 1,200 miles ( 2,000 km ) from Mexico to Canada . " I would say that this modeling come very nigh to conform to the geology , " Hildebrand said .
Sigloch also found another , more westerly slab wall connected to the Cascadia trench off Washington and Oregon , into which the last remnant of the Farallon crustal plate , now called the Juan de Fuca , is subducting .
Mihalynuk said the fresh framework will make waves , as it overturns 40 yr of accepted Wisdom of Solomon about the evolution of western North America . " It will take a while to turn the great unwashed around . That noetic ship has a spate of inactiveness , " he state . But for Mihalynuk , " this is one of those constantan moments . "
