ON CHRISTOPHER SCOTESE'S ESSAY
Atlas of Future Plate Tectonic Reconstructions: Modern World to Pangea Proxima (+250 Ma)
by
Christopher R. Scotese
March 1, 2018
An animated version of the maps can be viewed on YouTube at: https://www.youtube.com/watch?v=qrtl8oMn-U8
Introduction
In 1982, when I was a graduate student, I was asked by Discover magazine to produce a set of maps showing future plate motions (Overbye, 1982). I replied that I thought that I could make a reasonable extrapolation of current plate motions 50 million years into the future. However, that was not sufficient. The magazine wanted maps for 50 million, 150 million and 250 million years in the future! I thought about it for a few seconds, and then decided to give it a try.
I realized that simply projecting current plate motions 150 million or 250 million years into the future would not work. Too much can change during that length of time. In order to solve the problem of future plate motions I was required to ask some serious questions about plate dynamics. What drives the plates? Why do they move they way they do? These are important questions for anyone that makes plate tectonic reconstructions. We cannot make maps of the distant future, or the distant past
for that matter, unless we have a clear idea of the driving mechanisms of plate tectonics.
During the intervening years, I have come to realize that plates are driven by lithospheric forces, primarily “slab pull” and “ridge push” (Scotese, 1993, 2017, see Appendix I). Slab pull is the “tug” that plates receive from the subducting oceanic lithosphere. Ridge push is the “push” plates receive from the elevated mid-ocean ridges. This elevation imparts a gravitational “push” much like the push a skier receives going down a ski slope. The first set of maps imperfectly used these plate dynamics to propose where the plates might go in the future.
I published a second generation of future maps in the Atlas of Earth History (Scotese, 2001). This set of maps corrected a major mistake that I made in the first set of maps. It is common wisdom that the rifts of East Africa represent the first stage in the formation of a new ocean. While it is true that all intracontinental ocean basins begin their life as a continental rift, it is not true that all continental rifts form ocean basins. Some rifts are aborted or “failed” rifts. Sadly, the East African Rift system is one of these failures. The principal reason that a new ocean
will not open is that as Africa continues to drive northward towards Eurasia the East African Rift (and also the Red Sea and Gulf of Aden) will be caught up in that massive collision and the young, thin ocean floor will be squeezed shut.
The maps presented here represent my third attempt at predicting future plate motions. I finished these maps in 2004 and added plate boundaries in 2005.
In addition to maps for +50 million years, +150 million years, and +250 million years, I added eight additional maps representing plate configurations at 25 million year intervals into the future. Also, I changed the name of the supercontinent that appears at +250 million years from “Pangea Ultima” (which sounded awfully final) to “Pangea Proxima” (the next Pangea)
For the last 15 years the maps have mostly sat on my shelf because though they are informed speculation, they are not science. We can never really know what the future holds. However, because of the renewed interest in future plate tectonic scenarios (Battersby, 2017), I decided to provide a more detailed outline of my thought processes when I drew these maps. I think you will enjoy this speculative story of the
future wandering of our continents. The final version of the maps will appear in my book, “Earth History: The Evolution of the Earth System”, which I hope to finish in a few years.
The following section is made up of ten future plate tectonic reconstructions, each with an extended figure caption that describes the major plate tectonic features and events. The numbers in the figure caption refer to the circled numbers on each map. Yellow lines are mid-ocean ridges. Red lines are subduction zones. White arrows indicate the direction of plate motion. A set of maps without the numbers can be found Appendix II of this report.
In addition to a summary of important plate tectonic events, I have included a description of important geographic changes, e.g. new mountains or seas, and have estimated the global climate (i.e., icehouse, greenhouse or hothouse) and mean global temperature based on a climate model that I have applied to ancient times (Scotese, 2016).
Map 1. Modern World Plate Tectonics
New (Young) Mid-Ocean Ridges: Gulf of California (1), Red Sea and Gulf of Aden (2).
New (Young) Continental Rifts: East African Rift Zone (3), Lake Baikal (4), Basin & Range (Western USA) (5)
New Subduction Zones: the Capricorn subduction zone (6) in Central Indian Ocean
New Collisions: Arabia with Iran (Zagros Mts.) (7), Australia with Indonesia (Highlands of New Guinea) (8).
Continuing Plate Tectonic Processes: The Atlantic (9), Arctic (Eurasian Basin)(10) and Indian (11) Oceans continue to widen. The Pacific Ocean contracts slightly. Cocos-Nazca mid-ocean ridge (12) is subducted beneath Panama. Honduras (13) slides eastward as the Caribbean plate lengthens (E-W). Chile Ridge (14) subducts beneath southwestern South America. Subduction continues around Pacific “Ring of Fire”. Westward-directed subduction occurs beneath the Puerto Rican (15) and the Scotia
(16)arcs. Africa (17) and India/Australia (18) continue to move northward towards Eurasia.
Geography
High mountains run E-W across Eurasia (19) from the Pyrenees to the Burmese Ranges (20). Andean- type mountains rim the Pacific Ring of Fire.
Climate
Warming, bi-polar Ice House. Global Temperature 15 ̊C
Map 2. +25 Million Years in the Future Plate Tectonics
New Mid-Ocean Ridges: Gulf of California (1) widens. Zealandia (2) splits in two.
New Subduction Zones: A new island arc (3)forms along the northern edge of the Capricorn subduction zone in Central Indian Ocean. Capricorn subduction zone extends eastwards along the southern margin of Australia (4). Subduction begins along eastern South America (5). Subduction begins along eastern North America and (6) eastern Greenland and an island arc forms across the entrance to the Labrador Sea (7).
New Collisions: The Red Sea (8) and Gulf of Aden (9) close. Contraction and uplift (inversion) occur along the length of the East Africa Rift (10). Uplift of the Alps (11) is renewed. The Mediterranean Sea (12) begins to narrow as Africa is pilled northward by subduction in the eastern Mediterranean beneath the Hellenic arc. Iberia (13) rotates clock-wise closing the Bay of Biscay.
Continuing Plate Tectonic Processes: Extension continues in the Basin & Range (Western USA) (14). The North and Central Atlantic begins to contract as the Mid-Atlantic Ridge (15) is subducted beneath Greenland. The South Atlantic contracts as ocean floor is subducted beneath eastern South America (5). The Indian Ocean begins to contract as the Central Indian Mid-ocean Ridge is subducted northward beneath the Capricorn Trench (16). The oceanic gap between North and South Zealandia (2) widens. Honduras (17) slides eastward as the Caribbean plate lengthens (E-W). The Chile Ridge (18) is subducted beneath southwestern South America. Subduction around Pacific “Ring of Fire” continues. The northward movement of Africa and India (19) towards Eurasia (20) continues. The western margins of Australia (21) continue to collided with SE Asia.
Geography As Africa plows northward into Eurasia, mountains rise along the border of this collision zone (11). Island arcs and Andean-type mountains form along the east coasts of North America and South America. Boston, New York City, and Washington D.C. (22) are carried skyward by an
erupting volcanic mountain chain. The Basin and Range (14) continues to stretch and subside.
Shallow seas begin to invade western North America.
Climate Mild Green House. The Antarctic ice cap has melted, though a small region of continental glaciers remain in central Greenland. Global Temperature 20 ̊C
Map 3. +50 Million Years in the Future Plate Tectonics
New Collisions: The Red Sea and Gulf of Aden (1) are completely closed as Africa collides with Arabia. Ophiolites (remants of ocean floor) are obducted along the eastern shores of Egypt and Sudan. Uplift occurs along the length of the East Africa Rift (2). The northward thrusting Alps push into central France and central Germany (3). The Mediterranean Sea (4) is completely closed. Iberia (5) rotates clock- wise consuming ocean floor in the Bay of Biscay and collides along the southwestern margin of France. Honduras (6) collides with northwestern South America.
Continuing Plate Tectonic Processes: The Gulf of California (7) continues to widen. Baja California and portions of western California (8) slide northward towards the Gulf of Alaska. Stretching of continental lithosphere continues in the Basin & Range(9) (Western USA). The North and Central Atlantic Ocean (10) contract and the northernmost portion of the Mid-Atlantic Ridge is subducted
beneath Newfoundland (11). The central portion of the Mid-Atlantic Ridge is about to be subducted beneath the expanding Lesser Antilles island arc (12) and the northeastern margin of South America. Ocean floor in the South Atlantic is subducted beneath eastern South America (13). The Indian Ocean begins to contract as the western portion of the Southeast Indian Ridge (14) is subducted northward beneath the Capricorn Trench. Antarctica (15) is drawn northward by north-directed subduction beneath the Capricorn subduction zone. The oceanic gap between North and South Zealandia widens (16). The Chile Ridge is subducted beneath southwestern South America (17). The east coast of Madagascar (18) becomes a large transcurrent strike-slip fault (sinistral offset). Subduction around Pacific “Ring of Fire” continues. Australia begins to collide with southeastern China (19) closing the South China Sea.
Geography As Africa continues to plow northward into Eurasia, mountains rise along the northern border of this collision belt (3). The Red Sea and Gulf of Aden (1) collapse and are replaced by mountain ranges. The back-arc basins of Southeast
Asia (20) are replaced by mountain ranges. The Basin and Range (9) continue to stretch and subside. The collision of India with Asia (21)has stopped and the Himalayas and Tibetan plateau begin to erode.
Climate Cooling Greenhouse. Global Temperature 18 ̊C
Map 4. +75 Million Years in the Future Plate Tectonics
New Mid-Ocean Ridges: A rift forms in the Weddell Sea and Ross Sea (1) as the Western Peninsula of Antarctica separates from cratonic East Antarctica. A series of back-arc basins open north of Australia (2) as a result of Australia’s collision with SE China.
New Collisions: The northward thrusting Alps push into northern Germany and Poland (3). Australia collides with China (4) completely closing the South China Sea. Baja California and portions of western California (5) begin to collide with southern Alaska.
New Subduction Zones: Subduction begins along the South African (6) margin.
Continuing Plate Tectonic Processes:
continues in the Basin & Range (Western USA) (8). The North and Central Atlantic contract as the last portion of the Mid-Atlantic Ridge is subducted beneath Labrador (9) and the Greater Caribbean island arc. The southern portion of the Mid-Atlantic Ridge is about to be subducted beneath the eastern
margin of South America (10). The Indian Ocean begins to rapidly contract as the last portions of the Southeast Indian Ridge (11) have been completely subducted beneath the Capricorn Trench. Antarctica (12) is drawn rapidly northward by subduction beneath the Capricorn subduction zone. The oceanic gap between North and South Zealandia (13) widens. The Chile Ridge (14) is nearly subducted beneath southwestern South America. The east coast of Madagascar (15) is a large strike-slip fault with sinistral offset. Subduction around Pacific “Ring of Fire” continues. The East Pacific Rise (16) approaches the subduction zone along the west coast of South America.
Geography The collision of Africa with Eurasia (3) nears its peak. The Red Sea and Gulf of Aden (17) are replaced by high mountain ranges. The back-arc basins of SE Asia (4) are completely replaced by high mountain ranges. The Basin and Range stops stretching and the Californian mountain ranges are added to southern Alaska (5). The Himalayas and Tibetan plateaus (18) begin to erode.
Climate Bipolar Icehouse (similar to the Modern world). Global Temperature 14 ̊.
Map 5. +100 Million Years in the Future
Plate Tectonics
New Mid-Ocean Ridges: The Trans-Antarctic Ocean (1)opens as the Western Peninsula of Antarctica separates from cratonic East Antarctica.
New Continental Rift: A intra-continental rift forms in northeast Siberia (2) as Eurasia is pulled away from North America.
New Collisions: Baja California and portions of western California (3) collide with southern Alaska. The Solomon-New Hebrides trench (4) northeast of Australia collides with the Kamchatka-Aleutian subduction zone.
Continuing Plate Tectonic Processes: The North and Central Atlantic Oceans narrow as the last remnants of the Mid-Atlantic Ridge (5) are subducted beneath the east coast of North America. The Afro-Asian continent (6) is pulled toward North America. The last remnant of the Mid-Atlantic Ridge (7) in the South Atlantic is subducted beneath Argentina. As a consequence of the complete subduction of the Mid-
Atlantic Ridge, the Afro-Asian continent (8) is pulled towards South America. Antarctica (9) is drawn rapidly northward as the Southwest Indian Mid- Ocean Ridge (10) is subducted beneath South Africa. The Indian Ocean begins to close as the ocean floor north of Antarctica (9) is subducted beneath the Capricorn Trench. The east coast of Madagascar (11) is a large left-lateral strike-slip fault. Subduction around Pacific “Ring of Fire” continues. The East Pacific Rise (12) is subducted beneath western South America.
Geography The collision of Africa with Eurasia (13) has reached its peak. The Red Sea and Gulf of Aden (14) are replaced by high mountain ranges. The back-arc basins of SE Asia (15) are completely replaced by high mountain ranges. The Basin and Range (16) stops stretching and the Californian mountain ranges are added to southern Alaska (3). The Himalayas and Tibetan plateaus (17) begin to erode.
Warming Icehouse (only the Northern Hemisphere has an icecap). Global Temperature 16 ̊C
Map 6. +125 Million Years in the Future
Plate Tectonics
New Mid-Ocean Ridges: The Trans-Antarctic Ocean widens as the Western Peninsula of Antarctica separates from cratonic East Antarctica (1). The Verkhoyansk Ocean (2) opens as Chukotka, attached to North America, pulls away from Siberia.
Continuing Plate Tectonic Processes: Subduction continues along the eastern margins of North America, the Caribbean plate, South America, and the Scotia plate (3). The Atlantic Ocean begins to close as the Afro-Asian (4) continent is pulled toward North America and South America. The Indian Ocean (5) is nearly closed as the ocean floor north of Antarctica disappears into the Capricorn Trench.
Geography Antarctica (6) moves into tropical waters. The mountainous areas of East Africa, Iran, and Tibet (7) are rapidly worn down. High mountain ranges mark the collision belt between Australia and China. (8) The Himalayas and Tibetan plateaus (7) are less than half their original height.
Climate
̊C
Mild Greenhouse. Global Temperature 20
Map 7. +150 Million Years in the Future
Plate Tectonics
New Subduction Zones: Antarctica’s northward motion is halted due to its collision with the island arcs of the Capricorn Trench (1). A new subduction zone forms along the southern, trailing edge of Antarctica (2). This new Trans-Antarctic subduction zone (3) links up with the South African subduction zone to the west (3) and the South Australian subduction zone to the east (3). As a consequence, the Trans-Antarctic Ocean (4) is subducted northward and begins to close. The subduction zone along eastern Asia (5)extends northward and links with the subduction zone running northward, across the North Pole from Greenland.
New Collisions: Antarctica collides with the islands arcs of the Capricorn Trench (6), Sumatra, and northwest Australia closing the Indian Ocean. This is the first collision, in a series of four major collisions, that ultimately forms the supercontinent of Pangea Proxima. The southern half of Zealandia (7) is about
to collide with Java, Borneo and southwestern Australia.
Continuing Plate Tectonic Processes: Westward- directed subduction continues along the eastern margins of (8) North America, the Caribbean plate, South America, and the Scotia plate closing the Atlantic Ocean. The Trans-Antarctic Ocean begins to close as the Western Peninsula of Antarctica approaches East Antarctica. (4) The Verkhoyansk Ocean (9) opens as Chukotka and North America pull away from Siberia. The Verkhoyansk Ridge (10) links up with the Trans-Antarctic Ridge.
Geography The mountainous areas of (11) East Africa, Iran, northern India, Tibet and SE China occupy warm, wet, tropical latitudes are rapidly erode. A collisional mountain range lies along the northern margin of equatorial Antarctica (1) and an Andean-style mountain range (12) lies along its southern margin. Andean-style mountains rim the (13) western margins of North America and South Anmerica.
Climate
̊C
Mild Greenhouse. Global Temperature 21
Map 8. +175 Million Years in the Future
Plate Tectonics
New Collisions: The southern half of Zealandia (1) has collided with Java, Borneo and southwestern Australia.
Continuing Plate Tectonic Processes: Westward- directed subduction along the eastern margins of (2) North America, the Caribbean plate, South America, and the Scotia plate continues to rapidly close the Atlantic Ocean. The Trans-Antarctic Ridge is subducted beneath East Antarctica and the Trans- Antarctic Ocean rapidly closes. The Verkhoyansk Ocean widens as Chukotka and North America (4) pull away from Siberia. The subduction zone along eastern Asia (5)extends northward and links with the subduction zone running northward from Greenland.
Geography The mountainous areas of (6)East Africa, Iran, Tibet and SE China occupy warm, wet, tropical latitudes are rapidly worn down. The remnant of the northern Indian Ocean (7), which lies
east of Africa, south of India, and north of Antarctica is nearly cut-off from the other oceans begins to stagnate.
Climate Warming Greenhouse. Global Temperature 23 ̊C
Map 9. +200 Million Years in the Future
Plate Tectonics
New Collisions: The last remnants of the Trans- Antarctic seaway are subducted beneath Antarctica (1) and southwestern Australia as the Western Peninsula (2) of Antarctic rapidly approaches.
Continuing Plate Tectonic Processes: Westward- directed subduction along the eastern margins of (3) North America, the Caribbean plate, South America, and the Scotia plate has nearly closed the Atlantic Ocean. Newfoundland (4) is on a collision course with West Africa (Guinea). The Greater Caribbean island arc (5) and eastern Brazil are less than 1000 km from southern Africa. A circum-Pangean mid- ocean ridge system (6) encircles the assembling supercontinent. The subduction zone along eastern Asia (7) extends northward and links with the subduction zone running northward from Greenland. The Greater Caribbean – Eastern South American subduction zone intersects with the South African subduction zone (8). The Scotia Arc subduction zone
and the Trans-Antarctic subduction zone (9) approach each other.
Geography The mountainous areas of (10) East Africa, Iran, Tibet and SE China occupy warm, wet, tropical latitudes and have been worn down. The once lofty Himalayas (11) are reduced in size comparable to the Appalachian mountains. The MediPangean Sea, which lies east of Africa, south of India, and north of Antarctica is nearly enclosed and continues to stagnate.
Climate
Hot House. Global Temperature 24 ̊C
Map 10. +225 Million Years in the Future
New Collisions: Newfoundland (1) collides with West Africa. The eastern tip of Brazil (2) collides with the southern tip of Africa. The Greater Caribbean island arc (3) has collided and has been obducted along the southwestern margin of Africa. These collisions form a series of semi-isolated seas and seaways (Atlantic Sea (4), Caribbean Seaway (5), the Austral Sea (6), and the Medi-Pangean Sea (7)). The eastern tip of the Falkland Plateau (8) is about to be subducted beneath East Antarctica. The Western Peninsula of Antarctic (9) has collided with southwestern Australia.
Continuing Plate Tectonic Processes: The last bits of ocean floor in the Atlantic Sea (4) and Austral Sea (6) are being removed by subduction. The Eastern South American subduction zone (10) and the Trans- Antarctic subduction zone approach each other. A nearly complete circum-Pangean subduction zone (11) (the New Ring of Fire) encircles the assembling supercontinent (Pangea Proxima). A mid-ocean
ridge system (12) lies outboard of that subduction zone (the Circum-Pangean Mid-ocean Ridge)
Geography The remnants of the Atlantic Ocean is now a large inland sea (the Atlantic Sea (4)). A narrow Caribbean Seaway (5) separates Yucatan and Honduras from northern South America. The mountainous areas of (13) East Africa, Iran, Tibet and SE China occupy warm, wet, tropical latitudes and have been largely eroded. Only the remnants of the Himalayas, Tibetan plateau, and Afro-Eurasia mountain belt remain. The enclosed Medi-Pangean Sea (7), which lies east of Africa, south of India, and north of Antarctica continues to stagnate.
Climate
26 ̊C
Warming Hot House. Global Temperature
Map 11. +250 Million Years in the Future – “Pangea Proxima”
New Collisions: Greenland and North America (1) have collided with western Africa forming the west- central portion of Pangea Proxima. Florida and the southeastern United States (2) have collided with southwestern Africa. A narrow, stagnant sea (the Atlantic Sea) (3) is all that remains of the Atlantic Ocean. The northeastern and eastern margin of South America (4) has collided with South Africa and East Antarctica forming the southwestern portion of Pangea Proxima. Medi-Pangean Sea (5) is now completely enclosed by the continents that comprise Pangea Proxima.
Continuing Plate Tectonic Processes: A circum- Pangean subduction zone (6) (the New Ring of Fire) encircles Pangea Proxima. A mid-ocean ridge system (the Circum-Proxima Mid-ocean Ridge) (7) lies outboard of that subduction zone.
Geography The new supercontinent, Pangea Proxima, is surrounded by a global ocean, the Propanthalassic Ocean (8) (meaning “future”
Panthalassic Ocean). The mountainous areas of (9) East Africa, Iran, Tibet and SE China occupy warm, wet, tropical latitudes have been worn down. Only remnants of the Afro-Eurasia mountain belt and Sino-Australian mountains remain. The completely enclosed Medi-Pangean Sea (5) becomes a super- toxic inland sea that begins to poison the surrounding oceans, lands and atmosphere leading the next great extinction event.
Climate
28 ̊C
Extreme Hot House. Global Temperature
Figures & Maps
References Cited
Batterby, S., 20176. The Next Supercontinent, New Scientist, (October), p. 34 – 37.
Overbye, D., 1982. The Shape of Tomorrow, Discover Magazine, p. 20-25.
Scotese, 1993. Plate Tectonic Rules of Thumb, PALEOMAP Project, Department of Geology, University of Arlington, Arlington Texas.
Scotese, 2014. Appendix I. The 12 Rules of Plate Tectonics, in Atlas of Plate Tectonic Reconstructions, PALEOMAP Project, Evanston, IL, DOI 10.13140/2.1.1046.4967,
www.academia.edu/9712803/Atlas_of_Plate_Tectoni c_Reconstructions_Mollweide_Projection_Volumes_1- 6_PALEOMAP_Project_PaleoAtlas_for_ArcGIS_PALEO MAP_Project_Evanston_IL
Scotese, C.R., 2016. A Global Temperature Curve for the Phanerozoic, Geological Society of America Annual Meeting, Denver, Abstracts with Program,
Paper 74-31. https://www.researchgate.net/publication/309324 713_A_NEW_GLOBAL_TEMPERATURE_CURVE_FOR_ THE_PHANEROZOIC
The 12 Rules of Plate Tectonics
C.R. Scotese 12/19/93; updated 03/31/17
Rule I. Plates move only if they’re pushed or pulled, not dragged (mostly). • The mantle plays a passive role.
o The pattern of flow in the mantle is largely driven by lithospheric motions. • The motion of the mantle does not drive the plates (by mantle drag).
o The idea of organized convection cells upon which the plates ride is wrong (though intuitively
attractive – unfortunately!).
• Oceanic plates move faster than continental plates.
o Oceanic plates tend to have ridges (pushing) and attached subducting slabs (pulling).
o At the base of oceanic plates in the LVZ (low velocity zone), a region of partial melting that
provides ‘lubrication” at the base of the plates and mostly decouples an oceanic plate from
underlying mantle movement.
• Plates with a large area of continent move slowly (e.g., Eurasia) because they have a deep continental keel imbedded in the mantle.
o Continental plates are more likely to be caught in the mantle flow.
• Plates (oceanic or continental) that are surrounded by ridges tend to move slowly (e.g Africa, Antarctica).
o The push from the encircling ridges “cancels out”.
- Plates that are not driven by ridge push or slab pull do not move (Caribbean plate, Scotia plate).
- In all cases plate motion can be understood and predicted by “balancing the forces” that drive andresist plate motion (driving: slab pull & ridge push, resisting: mantle drag).Rule II. Subduction Rules.
• Slab Pull is more important than Ridge Push (80% vs 20% of driving force).
o Fast moving plates (>5 cm/yr), are attached to subducting slabs (e.g. Pacific, Nazca, Cocos,
Indian)
• The Phanerozoic speed limit is ~20 cm/yr (India, 65 Ma)
o Convenient yardstick: 10 cm/yr = 1° per million year = 111 km/my
• Where there is subduction, there is spreading or rifting. (The converse is not true.)
Rule III. Mid-Ocean Ridges are Passive Features.
• Mid-ocean ridges are there because the crust, which is weak in tension, breaks when it is pulled.
o Continental crust breaks first, because at the same depth it’s closer to its melting temperature.
o Oceanic crust rarely breaks; when it does it breaks where it has been weakened by a hot spot or subduction related volcanism.
- Mid-Ocean Ridges tend to align parallel to trenches.
- One ridge is all you need.o You’ll never see two ridges spreading side-by-side.
- Fracture zones point to the trench.
• A mid-ocean ridge that is not moving for 10’s of millions of years (in an absolute frame of reference), becomes “stuck” above a deep-seated mantle upwelling.
Rule IV. Subduction is Forever.
• Collision is the only way to stop subduction.
o You need big, gob-stopping continent-continent collision, otherwise subduction zone will
“jump over” the colliding terrane. (e.g Capricorn trench in Central Indian Ocean). • Subduction is hard to start.
o Most likely mechanisms: terrane collision or a “subduction virus”.
o A subduction zone that enters a new ocean can act as a trigger or focus for the start of
subduction along an old, cold, heavily loaded passive margin (e.g. Puerto Rican trench and
Atlantic passive margin).
• Subduction graveyards exist in the mantle.
o Old slabs can be detected by seismic anisotropy, which means that some seismic waves travel faster through cooler material (old slabs) than warmer material (mantle).
Rule V. Pacific versus Tethyan Subduction Systems
• Pacific Subduction Systems(PSS) are characterized by a ring of subduction with a spreading ridge in the middle (e.g. Ring of Fire).
o PSS are stable and can continue subducting for hundreds of millions of years. (As long as the central ridge doesn’t get too close to a trench and gets subducted!)
o They generate long-lived Andean-style margins or marginal back-arc basins. • Tethyan Subduction Systems (TSS) are asymmetric or “one-sided”.
o There is only one subducting margin (like Tethys).
o On the other side of “Tethys” is a passive margin.
o The ridge in the middle of Tethys must move towards the trench and is eventually subducted.
o Because there is no longer a ridge in the subducting ocean, a newrift must form.
o The rift forms in the continental plate that is now being “pulled “towards the Tethyan trench.
o These new slab pull forces either tear a chunk of continent away from the margin (India), or break the continent apart(breakup of Pangea).
o The latter in more likely to happen if the continent is weakened by old collision zones or new
hotspots.
Rule VI. Plates Subduct Normally.
• Oblique convergence is more work; orthogonal convergence(perpendicular to continental margin) is least work.
o Displaced terranes originate in areas of oblique convergence.
o The highest mountains along Andean-style margins are in areas of oblique convergence (In NA that’s Mt. McKinley).
Rule VII. The Style of Convergent margin depends of the absolute motions of the plates. • Andean margins – net convergence (~10 cm/yr)
• Western Pacific margins – net divergence (roll back 1-2 cm/yr)
Rule VIII. Island Arcs don’t ride their trenches across oceans.
• Back arc basins never evolve into wide (>30,000 km) ocean basins.
o You’ll never find an continental island arc in the middle of an ocean. o There is always a continent nearby.
o 90% of all ophiolites form in back-arc basins.
Rule IX. Slab Rollback Can Create Odd Intracontinental Ocean Basins
• Oceanic lithosphere can become trapped (encircled) by continents (e.g. Mediterranean, Arctic, Tethys north of Alps).
o Small, short-lived subduction zones can consume this ocean floor creating intracontinentalextension and small “odd ” ocean basins (e.g. Tyrhennian Sea, Pannonian Basin, Makarov Basin).
Rule X. Mantle Plumes (i.e. Hot Spots) are Important (Sort of)
• Hot Spots are derived from the core/mantle boundary (mostly).
• They provide a “good enough” reference frame for absolute plate motions.
o Some Hot Spots are Fixed; Some Aren’t • Hot Spots are organized in “proper groups”
o The Indian-Atlantic Proper Group has been fixed to spin axis since the Triassic.
o The Pacific Hotspots (Hawaii) have moved about 200 km in 100 my. • Hot Spots “help” break apart continents.
o They create areas of weakness in the continental lithosphere.
o Thermal uplift associated with mantle plumes causes uplift which helps to “push” the continents apart (like ridge push).
• Important: Mantle Plumes and hot spots do not break apart continents. They just make the job easier. The heavy lifting is done by slab pull forces.
• Hot spots under moving plates speed them up, a bit.
o They thin the continent’s “mantle keel” that usually slows plate motion.
o They “grease the skids” by melting the base of the lithosphere (decreasing mantle drag forces). • If mantle plumes and hot spots did not exist, the plates would still move, continents would still break apart, move across the globe and collide!
o But the things would be less interesting!
Rule XI. Continental Collisions are Important (Really Important) • A continental collision destroys a subduction zone.
o That causes the global balance of plate driving forces to change.
• Continental collision zones (sutures) provide long-lived zones of weakness in the crust that are likely to become future sites of continental rifting.
o A continent with many sutures will be weak and easily deformed (e.g. Asia following collision
with India).
• A continental collision typically takes ~50 million years.
Rule XII. Plate Tectonics is a Catastrophic System (but not chaotic).
• Plate motions are generally gradual; but every once and a while “WHAM”! o “Long periods of boredom interrupted by short moments of terror”
o Because plate boundaries are metastable. Geometries can interact in unpredictable ways to
produce global plate tectonic “events”.
• 2 Important Instabilities: 1) continent-continent collision & 2) ridge subduction.
o Continental collisions can cause new trenches to form.
o India’s rapid northward flight & the breakup of Pangea were due to ridge subduction.
o Both of these events cause rapid, global falls in sea-level
o Continental collision increases the volume of ocean basins because the area of continental lithosphere decreases.
o Ridge subduction increases the volume of ocean basins by decreasing the average age of the oceanic lithosphere.
o Ridge subduction both breaks supercontinents apart and brings them back together (Wegener / Wilson Cycle).
•Supercontinents form and break apart because of the metastable nature of plate evolution.
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