I. Global Patterns

A. Lifetime of an ocean

1. concept developed by J. Tuzo Wilson to account for opening & subsequent closing of a Late Precambrian-Paleozoic ocean to form the Appalachians

a. concept called the WILSON CYCLE

2. so how long does a typical ocean last? that is how long is the Wilson Cycle?

a. typical ocean width - 5000 to 10000 km

b. typical spreading rate - 1 to 5 cm/yr (10 to 50 km/my)

c. typical lifetime - 5 to 10 x 103 km/1 to 5 x102 km/my = 100 my to 1 by

d. average lifetime is probably 500 my

B. Present Mesozoic/Cenozoic episode

1. dominated by dismemberment of Wegener's Pangea

a. specifically, the Atlantic & Indian Oceans are opening at the expense of Panthalassa (ancestral Pacific)

2. Wegener's Pangea formed 300 my ago by a series of continental collisions

3. by 200 my ago (that is 100 my later), Pangea began to break apart

4. so 300 my of the 500 my duration of the Wilson Cycle have been expended

5. there are probably another 200 my left in the current episode, most of which will by dominated by the closing of the Atlantic & Indian Oceans

C. Past episodes

1. 800 to 300 my ago

a. previous Pangea (Pangea 4) formed about 800 my ago

b. was disrupted during the Latest Precambrian (Eocambrian) & Cambrian (until ~500 my)

c. began to close during the Ordovician - Taconic Orogeny along the East Coast of North America

d. culminated in formation of Wegener's Pangea (Pangea 5) 300 my ago

2. 1300 to 800 my ago

a. yet an earlier Pangea (Pangea 3) formed about 1300 my ago

b. was disrupted during the Late Proterozoic (mid-continent rifting in North America)

c. Grenville Orogeny at 1000 my marks closing & formation of Pangea 4

3. 2000 to 1300 my ago - disruption of Pangea 2 & formation of Pangea 3

4. 2500 to 2000 my ago - disruption of Pangea 1 & formation of Pangea 2

5. Archean - continental crust thinner & not incorporated into Pangea 1 until the end of the Archean

II. Pacific Ocean

A. The Pacific Ocean is the oldest ocean, but the seafloor is only as old as that in the Atlantic (Middle Jurassic)

1. Coincidental - in 50 my, the oldest Pacific crust will be Early Cretaceous, while the oldest Atlantic crust will still be Middle Jurassic

B. Mesozoic Sequence & Earlier

1. the Mesozoic magnetic anomalies lie in the northwest Pacific

2. the M-sequence anomalies form 2 bights (bends) that formed at 2 triple junctions & indicate that the Pacific plate was formerly bounded by 3 plates

3. note that the length of each successively younger magnetic anomaly increases

a. projected back into the Jurassic Quiet Zone, this trend indicates that the Pacific plate started as a point

b. in fact, the spreading centers that formed the Pacific plate probably began with initial lengths of a 1000 km or so, & they probably jumped into an area of older crust

4. extrapolating spreading rates determined from the Mesozoic Sequence into the Jurassic Quiet Zone suggests the the oldest ocean crust on the Pacific plate is ~190 my old (Middle Jurassic)

5. the characteristics of the Mesozoic Sequence anomalies indicate that they formed in the Southern Hemisphere, & have been transported the their present position in the Northern Hemisphere through the northward movement of the Pacific plate recorded by the Hawaiian Emperor Seamount Chain

C. Cretaceous Quiet Zone (~110 to 80 my)

1. the events affecting the Pacific plate during the Cretaceous Quiet Zone are not known

2. however, the northern bight is still apparent in the Cenozoic Sequence, although it has shifted to the south

3. the southern bight is also apparent in the Cenozoic Sequence in the South Pacific off New Zealand

D. Cenozoic Sequence (Latest Cretaceous included)

1. as the ancestral East Pacific Rise (Pacific-Farallon plate boundary) lengthened

a. the northern bight was pushed progressively north & was eventually subducted beneath Alaska, thereby eliminating the Kula plate

b. the southern bight was pushed progressively south & was eventually subducted beneath Antarctica, thereby eliminating the Phoenix plate

c. subduction of the triple junctions & ridges forming the Kula & Phoenix plate boundaries is indicated by seafloor getting older away from the Aleutian & Kurile- Kamchatka Trenches & from the Antarctic continental margin west of the Palmer Peninsula

i. these features probably subducted about Anomaly 20 time (~50 my)

ii. this about the time the bend in the Hawaiian-Emperor Seamount Chain formed

2. just before Anomaly 34 time (~90 my), the Pacific/Phoenix ridge worked its way into east Gondwana & New Zealand (and the Campbell Plateau & Lord Howe & Chatham Rises) separated from Australia-Antarctica, forming the southwest Pacific & Tasman Sea

3. in the eastern Pacific, the major event was the subduction of part of the East Pacific Rise by North America beginning about Anomaly 9 time (~35-30 my)

a. the San Andreas Fault was initiated at this time

b. for the next 20 my, the East Pacific Rise off North America progressively disappeared, until now there is only a short segment left (the Juan de Fuca & Gorda Ridges off the Pacific Northwest)

c. convection of the subducted ridge continues today, however, as the Basin & Range of the western U.S.

4. about 10 my ago (Anomaly 5 time), the East Pacific Rise began to reorient from a NW-SE trend, to its present NE-SW trend

a. by 5 my ago (Anomaly 3 time) the reorientation was complete & the Gulf of California opened

b. the Galapagos Speading Center initiated about 10 my ago as well

c. the Gorda, Cocos & Nasca Plates are the remnants of the Farallon Plate

i. the Chile Ridge is a remnant of the the NW-trending East Pacific Rise that is still spreading

ii. the extinct East Pacific Rise lies in the center of the Nasca Plate

5. the marginal basins in the western Pacific formed throughout the Cenozoic

III. Atlantic & Indian Ocean Tectonic History

A. Triassic (~215 my)

1. initiation of divergence leading to the dismemberment of Pangea 5

2. continental rifting along the western extension of Tethys between the east coast of North America & northwest Africa (see above)

B. Mesozoic Sequence & Earlier

1. 190 my (Middle Jurassic) - initial breakup of Pangea 5 begins

a. seafloor spreading in the central North Atlantic off eastern North America & NW Africa

i. based on extrapolation of spreading rates in the Mesozoic Sequence into the Jurassic Quiet Zone, & age of basalt flows in Mesozoic rift valleys in eastern North America & NW Africa

b. Pangea 5 breaks into Laurasia & Gondwana

i. Florida is left with North America

2. 165 my (M-25 time) - Gondwana begins to break up

a. seafloor spreading off eastern Africa & Antarctica

b. Gondwana breaks into western Gondwana (South America/Africa) & eastern Gondwana (Antarctica/Australia/India/Madagascar)

i. Jurassic seafloor off northwest Australia suggest that Indonesia left Gondwana at this time

3. 135 my (M-11 time)

a. western Gondwana separates into South America & Africa

i. Mesozoic anomalies off Argentina & Brazil & South Africa & Angola

b. eastern Gondwana separates into Antarctica/Australia & India/Madagascar (which is now probably part of Africa)

i. NE-SW trending Mesozoic anomalies off western Australia

C. Cretaceous Quiet Zone

1. 90 my - Laurasia begins to break up

a. seafloor spreading in the Labrador Sea & Bay of Biscay

i. Laurasia breaks into North America, Eurasia/Greenland, & Spain

2. 90 my - ridge in eastern Indian Ocean reorients to E-W

a. India separates from Madagascar & accelerates toward Asia

b. formation of Ninetyeast Ridge & Chago-Laccadive Ridge & Mascarene Plateau along transforms

D. Cenozoic Sequence (Latest Cretaceous included)

1. 55 my (Anomaly 24 time) - Greenland separates from Eurasia & joins North America

2. 55 my (Anomaly 22 time) - Antarctica & Australia break up

i. NOTE: These two events are critical for the climatic cooling that characterizes the Cenozoic

3. 45 my (Anomaly 20 time) - ridge in Indian Ocean reorients to present NW-SE trend as India starts to collide with Eurasia

4. 10 my (Anomaly 5 time) - Gulf of Aden begins to open

IV. Effect of Ocean Tectonic Events on Climate

A. Mesozoic Climate

1. generally warm

a. continents strattle the equator & none are at the poles

b. Equatorial currents nearly circle the world (flow west into Tethys)

i. complete circum-Equatorial ocean circulation through & North Atlantic established following separation of Pangea 5 into Laurasia & Gondwana

2. very dry during Triassic & Jurassic

wetter during the Cretaceous

a. Pangea 5 is so big moisture cannot reach the interior

b. furthermore, mountain ranges resulting from the formation of Pangea 5 create numerous rain shadows

c. by Cretaceous time, the continents have dispersed significantly & sea level is high

i. fast spreading along Mid-Ocean Ridges

B. Cenozoic Climate

1. Characterized by an overall cooling, culminating with the Quaternary Ice Age

a. temperature of bottom waters got colder (15 degrees C to -1 degree C)

b. temperature of tropical surface waters remains same (25 degrees C)

c. no continental glacial ice sheets at end of Cretaceous & beginning of Cenozoic

d. Antarctica & Greenland permanently covered with ice during Quaternary

i. continental glacial ice sheets periodically cover large parts of North America & Eurasia

2. Related to movement of the continents by plate tectonics, & to decreasing sea level resulting from decreasing rate of seafloor spreading along the Mid-Ocean Ridge

a. opening of northernmost Atlantic 55 my ago allowed cold deep water from the Arctic basin to flow south down the length of the Atlantic

b. at the same time, Antarctica was thermal isolated at the South Pole by the circumpolar ocean current following the separation of Australia from Antarctica

3. Northern hemisphere ice sheets appeared sporadically after 15 my ago, & permanent ice in Greenland was established 2.4 my ago

a. extremely cold (-1 degree C) bottom water began to form beneath glacial ice shelves adjacent to Antarctica once that continent was permanently ice covered 15 my ago

b. permanent ice in Greenland probably resulted from 2 Pliocene events:

i. uplift in the Rockies, which deflected moisture-laden Gulf air to the north

ii. uplift of the Isthmus of Panama, which cut off circum-Equatorial ocean circulation

iii. uplift of the Tibet Plateau also probably played a role

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