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LECTURE 2WHAT IS A CONTINENTAL MARGIN?

CEE 598, GEOL 593TURBIDITY CURRENTS: MORPHODYNAMICS AND DEPOSITS

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EVERY CONTINENT IS SURROUNDED BY A MARGIN

The margin consists of a shelf (out to ~ 100 m), slope, and rise (which tapers off into deep water).

shelf

slope

rise

continental plain (subaerial) shoreline

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THE POSITION OF THE SHORELINE VARIES WITH SEA LEVEL

http://www.teachingboxes.org/seaLevel/lessons/lesson4_SeaLevelCurveGraph.htm

Glacial low stand

Interglacial high stand

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DURING THE LAST GLACIAL MAXIMUM, THE SHORELINE

MOVED OUTWARD TOWARD THE SHELF-SLOPE BREAK

NASA World Wind

Shoreline at low stand (lowest sea level elevation during glaciation)

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shelf

slope

rise

continental plain (subaerial)

SO SEA LEVEL FLUCTUATIONS STRONGLY AFFECT CONTINENTAL MARGINS

6http://sio.ucsd.edu/png/science/

CONTINENTAL MARGINS COME IN TWO BASIC FLAVORS DEPENDING ON THE TECTONIC SETTING:

ACTIVE AND PASSIVE MARGINS

Passive margins tend to be broad, with extensive shelves.Active margins tend to be narrow, with constricted shelves.

7http://www.geoware-online.com/GeowareMercatorMap.jpg

CONTINENTAL PLATE TECTONICS

8http://www.eoearth.org/media/draft/b/b5/Cross_section_of_the_earth,_tectonic_plates.gif

MANTLE CIRCULATION DRIVES PLATE TECTONICS

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http://www.geoware-online.com/GeowareCentralNAtlanticMap.jpg

SPREADING MARGIN: ATLANTIC

OCEAN

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http://web1.hrhost.net/~huttonco/Other/Latinia/SAm_P&A750.jpg

PERU-CHILE MARGIN

11http://www.clas.ufl.edu/users/jmeert/sam.jpg

http://astro.wsu.edu/worthey/earth/html/im-geology/peru-chile-trench.gif

PERU-CHILE MARGIN

12Pratson and Haxby, 1997

CONTINENTAL MARGINS AROUND THE UNITED STATES

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CONTINENTAL SLOPE OFF NEW JERSEYDissected passive siliciclastic margin.

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CONTINENTAL SLOPE OFF GULF COAST FLORIDACarbonate margin: life is the sediment factory.

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CONTINENTAL SLOPE, NORTHERN GULF OF MEXICOHere the continental slope is governed by salt tectonics.

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CONTINENTAL SLOPE OFF NORTHERN CALIFORNIAThis is a characteristic active margin.

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CONTINENTAL SLOPE OFF OREGONActive margin with complicated tectonics.

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EACH MARGIN HAS ITS OWN SPECIFIC HISTORY AND ORIGIN

EXAMPLE: SALT TECTONICS!

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THE NORTHERN CONTINENTAL SLOPE OF THE GULF OF MEXICO IS RIDDLED WITH DIAPIRIC MINIBASINS

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HOW WAS THIS BASIN-AND-RIDGE TOPOGRAPHY CREATED?

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UNDERNEATH A LAYER OF SEDIMENT ~ 10 KM THICK IS A LAYER OF SALT ~ 1 KM THICK

Salt tectonism:Salt is a Newtonian fluid with a very high viscosity!Overlying sediment (mud) ~ a Bingham fluid!Salt is lighter than mud!

SALT LAYER

SEDIMENT LAYER

SEAFLOOR

Salt flows up, sediment flows down.Result: minibasins on surface.

Minibasins

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TURBIDITY CURRENTS HELP TO BOTH BUILD AND DISSECT CONTINENTAL MARGINS

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REFERENCE

Pratson, L.F. & Haxby, W.F. ( 1997) Panoramas of the seafloor. Scientific American, 276(6), 82-87.