Plate boundaries II

extension

Increasing heatcauses upliftand faultdevelopment, which in turncreates steep-sided valleysbounded by faults. A varietyof non-marineenvironmentsare created here.

Extrusion of basaltalong faults floodsthe valleys withmagma. Manyvolcanoes of theshield variety. Non-marine sedimentscontinue to accumulate.With continuedbasalt eruption thedensity of the crust in this regionincreases, andsubsidence, or sinking, occurs.Flooding by oceanwater creates a small ocean. Thetype of sedimentbecomes marine inthe basin, flankedby shelves.

The continued spreading of the new ocean continues. Fully marine,deep ocean sediment (clays and very fine-grained limestone) lies on top of basalt

Why do the oceans sit so topographically low when they are so

hot and buoyant?

The topographic difference between the continents and oceans is due to two factors: the thickness difference and the density difference…remember isostacy?

Not only is the thickness of the continents different from the ocean basins, so is the composition, and that controls density.For example:

We could ask the question,“how much lower will thegabbro sit in the mantlethan the granite?

What do we need to know in order to evaluate this question?

Thickness of granite continent = 28kmThickness of gabbro ocean crust = 7.9kmDensity granite = 2.67g/cm3

Density gabbro = 2.99g/cm3

Gravit.const = 9.81m/sec2 AND……

Note that the granite and gabbro are floating in the mantle….so we need to consider the thickness and density of mantle holding up thethinner crust of gabbro above the compensation level.

Is better thought of as:

The Earth’s crust is a series of blocks floating in theupper mantle

Where:Zc = thickness cont crust (28 km)Zo = thickness of ocean crust (7.8km)Zm = thickness of upper mantle (=x)∆H = difference in elevation

From: total P of cont = total P of ocean, or: (density ocean)(thick ocean)+ (density mantle) (thickmantle) = (density cont) (thick cont)Then:(2.99) (7.9)+ (3.3)(Zm) = 2.67(28)Zm = 15.5kmSo ∆H = 7.9+15.5+x= 28; x=4.6km

In other words, the oceans sit 4.6km below the continents,just because of density differences. This is an example ofPratt isostacy: elevation differences are the result of densitydifferences.

Meanwhile, back in rifted margins….

What is the geologic record of a rifted continental margin?

Ex, the geology of western vermont 600 million years ago….

A sequence of rocks thatrecord the rifting of a continent.

Overlying 100’s meterst ofbasalt is over 1km of sediment recording fluvialenvironments (blue andyellow-green) overlain bykm’s of sandy-shales representing continentalshelf environment (rusty red). Not shown here is the rock that overliesthe rusty-red shelf sediment;km’s of dolostone representingshallow ocean floor environment.Total thickness of sequence?~6km.

The geologic record of rifting:

• Extrusive volcanics

• Normal faults

• Non-marine sediments, over time covered by marine sediments

• Very thick sequences of sediments and volcanics because of the subsidence, or sinking that happens (due to isostacy)

How do you make what was continental crust more denseocean crust?

Here is a map showing arift forming in continentalcrust

Here is a cross section through the crust, showingthe formation of normalfaults and the intrusion ofmagma from the mantlealong these faults

The fault-boundedbasin floods withwater, sediment is deposited, alongwith continuedextrusion of maficvolcanic rocks

Rifting continues….

With continued rifting, more sediment islaid down on the crust…it causes thecrust to sink because of its weight.The ongoing extrusion of mafic magmacontinues to cause the lower densitygranite to be replaced by higher density basalt and gabbro. This causes further subsidence. Crust thatwas once only low density granite isnow a mix of granite and gabbro. The higher density causes the crust to sinklower into the mantle.

To summarize:• Continental crust converts to oceanic crust

during rifting, as more and more mafic gabbro and basalt is extruded and intruded into the granite. Over time it volumetrically overwhelms the amount of granite in the rift, and you have primarily just mafic igneous crust.

• This crust subsides (Pratt isostacy) as a function of its density, which means that it floats lower in the mantle, topographically lower than continental crust.

• By the time of step 2, the basin has also flooded with ocean water, and is called an ocean basin