Plate boundaries II extension

Increasing heat causes uplift and fault development, which in turn creates steep- sided valleys bounded by faults. A variety of non-marine environments are created here.

Extrusion of basalt along faults floods the valleys with magma. Many volcanoes of the shield variety. Non- marine sediments continue to accumulate. With continued basalt eruption the density of the crust in this region increases, and subsidence, or sinking, occurs. Flooding by ocean water creates a small ocean. The type of sediment becomes marine in the basin, flanked by 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 the gabbro sit in the mantle than the granite? What do we need to know in order to evaluate this question? Thickness of granite continent = 28km Thickness of gabbro ocean crust = 7.9km Density granite = 2.67g/cm 3 Density gabbro = 2.99g/cm 3 Gravit.const = 9.81m/sec 2 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 the thinner crust of gabbro above the compensation level. Is better thought of as: The Earth’s crust is a series of blocks floating in the upper 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) (thick mantle) = (density cont) (thick cont) Then: (2.99) (7.9)+ (3.3)(Zm) = 2.67(28) Zm = 15.5km So ∆H = 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 of Pratt isostacy: elevation differences are the result of density differences.

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 that record the rifting of a continent. Overlying 100’s meterst of basalt is over 1km of sediment recording fluvial environments (blue and yellow-green) overlain by km’s of sandy-shales representing continental shelf environment (rusty red). Not shown here is the rock that overlies the rusty-red shelf sediment; km’s of dolostone representing shallow 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 dense ocean crust? Here is a map showing a rift forming in continental crust Here is a cross section through the crust, showing the formation of normal faults and the intrusion of magma from the mantle along these faults The fault-bounded basin floods with water, sediment is deposited, along with continued extrusion of mafic volcanic rocks

Rifting continues…. With continued rifting, more sediment is laid down on the crust…it causes the crust to sink because of its weight. The ongoing extrusion of mafic magma continues to cause the lower density granite to be replaced by higher density basalt and gabbro. This causes further subsidence. Crust that was once only low density granite is now a mix of granite and gabbro. The higher density causes the crust to sink lower 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