Geology 12 - Historical Geology
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Tectonic Basins

Different plate tectonic settings produce distinctive basin environments where sedimentary rocks are deposited. 

Download a chart to use in class and an activity for practicing with these concepts:  Tectonic Basins chart and activity

Here is a diagram to help you understand how the tectonic basins are related to plate boundaries:



I.  Rift Valley
When a diverging boundary begins to open, it creates a narrow valley bounded by normal faults.  As well as the basalt that is erupted into the valley and injected into the rocks below to create new ocean floor, sedimentary rocks will also be deposited within the rift valley.  At first the rift valley will be dominated by river systems, including these environments: alluvial fans, river channels and floodplains.  The rift valley may fill with fresh water and become a lake.  As the rift opens, ocean water may invade.  The shallow waters of the rift valley can be ideal environments for limestones to form.

Diagram of an opening rift valley: at stage B the valley is dominated by rivers, and at stage C by shallow marine environments.

Here is a nice summary of sedimentation on divergent margins: be sure to look at the diagrams on the right side of the page, and click the links for each stage to see what each stage of rifting looks like.

Here are some images of what modern rift valleys look like:

II.  Passive margin
When rifting is successful, the new ocean basin gets wider and wider.  New ocean floor is generated at the diverging boundary, and the edges of the now-split continents get farther apart.  The now fully-developed mid ocean ridge is the site of hot new ocean floor, and so it floats high on the aesthenosphere beneath.  But the continents get cooler as they move away from the mid ocean ridge, and they subside.  The ocean rises over the edges of the continent, creating the passive margins.  They are called passive because they are not near a plate boundary, and there is very little geologic activity there - no volcanoes, no earthquakes.

If a passive margin is near a large river system, the sedimentary rocks deposited there will be detrital: sandstones and shales from the sediments carried by the river.  The sedimentary environments will include river channels, floodplains, deltas, beaches and the deposition in the shallow water of the continental shelf.

Because the passive margin is not near active mountain building with a lot of active erosion, the sedimentary particles in the system have been there for a long time.  They have been thoroughly weathered.  The result tends to be lot of quartz and other durable particles.

If the passive margin is far from a major river system, and the water is relatively warm, limestones can form there.

Passive margin basins include both the shallow water deposition on the continental shelf, and the deep water deposition on the continental rise below.  Deposition in this deep water environment is usually the result of turbidity currents - fluidized masses of sediment and water that tumble off the edge of the continental shelf, erode channels into the continental slope, and then are deposited on the continental rise and abyssal plain as the turbidity current loses energy.

Revisit the diagram of the diverging boundary.  The bottom stage has passive margins on each side at the edges of the continents.

Examples of modern passive margins:

III.  Intracratonic deposits

In the modern world, most of the ocean water is in the ocean basins.  There are very few places where sea water sits on continental material.  In the past, virtually the entire North American continent has been underwater.  Sometimes these vast continental seas have been very shallow, resulting in beach-like deposits over huge areas.  When there were extensive regions of the continent above water, the erosion of these areas shed clastic sediments into the continental sea, resulting in large sheets of sandstones and shales.  When there was less continent above water to erode, the deposits in these shallow seas were carbonates.  In general, intracratonic deposits are much like those of passive margins - sometimes detrital, sometimes carbonate - but they don't include the deep water facies of the passive margin.

One of the best examples of an intracratonic basin in the modern world is the Baltic and North Seas, which are simply ocean water sitting on the European continent.

IV.  Foreland Basin

Large mountain ranges have depositional basins on both sides.  These large mountain ranges can be created by subduction - volcanic mountain ranges - or by collisions - metamorphic mountain ranges.  Foreland basins can be complex systems.  They may include both the river deposits from the eroding mountain range, and marine deposits if the system includes a continental sea (called a foredeep).  For our purposes in this class, we will just consider the nonmarine deposits of the foreland basin system.

In the modern world, there are many foreland basin systems, including the interior of South America, east of the Andes, a subduction boundary; and the depositional basins bordering the Himalayas and Tibetan Plateau, a continental suture zone.  In both cases there are huge alluvial fans and massive river systems, with river channel deposits, floodplains and deltas.

Himalayan foreland basins:

V.  Forearc and Backarc Basins

In subduction boundaries, sediment is shed off the volcanic mountains, and some makes its way to the ocean to be deposited in shallow and deep water marine environments.  This sediment is poorly weathered, and contains many black minerals and volcanic rock fragments.  In continent-ocean subduction boundaries, this sediment is deposited in the ocean above the trench itself.  In ocean-ocean subduction boundaries, the sediment is deposited in the forearc - above the trench - and in the backarc - on the opposite side of the volcanic island chain from the trench.

There are many active subduction zones in the world today with well-developed forearc and backarc basins.  The Cascade Mountains of the US Pacific Northwest sit in a climate zone with massive amounts of rain and rapid weathering, so the oceanic treanch has been completely overrun and buried by the forearc basin.  Japan is an excellent example of a volcanic island arc with a forearc and backarc basin.

Cascade subduction zone:

Japan:
VI.  Trench

As an oceanic plate sinks in a subduction zone, the sediment on that plate is scraped off and the sediment is plastered onto the edge of the overlying plate.  As this process continues, more sediment is scraped off and underplates the previously accreted material.  Over time an accretionary wedge forms.  These rocks are subjected to higher pressure as they are drawn deeper into the subdcution zone.  The rocks formed higher in the wedge tend to be greenschist facies rocks; those formed lower down tend to be blueschist.  Because of the scraping and churning within the accretionary mass, these rocks often contain chunks of many things: bits of the downgoing oceanic plate, blueschist chunks that have been recycled upward from deep in the wedge, remnants of volcanic islands that once sat on the downgoing plate.  This characteristic mixture of material is called a melange.

Accretionary wedge:

VII.  Ophiolite

This last category is not really a depositional basin, but a recognizable sequence of rock that we understand as a slice of ocean floor.  An ophiolite consists of this sequence (top to bottom):

If we see ophiolites in continental rock, it means something horrible has happened to emplace a bit of ocean floor on the continent.  Typically this is the result of a collision of some kind - either two continents colliding, or a smaller chunk - an island arc or continental fragment - being subducted and accreting onto a continent.  Sometimes fragments of ocean floor are incorporated into the melange of an accretionary wedge.

Ophiolite images: