Large rounded chunks of seafloor are included within the serpentinite mélange. Serpentinite is a metamorphic rock that is created through the hydrous alteration of ultramafic rocks under great pressure. One type of rock that makes mélange in subduction zones is serpentinite. This is what is referred to with the French term “ mélange.” Translated, it means “mixture.” Mélange is a sheared-out mixture of heterogeneous seafloor scrapings. All of this is stirred up and mashed ocean floor sediment metamorphosed as one plate dove beneath another. The Franciscan Complex is a body of rock that, as you can see below, is in part made up of a metamorphic rock called serpentinite (formed under very intense pressures when mantle peridotite reacts with water), along with chunks of metamorphically altered ancient basaltic seafloor mixed with shelf and seafloor sediments such as cherts and mudstones. The accretionary prism units, including the mélange, are indicated in blue. The geologic setting of the Franciscan Complex. Consider what kinds of other rocks you might seek nearby if you wished to confirm your analysis of an outcrop of Franciscan Complex. In this case, the subduction was a classic situation of oceanic-continental convergence. The subduction zone is no longer active it shows us rocks that formed during the Mesozoic Era. This complex of rock is a part of the accretionary wedge, or mélange, associated with a subduction zone. The first image below is of an artist’s rendering of an idealized subduction zone displaying the “Franciscan Complex” in situ. Convergent Boundaries Continental-Oceanic Convergence: The Franciscan Complexįor a more in-depth geological analysis of this plate boundary, read this case study on the Franciscan Complex here. In this VFE, we will virtually travel to examples of all of the major types of plate boundaries, including:ġ) Convergent Boundary, Continental-Oceanic (Franciscan Complex, San Francisco, USA)Ģ) Convergent Boundary, Oceanic-Oceanic (Semail Ophiolite, Oman)ģ) Convergent Boundary, Continental-Continental (Himalayas)ĥ) Divergent Boundary, Continental (East African Rift)Ħ) Transform Boundary (San Andreas Fault)Īt each of these sites, you will be asked to use your powers of observation to explore some of the field geology produced at such locations. The Himalayas and Tibetan plateau trend east-west and extend for 2,900 km, reaching the maximum elevation of 8,848 metres (Mount Everest – the highest point on Earth).Plates and plate boundaries with a general sense of motion identified. However the forces of weathering and erosion are lowering the Himalayas at about the same rate. The Himalayas are still rising by more than 1 cm per year as India continues to move northwards into Asia, which explains the occurrence of shallow focus earthquakes in the region today. The thickening of the continental crust marked the end of volcanic activity in the region as any magma moving upwards would solidify before it could reach the surface. The continental crust here is twice the average thickness at around 75 km. This caused the continental crust to thicken due to folding and faulting by compressional forces pushing up the Himalaya and the Tibetan Plateau. The Eurasian plate was partly crumpled and buckled up above the Indian plate but due to their low density/high buoyancy neither continental plate could be subducted. (Note that in the above animation the continental plates are shown to collide at 10 Ma this should instead read 50 Ma.) This slowdown is interpreted to mark the beginning of the collision between the Eurasian and Indian continental plates, the closing of the former Tethys Ocean, and the initiation of Himalayan uplift. These scraped-off sediments are what now form the Himalayan mountain range.įrom about 50-40 Ma the rate of northward drift of the Indian continental plate slowed to around 4-6 cm per year. At this time Tethys Ocean floor would have been subducting northwards beneath Asia and the plate margin would have been a Convergent oceanic-continental one just like the Andes today.Īs seen in the animation above not all of the Tethys Ocean floor was completely subducted most of the thick sediments on the Indian margin of the ocean were scraped off and accreted onto the Eurasian continent in what is known as an accretionary wedge (link to glossary). 80 Ma India was 6,400 km south of the Asian continent but moving towards it at a rate of between 9 and 16 cm per year. The supercontinent Pangea began to break up 200 Ma and India started a northward drift towards Asia. 225 million years ago (Ma) India was a large island situated off the Australian coast and separated from Asia by the Tethys Ocean.
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