科研之友
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摘要
Seismic record from 1975 to 2003, surface tectonics and crustal seismic velocity in Hindu kushi-Pamier-west China area show that a continental deep subduction is acting along Tethys continent-continent collision zone. Both lower crust and upper mantle are brought down to a depth more than 200 km, while mid-upper crust is overburdened by multiple back-thrusting in Pamier area. The continental deep subduction is restricted by Qiaman left-lateral strick slip faults in the west and Karakoron right-lateral strick slip faults in the east. Three seismic swarms occur along the deep subduction zone in vertical direction. The tipper seismic swarm presents in depth between 30 and 50 km, responding to the back-thrusting overburden of mid-upper crust. The middle swarm is concentrated around 90 similar to 120 km in depth comparable to a change position where the dipping angle of subducted slab becomes steeper downwards. The lower swarm occurs in a depth from 180 to 200 kin, representing the edge of the slab. Geometry of deep subducted slab in Pamier area shows a wedge narrowing downwards, with a change in subduction angle from 20 similar to 30 to 60 similar to 70 in 80 similar to 120 kin. Two numerical simulations have been used in the discussion about dynamics of the continental deep subduction. Two-dimensional numerical simulation with a given temperature field and negative buoyancy indicates that: (1) the mantle convection drag force plays an important control role to subducted depth and subducted rate. Its change from 100 MPa to 20 MPa results in decreases of the subducted depth from 231 kin to 151 km and subducted rate from, 10.79 mm/a to 5.46 mm/a; (2) The subducted depth of a subducted slab with a 30 degree angle is 25 similar to 50 kin more than that with a 45 degree angle. (3) Under the combined action of three kinds of dynamic load including a 10 similar to 30MPa push force of the mid-ocean ridge, negative buoyancy and the mantle convection drag force of 100MPa, the actual vertical displacement of the subducted continental crust can reach 117km and its final depth is 147km; the corresponding data for the subducted oceanic crust are 162km and 231km, respectively. In another model of coupled lithosphere and asthenosphere the LDDA and regional division methods are used. The model is characterized by the simple border conditions and the automatically acted effects between lithosphere and asthenosphere during subduction, but the mechanic parameters of different depths in different slabs must be given and calculating process is more complex.
