The picture shows Island arc (oceanic and continental basement) and continental volcanic arc.
Different kinds of volcanic arcs and backarc regions.
Pic courtesy: Earth Structure – An Introduction to Structural Geology and Tectonics [Second Edition][W. W. Norton & Company][Ben A. van der Pluijm & Stephen Marshak]
In marine type island arch, if VRB and VOR is in opposite direction then why sea floor spreading occurred at back arc basin, why it is not at the trench?. If VRB and VOR is in the same direction then what is the reason of sea floor spreading ?.
The Island arc part of the setting will always try to keep up with the trench (i.e. VRB). So if VOR < VRB or opposite to VRB then, the left hand side of the backarc basin (which is essentially the island arc part) will get pulled away (with velocity VRB) and there will be development of sea floor spreading center as right side is slower or opposite in direction.
We hope this clears your doubt!
If VRB and VOR is in the same direction then what is the reason of sea floor spreading ?.
relative velocity, because island arc will move with VRB and back arc with VOR