Have you ever imagined the processes how hydrocarbon was formed at the first time? It was started from the very early process which is called basin formation. Basin is a convex-formed area of earth crust where sediment can be accumulated and form vertical succession of sedimentary facies. After basin formation process, the deposition process of sediment material filled in basin enable to affect hydrocarbon potential in its basin which takes long time to accumulate. So, when we have so many oil and gas production now, it means that we have through very long period of hydrocarbon formation. Let’s we get in to the time machine and go back to the past to explain about basin formation.
We all know that we have some kinds of basin based on its formation processes. Fore arc basin, Back arc basin, and the other basins. They have their own characteristics as the result of their formation processes. Now, AAPG UGM -SC will bring you to the unforgettable experience called as “The Formation of Pull-Apart Basin”.
What is Pull-Apart Basin?
Pull-Apart Basin can be simply defined as a basin which is formed by pull force which comes from extensional forces in shear zone. Pull-Apart basin is usually bounded by two similar strike slip fault / wrench fault. The formation of this basin depends on the configuration of the strike slip fault. The combination must be a left stepping sinistral strike slip fault or right stepping dextral strike slip fault. Those configurations will create pull-force as the result of stress distribution in shear zone.
THE DEVELOPMENT OF PULL-APART BASIN
There are some mechanisms which form the pull-apart basin, they are.
1. RIEDEL SHEAR MECHANISM
The shear zone is a zone which is still influenced by shear stress in deformation activities. In shear zone we’ll find many kinds deformations. Where we can find the shear zone? In this case, we can find shear zone in a zone of strike slip fault deformation or between two strike slip faults. Shear zone is characterized by highly-deformed rocks and showing some rotational deformations. Deformations in shear zone can be explained based on Riedel shear mechanism (figure 3).
The main strike slip fault is called PDZ (Principal Displacement Zone) generating the Riedel Structures as the result of the main displacement. Then after the formation of Riedel faults, a new fault develops parallel with the PDZ (Principal Displacement Zone) direction. This fault creates the pulling-forces which finally form pull-apart basin.
2. EN ECHELON FAULTS MECHANISM
En echelon faults develop their unique configurations. Those configurations form as systematic arranged faults. En echelon faults forming pull-apart basin is an en echelon right-stepping dextral strike slip fault or en echelon left stepping sinistral strike slip fault. The mechanism can be seen on figure 3. The configuration creates pulling-forces in the area between two faults. The pulling forces are created because of the stress distribution in shear zone of those two faults (figure 4). Those extensional forces create a series of normal faults perpendicular to the forces direction. Then graben can be formed by those series of normal faults (figure 5).
Now we have learned how pull-apart basin is formed, then how is Central Sumatra basin formed? Find the answer in the next AAPG UGM-SC journey.
Find more explanation about forming of pull apart basin by illustrating video here
Created By :
( Vice Head of Course & Workshop Division )