Woodlark Basin

The Woodlark Basin is very young and only started spreading around 3 million years ago, so that there is very little sediment in the basin relative to most oceanic basins, which can have thousands of meters of sediment fill close to continental margins. The basin has a maximum thickness of 1500 meters of sediment fill in the deepest section of the North Moresby graben, however most of the basin is covered in less than 1000 meters of sediment.[3] This is great for researchers because different features, such as faults and spreading centers, can easily be seen in satellite images of the basin due to the lack of sediment buildup.[3]

Figure 2: Above is an image of the Woodlark Basin (boxed in area in red) on a map of the plate tectonics in the region surrounding it

Figure 3: The image above shows the basic faulting in the woodlark basin. The purple line represents the transform that marks the northern boundary of the basin, the red lines represent the spreading centers in the middle of the basin, and the yellow line represents the subduction zone where to the east of the basin.

The Woodlark Basin originally began to open as a sphenochasm with a pole near the tip of eastern Papua about 20 million years ago. A sphenochasm is a triangular gap of oceanic crust separating two continental blocks and converging to a point. This was caused by left-lateral shear in the region induced by a change in the relative motion pole of the Indo-Australian plate and Pacific Plate. The basin opened only a few degrees at this time, then stopped for a period of time. Then the rifting resumed in the entire basin about 3 million years ago, based on magnetic anomaly data. Then about 1 million years ago, the spreading center in the western basin shifted to the Woodlark Rise.[1] The Woodlark Rise is the northern section of the Woodlark Basin that separates the basin from the bordering Soloman sea to the north.[4] This spreading rift marks the southern boundary of the Solomons Plate which is bounded by subduction zones in the north and east (the New Britain and northern Solomons Trenches, respectively) and in the west by a combination strike-slip rifting (dip-slip) boundary in eastern Papua (New Guinea). A vector triangle solution near the Solomons Trench-Woodlark Rift triple point gives underthrusting of the Solomons Plate beneath the northern Solomon Trench in a northeasterly direction at about 11 centimeters per year.[1] Figure 2 shows each of the tectonic plates surrounding the Woodlark Basin along with the relative plate motion and direction that each plate is moving.

The Woodlark Basin is one of very few places on the earth to have active low angle normal faults. A low angle normal fault is a normal fault that has a dip of less than thirty degrees (dip <30°). This is a topic that is currently debated heavily, the low angle normal fault paradox. Low angle normal faults have been proposed to be a key structural feature for crustal extension.[5] In the Woodlark Basin the active low angle normal faults have a dips between 15°-35°, and are all located in the small range of 150.5° E to 152.5° E.[6] This is a very interesting area in the Woodlark because it is the area where sea-floor spreading makes the transition to continental rifting.[6] However, some scientist do not believe that low angle normal faults exist actively in the world. They instead believe that the low angle normal faults found are old normal faults that are no longer active and have been rotated to a different orientation. This would make a normal fault look like it was a low angle normal fault that is no longer active.

The Woodlark Basin is actively spreading in a mostly north and south direction. This is shown in Figure 3 by the red lines that stretch across the middle of the figure. The spreading centers are offset by small transform faults, this is what causes the spreading center to be uneven across the basin. Also the rate of spreading in the eastern section of the basin is significantly higher than the rate of spreading to the western side of the basin.[7] This statement is easily backed up by satellite imaging, which can be seen in figure 3. This figure helps scientist and researchers to visually notice that the spreading rates across the basin are uneven. As shown in the figure you can see the eastern side of the basin has opened up much faster than the western side. This is only easily visible in satellite imaging due to the relatively small amount of sediment build up in the basin.[3]