Battle of the Back-bulge
…Backbulge basin, that is. The term is in common use in the stratigraphy/paleosedimentology community for sedimentary rocks deposited on the foreland side of a forebulge:
There’s a little exaggeration in this cartoon, thoughFirst, a flexural foredeep is, even in the presence of an orogenic wedge, asymmetric, as this simple calculation shows:
That triangle near 100 km distance with the red lines underneath and the yellow line on top is the forebulge basin (here left empty with air; if filled, it is deeper and gets wider). Another exaggeration is to the right of the forebulge, where the back bulge basin is shown:
In this case, with a foredeep 230m deep, the forebulge is 42m high and the back bulge basin is just 1.8m deep. Now one of the notions of the back bulge basin is that the sediments overtop the forebulge in some situations (like that diagramed above), but then instead of an amplified back bulge basin, you get something like this:
Here the blue line is the top of the thrust wedge+sediments on top and in the foredeep and the red is the original plate surface. The forebulge fades into insignificance in this scenario; the back bulge basin is simply an extension of the foredeep. Note the foredeep retains an asymmetric shape (the toe of the thrust wedge is at x=100km). So not quite as was drawn in the first figure.
If you really push, you can get the back bulge basin to about 10% of the thickness of the foredeep instead of the 1% value while leaving little sedimentation on the forebulge:
Go much beyond that, and the forebulge essentially will vanish.
Now to be fair, DeCelles and Giles suggest that back-bulge basins probably accumulate more sediment than simple flexure would suggest because of broad subsidence from dynamic effects or a coupled slab or higher sedimentation (though we see here that higher sedimentation might not get the right result). But tracking through the literature, it is clear that “back-bulge basin”is often felt to be controlled flexurally.
OK, what does this mean?
Consider one of the earlier interpretations of the sedimentation in front of the Roberts Mountain Allocthon in the Antler orogeny including a back bulge basin but Giles and Dickinson in 1995 in SEPM Special Publication 52:
Here, the foredeep, which was apparently starved for sediment, only has 60m of sediment, the forebulge accumulated no sediment, and yet the back bulge basin accumulated over 100m of sediment. Given the forebulge stayed out of water and the back bulge basin similarly didn’t fill up the whole way, this seems quite peculiar. The foredeep in this situation had to be quite deep. This geometry continued for quite some time. At the time step after the last one where the Diamond Range is interpreted to be the forebulge, that forebulge suddenly moves eastward more than 100 km, with the old forebulge now accumulating more than 220m of sediment. This happens in association with only 100m of sediment being added to the foredeep, an addition that changes the facies to shallow water facies.
Two things stand out: the thicknesses simply don’t look like they work, and the episodic stability of the forebulge makes little sense. Stickiness of the forebulge has been noticed before (e.g., Waschbusch and Royden, Geology, 1992), but it seems that really considering the magnitude of the sedimentation in the back-bulge basin hasn’t been a priority. When a back bulge basin is accumulating over 400m of sediment (and it isn’t clear what the relationship is between the thicknesses Giles and Dickinson report and the original depositional thickness), you’d think that the foredeep would have to be about 4000m deep–yet shallow water facies show up after only about 500-600m of sediment accumulates.
This isn’t to say all the forebulges and back-bulge basins in the literature are wrong, but it is to point out that the assumption that these are simply flexural features needs to be vetted. In those cases where the numbers don’t add up, what is going on? Simple appeals to migrating dynamic topography are frequently difficult to justify; the wavelengths are often wrong and the means of a progressing subsidence are difficult to construct.
One suspects that many features identified from stratigraphic arguments as forebulges are more complex features, and many back-bulge basins might well have nothing to do with the forebulge they are inferred to be behind. Just knowing where the sediments were deposited isn’t enough.