as for back as the present ocean basins have existed 讲不通，应改为：as far back as the present ocean basins have existed。

as far back as：固定习语。意为“早在...时候；追溯到...时候”。例如：

The band made their first record as far back as 1980. 早在1980年，这个乐队就灌制了他们的第一张唱片。

As far back as the Roman Empire, Europe has been Christian. 早在罗马帝国时代,欧洲就有基督教了。

as far back as the present ocean basins have existed 的意思是：“早在目前海洋盆地存在时”。不过我认为，原句使用完成时have existed不合适，应改为一般过去时existed。