Not sure what you mean by "end of the left side of the great fault". If, by 'left side', you mean the part of the US that was and still is west of the fault, it of course is still there, as that arm of the 3-prong fault never actually opened. It was/is a "failed" arm or failed rift. As spreading proceeded along the other two arms it stopped along the New Madrid arm. This was 600+ million years ago, during the breakup of early supercontinent Rhodinia. It was before supercontinent Pangea, which began breaking apart 'only' about 200 million years ago, eventuslly giving birth to the Atlantic Ocean. North and South America had been stuck to Africa at that time, and was South of the equator.
The PBS broadcast has been showing a great upside down magma red U starting in the upper midwest, curving over the great lakes and coming back down east of the great lakes. The end point showed on the left side was above the New Madrid area by hundreds of miles. The 600 mya you quote is also their figure, and they did refer to it as a failed rift. That is all I know. Hope you can find the broadcast some time, I have seen it twice in the past 2 weeks, don’t remember the exact name. Perhaps “Building North America” or something like that? I am 80 and memory not as good as when younger.