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For my world, I would like a coastal mountain range that partway along its length is offset by a few (say, 100) kilometres, forming a pass through the mountains, like so:

Map of mountains

(N.B. I've drawn this very square and regular to illustrate my point)

From what little I know of plate tectonics, this would require the mountains to be created by a normal continental plate (yellow) meeting an oceanic plate (blue) along one fault line (red), and then a second fault line perpendicular to the first along which one part of the plates slips (so in effect four plates meeting at one point):

Map of tectonic plates

My question is therefore:

  1. Is this arrangement of tectonic plates, giving rise to this formation, plausible?
  2. If not, is there another phenomenon which might give rise to the same formation?

I had in mind a roughly Alps sized range placed in a similar manner to the Andes, but these are less important than the existence of this feature.

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  • $\begingroup$ Please make it clear that the plate tectonics part is just your hypothesis of how such a geographical feature may be formed and it's not relevant to the answers. And by the way, mountain ranges do not form exactly on top of plate boundaries, but rather on one (or possibly both) plates near the boundaries. (To understand what "near" means consider the Himalayas and the Tibet plateau formed on the Asian plate "near" the boundary with the Indian plate.) $\endgroup$
    – AlexP
    Feb 28, 2018 at 11:38
  • $\begingroup$ @AlexP the plate tectonics hypothesis is relevant, as I'm asking for a reality check on it. I understand about mountain ranges not forming directly above plate boundaries, but I thought drawing them like this would be clearer. $\endgroup$
    – walrus
    Feb 28, 2018 at 11:55
  • $\begingroup$ Consider how far east the Rockies extend from the coastal fault lines before assuming mountains only form above faults. $\endgroup$
    – rek
    Mar 1, 2018 at 0:48
  • $\begingroup$ @rek to quote myself, from the comment right above yours: I understand about mountain ranges not forming directly above plate boundaries, but I thought drawing them like this would be clearer $\endgroup$
    – walrus
    Mar 1, 2018 at 11:36
  • $\begingroup$ So you're contradicting yourself and it's somehow my fault for not correctly interpreting, got it. $\endgroup$
    – rek
    Mar 2, 2018 at 1:16

3 Answers 3

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It's not geologically impossible. If you want something that looks somewhat similar, there's the transverse faults located along seafloor spreading ridges that offset the ridges by sometimes long distances. Here's an example from the East Pacific Riseenter image description here

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    $\begingroup$ As ever, nature's answer to "is this plausible?" is "here's one I made earlier". $\endgroup$
    – walrus
    Mar 1, 2018 at 11:59
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    $\begingroup$ @walrus Sometimes I feel like the more preposterous the idea, the more likely Nature's already done something even more insane. $\endgroup$
    – Andon
    Mar 12, 2018 at 2:49
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Perhaps something like Columbia River?

Between The Dalles and Portland, the river cuts through the Cascade Range, forming the dramatic Columbia River Gorge... the Columbia cuts through the range nearly a thousand miles from its source in the Rocky Mountains.

While not between tectonic plates, it's between crustal blocks.

The river is the blue line

The picture is from here, the river is (approximately) the blue line.

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    $\begingroup$ @AlexP the question isn't about the river, it's about an offset gap in a mountain range and whether it could be formed by plate tectonics like those described. It's the second part that I'm specifically asking for a reality check on - it's not a 'handwaving justification' $\endgroup$
    – walrus
    Feb 28, 2018 at 11:47
  • $\begingroup$ @AlexP: For a river flowing obliquely through a coastal mountain range, consider the Sacramento river flowing into San Francisco Bay. The river flows south, makes a nearly 90 degree turn to flow west through the coast range around Suisun Bay, then turns the water turns south again in the Bay before going west through the Golden Gate. I don't know the geology well enough to be sure, but on the map it looks like the range is offset a few miles east-west, too. $\endgroup$
    – jamesqf
    Feb 28, 2018 at 19:12
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It depends on how married you are to the idea that they must literally be the exact same mountain range. From the strictest geographic perspective, once the mountain chain is no longer in a single unbroken line, then it's considered two separate ranges anyway, even if they have the same geological origin.

Based on the first map, which is very similar to the west coast of North America, then yes, there is another process by which a similar topography can happen that would not involve the transverse fault running horizontally through your second map.

As the North American Plate moved west, it created chains of volcanic islands in front of it, called island arcs.

https://en.wikipedia.org/wiki/Island_arc

Eventually, the North American Plate plowed into the islands and they glued themselves to the front of it, as mountain chains. In geology, this is called accretion.

https://en.wikipedia.org/wiki/Accretion_(geology)

As North America has continued moving west, the former-islands-now-mountains have continued to get taller, especially in western Canada. You can get an idea of what this process would have looked like, over geologic time, here:

https://www.theatlantic.com/technology/archive/2013/09/what-did-the-continents-look-like-millions-of-years-ago/279892/

So, going back to your first map, what you would have would technically be two separate mountain ranges, very close together. They would have been two island arcs, very close together, that were accreted onto the main continent. This could result is a valley with a river system similar to yours between them, but the ranges could also meet somewhere off the top of the map, the way the Coastal Ranges meet up with the Sierra Nevada.

Edited to add: This way, you don't have to have a separate explanation for how the island to the south of your western mountain chain formed. Since it would be part of western island arc, it would make sense for the island to be slightly behind the eastern range. You wouldn't even need to say that it was still accreting - similar to Vancouver Island, you could have the leading edge of the continental plate be just to the west of the island and its associated range.

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  • $\begingroup$ Thanks, it's always good to have options when worldbuilding, and as you said, those island arcs are helpful for explaining how those islands got there. $\endgroup$
    – walrus
    Mar 12, 2018 at 9:24
  • $\begingroup$ Glad to be helpful! $\endgroup$
    – KernelOfChaos
    Mar 12, 2018 at 22:27

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