Everything to do with geoscience has some connection with “relief” [not release] of surface curvature on an Expansion Tectonic Earth. Every joint, fracture, fault, fold, sedimentary structure, magmatic intrusion, erosional and depositional cycle, biotic change, and so on involves, and is a result of, gravity induced relief of surface curvature of the earth’s crusts over time.
Relief of surface curvature operates in four dimensions, it cannot be viewed as a simple cross section of part of the earth. You must think in four dimensions to fully appreciate the significance of relief of surface curvature. Specific examples; look out of your window, every outcrop, every road cutting, every mountain range, everything you see in the landscape has to do with relief of surface curvature.
So, what are the mechanisms? Firstly, visualise an ancient earth at 50% of the present earth radius – i.e. the Permian Pangaean supercontinent, where supercontinental crust covered the entire earth surface and there were no modern seafloor crusts. Now, double the radius of the earth to the present-day size. This is where the seafloor crusts come in. The seafloor crusts are simply quenched and exposed mantle rocks which form only after the continental crusts breakup, so, for the moment, remain focussed on the continental crusts.
Over this 250 million year time span the relatively thick, deep rooted, continental crusts have to continually re-equilibrate to changing surface curvature, vaguely somewhat like plasticine or a clay coating on the surface of a balloon. It does this by stretching, folding, distorting, faulting, thrusting, fracturing, tearing, and so on. In addition, the exposed crusts are periodically being eroded and the sediments redeposited elsewhere. Overprinting this ever-changing process, plant and animal species are trying to survive, only to be forced to evolve, adapt, or perish as the landscape changes ever so slowly over time, and climate zones are being fragmented and displaced.
This is all very well for the continental crusts, but the relatively thin seafloor crusts are also subjected to changing surface curvature, albeit in a somewhat different way. The seafloor crusts are volcanic rocks emplaced over a set period of time at a set ancient earth radius. As the radius of the earth increases, the older seafloor crusts must also re-equilibrate to changing surface curvature. Unlike the continental crusts, the seafloor crusts have an inherent weakness, a vertical intrusion-related fabric which allows the crusts to flex and distort more readily during relief of surface curvature. In addition, new lava may be intruded within dilatant areas, further healing and sealing the crusts, along with high heat flow.
The active margins that you mention are generally more complex because they act as an interface between the relatively thick, deep rooted, continental crusts and the relatively thin, shallow rooted, seafloor crusts. The North-western Pacific Ocean, in particular, has vast areas of older seafloor crust which must undergo similar amounts of fragmentation, stretching, and distortion as the continental crusts. I sometimes use the term crustal interaction for these active margins, whereby the various continental and seafloor crusts have a complex interplay of vertical and translational motions to accommodate for relief of surface curvature. Unlike the passive margins, because of the contrasting continental and seafloor crustal settings along the active margins there is also a complex stress regime. Extension between thick continental crusts and thin seafloor crusts sets up deep, mantle tapping dilatant zones, which, on an expansion Tectonic Earth, represent juvenile mid-ocean-rift zones. In the distant future these dilatant zones will progressively open further and take on the characteristics of mature MORs.
For the purpose of your query, the geodynamics of the active margins is juvenile by comparison to the more mature MORs, hence have somewhat different characteristics – e.g. the deep-rooted plumes that Florian portrayed in his figure in a previous post. These active margins are generally in close proximity to continental margins, hence have a more dynamic crustal interaction during relief of surface curvature. Relatively thick crusts trying to do their own thing adjacent to relatively thin crusts which are in turn trying to do something completely different.
Cheers,
James