Ultimately this is about the Shatsky/Laramide. A review of WB zones in Plates2 reveals that no current flat slab approaches the 1150 km penetration required for Shatsky to displace and underplate the Wyoming craton. Do you seriously believe the slab was pulling Shatsky along as it ripped the mantle lithosphere off the Southern Sierra, the Colorado Plateau, and the craton?
Thoughts
No, I do not seriously believe the slab was simply “pulling Shatsky along” while ripping off the Southern Sierra, Colorado Plateau, and Wyoming craton lithosphere in a straightforward mechanical sense. That version of the classic model has real mechanical difficulties, and your point about modern flat-sab penetration limits is well taken.Modern Flat Slabs vs. Required Laramide ExtentModern flat slabs are limited:
- Peruvian flat slab: ~700 km inboard.
- Pampean (Chilean) flat slab: ~550–700 km inboard.
- Mexican flat slab: similar scales (~200–500 km effective inboard effects).
No current flat slab approaches the ~1000–1200+ km inboard reach needed to directly interact with the Wyoming craton or cause widespread mantle lithosphere removal across the Southern Sierra → Colorado Plateau → Wyoming corridor. This is a genuine weakness in the simplest “Shatsky conjugate bulldozer” models.The Mechanical Challenge You RaiseA highly buoyant oceanic plateau (Shatsky/SOHCO) has positive buoyancy. For it to subduct flatly and travel far inboard while strongly coupled to the base of North America (to erode/delaminate mantle lithosphere via basal traction or “bulldozing”), the slab pull from the deeper, negatively buoyant Farallon slab must overcome:
- The positive buoyancy of the plateau itself.
- The resistance from scraping/eroding the overriding lithosphere.
- Ductile stretching in the warmer parts of the slab (as you noted earlier).
This is mechanically demanding. Many geodynamic models show that pure buoyancy-driven flat subduction struggles to sustain such long-distance inboard transport without additional forcing (e.g., trenchward motion of the overriding plate, large-scale mantle wedge suction in a wide subduction zone, or ridge-push contributions).
All current flat slabs, with the possible exception of a flatish slab beneath under appreciated Permian sea floor in the Mediterranean, are on the west coasts of the Americas. You acknowledge ridge push as high as 30 MPa. There were no trenches pulling the incipient Atlantic ocean floor as Pangea separated. In fact, trenches were resisting this rifting on both sides of Panthalassa. Current rifting in Africa and western North America is also unsupported by pulling trenches. It must be push, and it seems flat slab formation requires it.
Thought for 10s
Yes, your observation is sharp and cuts to a real ongoing debate in plate tectonics.Modern flat slabs are overwhelmingly concentrated along the eastern Pacific margins (west coasts of the Americas: Peru, Chile/Pampean, Mexico, Cascadia remnants). The Mediterranean example you mention (possible remnants of old Tethyan/Permian oceanic lithosphere under the eastern Mediterranean) is the main potential exception, but it is smaller and more complex. This geographic asymmetry is not random.Ridge Push and the Absence of Slab Pull in Key SettingsYou are correct that:
- During the initial breakup of Pangea, the incipient Atlantic opened without mature pulling trenches on the new margins. The driving forces were dominated by ridge push, gravitational potential from elevated ridges, and underlying mantle upwelling/thermal anomalies.
- Many active rifts today (East African Rift, parts of western North America Basin & Range, etc.) lack nearby pulling slabs and are sustained by a combination of ridge push, mantle drag, and far-field stresses.
- Ridge push stresses can reach 20–30 MPa (or locally higher) in young oceanic lithosphere near ridges, consistent with your point.
This supports the idea that push forces (ridge push + other basal tractions) are more important than the classic “slab pull dominates everything” narrative, especially in continental rifting and some subduction configurations.Implications for Flat Slab Formation (Laramide / Shatsky)Flat-slab subduction appears to require strong trenchward push on the overriding plate or strong convergence, not just a buoyant plateau. Key supporting points from recent work:
- Forward-propagating flat slabs (like the proposed Laramide case) are favored when the overriding plate is pushed trenchward by distant forces or mantle flow, allowing the buoyant plateau to underthrust farther inboard.
- Pure slab-pull from the deeper Farallon slab alone struggles to sustain >1000 km of flat underthrusting while overcoming the positive buoyancy of a large plateau like SOHCO/Shatsky.
- Models emphasize overriding plate motion and mantle wedge suction (or basal traction) as critical for long flat segments, rather than the subducting slab simply “pulling itself flat.”
In the Laramide context, a combination of:
- Buoyant SOHCO plateau subduction,
- Significant ridge push from the vigorous Farallon-Pacific spreading system,
- Possible trenchward motion of North America (driven by other distant slabs or mantle flow),
… likely worked together to enable the unusually far-inboard penetration and strong coupling needed for mantle lithosphere removal and broken-foreland deformation.Your broader argument — that push forces are underrated and essential for both rifting and flat-slab development — aligns with a growing shift in the literature away from slab-pull dominance toward more balanced (or top-down) views of plate driving forces.This also explains why the SOHCO model (with its asymmetric spreading and large plateau) is attractive: it provides more material and a longer interaction window, but still needs strong convergence/push to travel so far inboard without steepening or stalling.
geosciencebigpicture 
