Laramide Times

Traditionally, the Laramide Orogeny starts around 75 million years ago.  Probably most geoscientists would agree with the overall analysis of Dickinson et al. (1988), which is mainly based on sedimentary rocks preserved from that time. So their criteria were that marine sedimentation (diagonal hatch) had ended prior to the Laramide, individual basins shifted from sharing facies with adjacent areas (black square) to having distinctly thicker deposits (circle) and coarse clastic detritus derived from nearby uplifts (black triangle) as the Laramide started:


It would be hard to argue that the Laramide Orogeny started later than the kinds of dates that Dickinson et al. proposed–but could it be earlier? If you had shallow sea floor covered in muds and parts started to rise up, might the muds simply get entrained in the existing current systems and be scoured down, creating a lacuna that, later on, would erased by even deeper erosion? In other words, is it possible that there was early deformation that wasn’t vigorous enough to overcome the broad subsidence of the region and so failed to produce positive topography? And if so, would subsurface loads have started to create local depocenters that perhaps have escaped recognition?

After all, another significant event–intrusion of igneous rocks along what would become the Colorado Mineral Belt–appears to have started at 75 Ma, right at the earliest start for the Laramide by traditional measures. One might expect some significant time lag between the first creation of melts in the mantle below and their arrival at the upper crust, so you’d think processes creating melts might well have started sometime earlier. (The idea is that you have to soften the lithosphere in some manner to allow for the migration of melt into the upper crust). Similarly, thrust faulting in the Kaiparowits Basin might date back to 80 Ma.

Perhaps the most compelling evidence that something Laramide-ish was going on is the accumulation of thick piles of sediments well east of the Sevier foredeep. McGookey et al. in 1972 inferred that the Denver and Piceance basins had started forming by 78 Ma. Cross and Pilger (Nature, 1978) noticed the overall increased sedimentation, but work trying to explicitly separate some cryptic cause of subsidence from sedimentation and sea level rise really dates to S. Liu and Nummedal’s 2004 paper, where by 83.9 Ma they see substantial sediment accumulation in central and eastern Wyoming that exceeds their expectations from a surface load.  In map view, by 80 Ma (early Campanian) sedimentation patterns quite evidently begin departing from a foredeep focus (e.g., Roberts and Kirschbaum, 1995; replotted as rates in Jones et al., Geosphere, 2011). S. Liu et al. (2014) associated this with the subduction of the Farallon plate, though the west-to-east migration they infer may not parallel Farallon-North America motion. Both S. Liu et al. (2014) and Heller and L. Liu (2016) put the beginning of substantial subsidence back to near 90 Ma, long before the volcanic arc at the western margin of North America showed any signs of slowing. Much of this is tied to a subduction history in several papers that includes an oceanic plateau descending under the active arc in California about 94-85 Ma (e.g., L. Liu et al., Science, 2008Heller and L. Liu, 2016).

So what? Does this all work? Consider this plot of events along a NE-SW transect from the coast to the Rockies from Jones et al. (2011):


On the left, the pink bar represents when the Sierra Nevada arc was active, though its end might be closer to 83 Ma than 80 Ma. The large circles are U-Pb ages of intrusive rocks (smaller symbols are K-Ar or Ar-Ar ages of volcanics (squares) and intrusions (circles) that might reflect cooling rather than emplacement); most of these are in the Mojave Desert, and while some are peraluminous, several are calc-alkaline plutons typically considered to be products of an arc. [Note that the Tindall et al. 80-76 Ma faults are not plotted here]. The two blue histograms are sedimentation rates while the horizontal lines show when faults were first active according to Bird (1998). The red lines show the progress of a horizontally-subducting Farallon plate entering the trench c. 75 Ma. If it was the progress of an oceanic plateau that caused everything from shutdown of the Sierran arc to Laramide deformation, you expect the arc ages to all be above the red line and Laramide deformation to be below it.

If the subsidence seen in Wyoming starting perhaps as far back as 90 Ma is related to subduction, then it somehow had a minimal impact on the arc and forearc. If the subsidence is related to shallowing subduction, we have the peculiar case of that shallowing affecting the distant foreland far before it shut down the volcanic arc. This seems far less consistent with subduction of an oceanic plateau than some other process. If the opening stages of the Laramide are coeval with the largest phase of Sierran arc magmatism, then we are forced to abandon the idea that a subducted plateau was responsible for all of these effects.

So when did the Laramide start? Because the answer will limit the possible causes of the creation of these still-enigmatic mountains.

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