The 85 Ma Trainwreck: Introduction

It used to be when we thought what the southwestern U.S. looked like at 85 million years ago, back in the Cretaceous, we thought things were pretty simple.  There was a nice volcanic arc running from the Sierra down through the Mojave into the (restored) Peninsular Ranges. To the east was a fold-and-thrust belt extending nicely from well up in Canada down to the Las Vegas area in continuous fashion before taking a left turn to angle more erratically into more complex geology across Arizona and New Mexico. East of that was the foreland, its western edge bowed down under the weight of those fold-thrust mountains and the torrent of sediment washed off those ranges toward the inland sea that stretched up from Texas towards the Arctic Ocean. The only real wrinkle in this had to do with where the troublesome exotic terranes now in British Columbia were at the time.

Now it feels like 85 Ma is instead a pivot for everything about the western U.S.  A number of puzzling things seem to be going on around this time.

• Emplacement of the first of the subducted oceanic/forearc schists (the Catalina and San Emigdio Schists) appears to occur about 85-90 Ma [Jacobson et al., 2011]
• The Sierra Nevada sees the culmination of the most massive plutonic episode in its history just prior to a complete end to plutonism [e.g., Ducea, 2001]
• The Mojave Desert will continue to see both peraluminous and metaluminous magmatism for another 10 million years despite the inferred emplacement of most of the Rand Schist over that time. [e.g., Barth et al., 2013]
• Shortening in the hinterland appears to be a short-lived interval between extensional episodes [Wells et al., 2012]
• The Sevier fold-and-thrust belt in the vicinity of Las Vegas is dead or dying–some put its end before 90 Ma [e.g., Fleck et al., 1994]
• Perhaps a northwest-trending thrust belt has instead invaded the area about this time. [Pavlis et al., 2014]
• Canyons start to form in the southwesternmost Colorado Plateau [Flowers and Farley, 2012].
• In contrast, the simple foredeep geometry of sediment accumulation in the foreland begins to broaden out, suggesting a “dynamic subsidence”. [Liu and Nummedal, 2004]

By 75 Ma, it is really clear that things have gone super strange.  The Sierran arc is dead with no real replacement. Magmatism at that latitude is limited to smaller intrusions in the Colorado Mineral Belt and some peraluminous (two-mica) granites in Nevada, eastern California and parts of Arizona. In contrast, the Mojave continues to see plutonic activity with both peraluminous and metaluminous magmatism, seeming to be the southwestern end of a long linear belt of unusual magmatism. The Sevier hinterland has several examples of significant decompression accompanied by more limited evidence of extension. The foredeeps in front of the fold-thrust belt seem to have ceased to accumulate much sediment, which is now captured in broader basins farther east. Laramide uplifts are certainly underway.

Although 75 Ma is often taken to be the start of the Laramide orogeny [Dickinson et al., 1988], it increasingly seems like the orogeny’s origins lurk in the preceding 10 million years. What exactly was going on?

Some of the things we don’t really have a solid handle on:

• Where was the Insular Superterrane? [We might have a more decent view of the Intermontane Superterrane due to work in Idaho over the past decade].
• What really was the plate geometry? The Kula plate dates back to about this time; were there other plates to the east of the Kula-Farallon-Pacific triple junction?
• Do we have a good handle on where plutonism was active in the Mojave/Sonoran deserts?
• How much of the Cretaceous decompression events are from extension vs. some kind of intra-crustal pseudo-convection?
• How could the southern Nevada part of the fold-thrust belt be inactive?
• How can schists be emplaced against the middle continental crust immediately below fresh plutonic rocks?

The Laramide Orogeny made North America look the way it does today; this time period holds the keys to understanding how it could happen. Some way-out ideas are kicking around [e.g., Hildebrand, 2009, 2013; Sigloch and Mihalynuk, 2013], though some are moderating a bit [Sigloch and Mihalynuk, 2017]. Conversely, most earth scientists have gravitated to some flavor of subduction of an oceanic plateau as the cause of all the misadventures near the start of the Laramide orogeny. That all of these have pretty substantial flaws shows that the community is struggling to understand just what was going on at the start of the Laramide.  Hopefully over the next few months we can explore some of these topics in some detail…

85 Ma Trainwreck pages

• Introduction (this page)
• Schists
• Igneous Activity
• Deformation
• Mojave Mojo
• Crustal structure
• Sedimentation