Location, location, location: The variable lifespan of the Laramide orogeny Peter Copeland1, Claire A. Currie2, Timothy F. Lawton3, and Michael A. Murphy1 1Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204, USA 2Department of Physics, University of Alberta, Edmonton, Alberta T6G 2E1, Canada, 3Centro de Geociencias, Universidad Nacional Autónoma de México, Querétaro 76230, México ABSTRACT our boundary passes through the northwest edge The Laramide orogeny had a spatially variable lifespan, which we explain using a geody- of the classic Laramide province (DeCelles, namic model that incorporates onset and demise of flat-slab subduction. Laramide shortening 2004), we designate this line as the northwest and attendant uplift began in southeast California (USA) at ca. 90 Ma, swept to the northeast to margin of the region we consider below. arrive in the Black Hills of South Dakota (USA) at ca. 60 Ma, and concluded in South Dakota Figure 1B shows the age of the various phe- within ~10 m.y. During subsequent slab rollback, the areal extent of Laramide deformation nomena listed above versus distance along a decreased as the eastern edge of active deformation retreated to the southwest rapidly from line of projection, A-A′ (Fig. 1A), which has a ca. 55 to 45 Ma and more slowly from ca. 45 to 40 Ma, with deformation ultimately ceasing bearing of 045 and passes through the middle of in the southwestern part of the orogen at ca. 30 Ma. Geodynamic modeling of this process the classic Laramide province (DeCelles, 2004). suggests that changes in the strength of the North America plate and densification of the Far- This approach compresses the spatial complex- allon plate played important roles in controlling the areal extent of the Laramide orogen and ity of a continent-scale orogen onto a single line hence the lifespan of the orogenic event at any particular location in western North America. that may not be the best choice for all points portrayed, but we conclude that the synoptic INTRODUCTION sedimentation, estimates of the time of initia- utility of the diagram outweighs the shortcom- The Laramide orogeny most generally refers tion and cessation of Laramide-style deforma- ings of the approach. For details of our methods to basement-involved deformation and inter- tion, the estimated time of attainment of maxi- and sources of data, see the Data Repository. montane basin development that took place in mum surface elevation, and the inferred timing Estimates of the age of the youngest marine the Rocky Mountain region (western United of transit of the thickest part of the conjugate strata in the study area reveal a northeastward States) between ca. 80 and 40 Ma (e.g., Bird, Shatsky Rise (CSR) beneath the region during migration of the shoreline during the interval 1998; Saleeby, 2003; DeCelles, 2004; English subduction of the Farallon (FA) plate (see the 100–60 Ma (Fig. 1B). This distribution of marine and Johnston, 2004; Yonkee and Weil, 2015). GSA Data Repository1). The locations have strata in space and time could be explained if Deformation produced uplifts within a formerly been restored from their modern-day positions the surface of the continent sloped from a high extensive foreland basin that developed adjacent to the approximate locations they occupied at in the southwest to low in the northeast and to thin-skinned thrust sheets of the Sevier oro- ca. 25 Ma. We note a substantial northeast shift sea level was falling, but the long-term eustatic genic belt (Armstrong, 1968). Laramide fore- of magmatic age north of a line from northern curve indicates that sea level was essentially land disruption was a largely amagmatic event California through Idaho and into Montana (Fig. unchanged during this time interval (Haq, 2014). that affected a broad region between southern 1A) relative to magmatic ages south of this line. Therefore, a regional tectonic cause is needed to Montana (USA) and northern Mexico (Dickin- It is likely that this shift in magmatic activity explain the distribution of marine rocks. son and Snyder, 1978). Time-equivalent shorten- is due to the presence of an embayment in the The eastward-then-westward sweep of mag- ing in central Mexico was accompanied by arc margin of western North America (Yonkee and matism across western North America (Fig. 1B) magmatism and diverse structural styles includ- Weil, 2015; Schmandt and Humphreys, 2011), a has been interpreted as a consequence of pro- ing ramp-flat thrusting and large-scale buckle holdover from the development of a Proterozoic gressive shallowing of the subducting Farallon folding (Guzman and de Cserna, 1963); coeval transfer zone (Lund, 2008). The lack of Creta- plate in the Late Cretaceous and Paleogene fol- deformation in Canada was thin-skinned (Bally ceous arc magmatism in Oregon and Washing- lowed by slab break-off or slab rollback in the et al., 1966). The mechanism for the distinctive ton reflects the ca. 50 Ma accretion to North Eocene and Oligocene (Coney and Reynolds, basement-involved deformation that character- America of the Siletzia terrane, a large igneous 1977; Dickinson and Snyder, 1978; Saleeby, ized the classic Laramide province constitutes a province or plateau (Fig. 1A; Wells et al., 2014). 2003; Liu et al., 2010). The presence of a buoy- geodynamic conundrum that geoscientists have The boundary we note in Laramide magmatism ant oceanic plateau (the CSR) within the Faral- struggled to explain for more than 100 years. is coincident with variations revealed by body- lon plate has been suggested as an important wave tomography of the western United States contributing factor in the decrease of the plate’s OBSERVATIONS (Schmandt and Humphreys, 2010), suggesting subduction angle (e.g., Liu et al., 2010; Heller To understand the temporal variability of the a tear in the Laramide slab (Humphreys, 2009; and Liu, 2016). Along our line of projection, a Laramide orogeny at a regional scale, we com- Schmandt and Humphreys, 2010; Colgan et al., close temporal correspondence exists between piled observations relevant to the evolution of 2011). Based on these observations and because the passage of the thickest part of the CSR and marine sedimentation, magmatism, deforma- the initiation of Laramide-style deformation, and tion, and surface elevation across the Laramide the youngest marine sedimentation (Fig. 1B). 1 GSA Data Repository item 2017058, sources of region and beyond. Figure 1 shows the distri- data and details of methods used, is available online In addition, close spatial-temporal corre- bution of igneous rock, estimates of the time at www.geosociety.org /datarepository /2017 or on spondence exists between cessation of Laramide of the transition from marine to nonmarine request from [email protected]. shortening and age of attainment of maximum GEOLOGY, March 2017; v. 45; no. 3; p. 223–226 | Data Repository item 2017058 | doi:10.1130/G38810.1 | Published online 23 January 2016 GEOLOGY© 2017 Geological | Volume Society 45 | ofNumber America. 3 For| www.gsapubs.org permission to copy, contact [email protected]. 223 surface elevation (Fig. 1B); in the study area, the flat slab then coincides with an increase in BC AB SK these events are oldest in the northeast and plateau density via eclogitization. In the model, (A(A) become younger to the southwest, contrary to progressive plateau eclogitization is imposed the trend for the initiation of Laramide deforma- from 58 to 48 Ma to match geological observa- 50°N ID MT ND tion. The temporal trends for the initiation and tions of the timing of cessation of deformation cessation of shortening indicate that the lifespan (Fig. 1B). Sinking also depends on the strength WA OR SD of the Laramide orogeny varied systematically of the continental mantle lithosphere. The con- AA’’ WY with distance from the trench, with deforma- tinental mantle lithosphere may weaken during NE tion persisting for as long as 60 m.y. (from the flat-slab subduction due to the infiltration of CA NV UT CO Turonian to the Oligocene) in parts of Califor- hydrous fluids released from the oceanic plate. 0°N 4 nia, Arizona (USA), and Sonora (Mexico), but The primary source of fluids is likely dehydra- KS occurring for perhaps <10 m.y. of the Paleocene tion reactions within the oceanic mantle litho- AZ OK TX in eastern Wyoming and South Dakota. sphere (Currie and Beaumont, 2011); additional fluids would be released during crustal eclogiti- NM BBCCNN A GEODYNAMIC MODEL zation. We simulate this by gradually weaken- a A Ma a N MaM Following the hypothesis that Laramide ing the continental mantle lithosphere closest 5 M 775 5 a 30° 665 Ma 5 M COA shortening was a consequence of the traction to the margin from 75 Ma until 50 Ma. With 555 a BCS Ma SON CHH 4455 M between the base of the North America (NA) the combination of eclogitic densification of the 440000kkmm SIN DUR plate and the top of the Farallon plate (e.g., Yon- Farallon lithosphere and hydration weakening of 120°W 110°W 100°W kee and Weil, 2015; Heller and Liu, 2016), we the NA lithosphere, the flat slab decouples from A AA’’ suggest that the southwestward migration of the the continent and sinks as subduction continues. 120 inboard deformational edge (Fig. 1B) was a con- 100 (B) sequence of a narrowing of the zone of FA–NA DISCUSSION ) lithospheric interaction by progressive rollback The results of this lithospheric model com- 80 Ma of the Farallon plate from northeast to southwest pare favorably with observations of surface beginning at ca. 55 Ma and continuing into the geologic phenomena. The limit of easternmost e( 60 Oligocene. To investigate the possibility of slab FA-NA interaction tracks both the north- Ag 40 rollback after shallow subduction as much as eastward migration of the Laramide defor- 1900 km inboard of the trench, we developed a mation front and shoreline from ca.