An official website of the United States government
Western North America is the archetypical Cordilleran orogenic system that preserves a Mesozoic to Cenozoic record of oceanic Farallon plate subduction-related processes. After prolonged Late Jurassic through mid-Cretaceous normal-angle Farallon plate subduction that produced the western North American batholith belt and retroarc fold-thrust belt, a period of low-angle, flat-slab subduction during Late Cretaceous−Paleogene time caused upper plate deformation to migrate eastward in the form of the Laramide basement-involved uplifts, which partitioned the original regional foreland basin. Major questions persist about the mechanism and timing of flat-slab subduction, the trajectory of the flat-slab, inter-plate coupling mechanism(s), and the upper-plate deformational response to such processes. Critical for testing various flat-slab hypotheses are the timing, rate, and distribution of exhumation experienced by the Laramide uplifts as recorded by low-temperature thermochronology. In this contribution, we address the timing of regional exhumation of the Laramide uplifts by combining apatite fission-track (AFT) and (U-Th-Sm)/He (AHe) data from 29 new samples with 564 previously published AFT, AHe, and zircon (U-Th)/He ages from Laramide structures in Arizona, Utah, Wyoming, Colorado, Montana, and South Dakota, USA. We integrate our results with existing geological constraints and with new regional cross sections to reconstruct the spatial and temporal history of exhumation driven by Laramide deformation from the mid-Cretaceous to Paleogene. Our analysis suggests a two-stage exhumation of the Laramide province, with an early phase of localized exhumation occurring at ca. 100−80 Ma in Wyoming and Montana, followed by a more regional period of exhumation at ca. 70−50 Ma. Generally, the onset of enhanced exhumation occurs earlier in the northern Laramide province (ca. 90 Ma) and later in the southern Laramide province (ca. 80 Ma). Thermal history models of selected samples along regional cross sections through Utah−Arizona−New Mexico and Wyoming−South Dakota show that exhumation occurred contemporaneously with deformation, implying that Laramide basement block exhumation is coupled with regional deformation. These results have implications for testing proposed migration pathway models of Farallon flat-slab and for how upper-plate deformation is expressed in flat-slab subduction zones in general.
more »
« less
This content will become publicly available on November 13, 2025
Evidence for a Late Cretaceous to Paleogene basement-involved retroarc wedge in the southern U.S. Cordillera: A case study from the northern Chiricahua Mountains, Arizona
Late Cretaceous to Paleogene contractional deformation in the southern U.S. Cordillera is commonly attributed to the Laramide Orogeny, in part because of the prevalence of moderate- to high-angle, basement-involved reverse faults. However, it is unclear if the tectonic models developed for the archetypal Laramide foreland belt in the U.S. Rocky Mountain region are applicable to the southern U.S. Cordillera. New geologic mapping of the northern Chiricahua Mountains in southeast Arizona, USA, indicates the presence of an originally sub-horizontal thrust fault, the Fort Bowie fault, and a thin-skinned ramp-flat thrust system that is offset by a younger thrust fault, the Apache Pass fault, that carries basement rocks. Cross-cutting relationships and new geochronologic data indicate deformation on both faults occurred between 60 Ma and 35 Ma. A biotite 40Ar/39Ar plateau age of 48 Ma from the hanging wall of the basement-involved Apache Pass fault is interpreted to record erosion related to reverse fault movement and rock uplift. The presence of thrust faults in southeast Arizona raises the possibility of a latest Cretaceous−Eocene retroarc orogenic wedge that linked the Sevier and Mexican thrust belts to the north and south, respectively. Basement-involved deformation does not rule out the presence of a retroarc wedge, and many Cordilleran orogenic systems include basement-involved thrusting.
more »
« less
- PAR ID:
- 10560181
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geological Society of America Bulletin
- ISSN:
- 0016-7606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Low‐temperature thermochronometric data can reveal the long‐term evolution of erosion, uplift, and thrusting in fold‐thrust belts. We present results from central Idaho and southwestern Montana, where the close spatial overlap of the Sevier fold‐thrust belt and Laramide style, basement‐involved foreland uplifts signify a complex region with an unresolved, long‐term tectono‐thermal history. Inverse QTQt thermal history modeling of new zircon (U‐Th)/He (ZHe,n = 106), and apatite (U‐Th)/He dates (AHe,n = 43) collected from hanging walls of major thrusts systems along a central Idaho to southwestern Montana transect, and apatite fission track results from 6 basement samples, reveal regional thermal and spatial trends related to Sevier and Laramide orogenesis. Inverse modeling of foreland basement uplift samples suggest Phanerozoic exhumation initiated as early as ∼80 Ma and continued through the early Paleogene. Inverse modeling of interior Idaho fold‐thrust belt ZHe samples documents Early Cretaceous cooling at ∼125 Ma in the Lost River Range (western transect), and a younger cooling episode in the Lemhi Arch region (mid‐transect) at ∼90–80 Ma through the late Paleogene. This cooling in the Lemhi Arch temporally overlaps with cooling in southwestern Montana's basement‐cored uplifts, which we interpret as roughly synchronous exhumation related to contractional tectonics and post‐orogenic collapse. These data and models, integrated with independent timing constraints from foreland basin strata and previously published thermochronometric results, suggests that middle Cretaceous deformation of southwestern Montana's basement‐cored uplifts was low magnitude and preceded tectonism along the classic Arizona‐Wyoming Laramide “corridor.” In contrast, Late Cretaceous and Paleogene thrust‐related exhumation was more significant and largely complete by the Eocene. The basement‐involved deformation was contemporaneous with and younger than along‐strike Sevier belt thrusting in central Idaho.more » « less
-
The beginning of the Laramide orogeny is a pivotal time in the geological development of the western United States, but the driving mechanism responsible for mountain building, basin formation and ore mineralization is controversial. Most prominent models suggest this event was caused by the collision of an oceanic plateau with the Southern California Batholith sector of western North America at ca. 88 Ma which caused the angle of subduction beneath the continent to shallow. This subhorizontal (flat) subduction is thought to have led to shut-down of the arc, crustal cooling, and the formation of deep, basement-involved thrust faults that penetrated far into the continental interior. In contrast to these predictions, we show that the Southern California Batholith experienced a magmatic surge from 90 to 70 Ma, the lower crust was hot (835-750°C) and partially molten, and cooling occurred after 75 Ma. These data contradict plateau underthrusting as the driving mechanism for early Laramide deformation at 90-80 Ma; therefore, the Laramide orogeny cannot have been initiated by flat-slab subduction. We propose that the Laramide orogeny is best explained as a two-stage orogeny consisting of: 1) an arc magmatic ‘flare-up’ phase associated with sinistral-reverse ductile shearing in the Southern California Batholith from at 90-75 Ma and coeval dextral-transpression north of the Garlock fault, and 2) a widespread mountain building phase in the Laramide foreland belt from 75-50 Ma. Only that latter phase is linked to flat-slab subduction beneath the Southern California Batholith.more » « less
-
In the eastern San Gabriel Mountains, located north of Los Angeles, California, the late Cenozoic Cucamonga thrust has uplifted and exposed the lower crustal root of the Mesozoic Southern California Batholith. We use structural data and U-Pb zircon analyses from these exposures to document changes in the style of intra-arc deformation in the batholith as the Laramide Orogeny began during the Late Cretaceous (at or after ~90 Ma). At the base of the uplifted section, a 4 km-thick package of metasedimentary rock records the intrusion of amphibolite, charnokite and other dikes of probable Jurassic to Early Cretaceous age. The oldest gneissic fabrics (S1, S2) in these rocks record Early Cretaceous partial melting, granulite-facies metamorphism, and top-to-the-S and -SE (present day reference frame) reverse motion on surfaces that dip moderately to the N and NW. These structures define a D1/D2 thrust system that formed on the trench side of the arc and was active during the Early Cretaceous. From 89-77 Ma this thrust system was reactivated by oblique-slip shear zones (D3) that record sinistral-reverse displacements on N- and NW-dipping surfaces. The timing of deformation in these latter shear zones is indicated by the age of 90-85 Ma syn-kinematic intrusions of the Tonalite of San Sevaine Lookout. After emplacement of the tonalite, the lower crustal section was deformed by a series of S-vergent, overturned folds. The emplacement of granodioritic dikes into the axial planes of some of these folds suggests that they formed during the latest stages of D3 transpression and tonalite emplacement. Superimposed on all these structures are a series of ductile-to-brittle thrust faults and folds that appear to be related to formation of the late Cenozoic Cucamonga thrust fault at the southern edge of the San Gabriel mountains. These data show that the Southern California Batholith in the San Gabriel Mountains records a tectonic transition from Early Cretaceous reverse faulting and crustal imbrication on the trench side of the arc to Late Cretaceous transpression and oblique sinistral-reverse deformation during a magmatic flare-up from 89-77 Ma. Another major episode of shortening and crustal imbrication occurred during the late Cenozoic when the Cucamonga thrust uplifted the San Gabriel block.more » « less
-
Abstract Understanding the effects of flat slab subduction on mountain building, arc magmatism, and basin evolution is fundamental to convergent‐margin tectonics, with implications for potential feedbacks among geodynamic, magmatic, and surface processes. New stratigraphic and geochronological constraints on Cenozoic sedimentation and magmatism in the southern Central Andes of Argentina (31°S) reveal shifts in volcanism, foreland/hinterland basin development, sediment accumulation, and provenance as the retroarc region was structurally partitioned during slab flattening. Detrital zircon U‐Pb age distributions from the western (Calingasta basin), central (Talacasto and Albarracín basins), and eastern (Bermejo foreland basin) segments of the retroarc basin system preserve syndepositional volcanism and orogenic unroofing of multiple tectonic provinces. Initial shortening‐related exhumation of the Principal Cordillera at 24–17 Ma was recorded by the accumulation of distal eolian deposits bearing Oligocene–Eocene zircons from the Andean magmatic arc. The Calingasta basin chronicled volcanism and basement shortening in the Frontal Cordillera at ~17–11 Ma, as marked by an upward coarsening succession of fluvial to alluvial fan deposits with a sustained zircon U‐Pb age component that matches pervasive Permian‐Triassic bedrock in the hinterland. An ~450 km eastward inboard sweep of volcanism at 11 Ma coincided with the inception of flat slab subduction, and subsequent thin‐skinned shortening in the Precordillera fold‐thrust belt that exhumed wedge‐top deposits and induced cratonward (eastward) advance of flexural subsidence into the Bermejo foreland basin. This foreland basin was structurally partitioned as basement uplifts of the Sierras Pampeanas transformed a fluvial megafan sediment routing network into smaller isolated alluvial fan systems fed by adjacent basement blocks.more » « less
