Evolution of the Jura-Cretaceous North American Cordilleran margin: Insights from detrital-zircon U-Pb and Hf isotopes of sedimentary units of the North Cascades Range, Washington

Abstract

The U-Pb age and Hf-isotope composition of detrital zircons from Jurassic to Upper Cretaceous sedimentary rocks adjacent to the southern North Cascades-Coast Plutonic Complex continental magmatic arc document shifting provenance, the tectonic evolution of the arc system, and translation along the continental margin. Systematic changes in the detrital-zircon data provide insight that the western margin of North America evolved from: marginal basins adjacent to continent-fringing oceanic arcs (ca. 160-140 Ma); forearc basins adjacent to mid-Cretaceous (ca. 120-90 Ma) Andean-type continental arcs; and addition of a cratonic source to forearc and accretionary wedge units to Cordilleran arc systems in the mid-Late Cretaceous (ca. 85 Ma). Jurassic Methow terrane, Nooksack Formation, and western mélange belt units dominantly contain detrital zircons derived from accreted oceanic terranes, whereas Lower Cretaceous strata from the same units have age peaks that correspond to known pulses of magmatism in Cordilleran continental magmatic arc systems. The age peaks and Hf-isotope signature of the Jurassic and Lower Cretaceous strata are comparable to multiple sources exposed along the margin. In contrast, the Upper Cretaceous western mélange belt has distinct Precambrian zircon populations and unradiogenic Late Cretaceous zircons that are more similar to southwestern than northwestern Laurentian sources. Statistical comparisons confirm provenance similarities between rocks of the North Cascades and those 700-2000 km to the south and, thus, support marginparallel translation from as far as the latitude of southern California.

Transfer of Metasupracrustal Rocks to Midcrustal Depths in the North Cascades Continental Magmatic Arc, Skagit Gneiss Complex, Washington

“…Plot of age and Hf‐isotope data for major Cordilleran arc segments modified from Sauer et al (). Age data sources: Coast Plutonic Complex, Gehrels et al (); North Cascades, Miller, Paterson, et al, ; Idaho Batholith, Gaschnig et al (); Sierra Nevada, Chapman et al (); Mojave, Needy et al ().…”

“…These results are between values expected for Late Cretaceous arc magmas and reworked Proterozoic crust. There are two potential localities where Late Cretaceous plutons intrude Proterozoic crust: southwestern Laurentia, where “anorogenic granites” intruded Proterozoic basement in the Mojave desert, and northwestern Laurentia, where the southern Idaho batholith intruded the Lemhi Subbasin of the Belt Supergroup (Garver & Davidson, ; Gaschnig et al, ; Sauer et al, ) (Figure ). For the Proterozoic zircons, the Hf‐isotope data from both northwestern and southwestern Laurentia cover a similar range and do not differentiate between the two sources (cf.…”

“…It is interpreted to have formed as a forearc basin (Barksdale, ; Coates, ) but reached a backarc position with respect to the North Cascades by circa 91 Ma (Miller, ). The detrital zircon signature of the Jurassic Methow section is characterized by unimodal peaks between 170 and 150 Ma (Sauer et al, ), whereas the Cretaceous strata have distinctive bimodal age peaks at circa 115 and 160 Ma with very minimal (<1%) input from cratonic sources (DeGraaff‐Surpless et al, ; Surpless et al, ).…”

“…The WMB is dominantly composed of a weakly metamorphosed argillite matrix with blocks of marble, metagabbro, arkosic sandstone, metadiabase, and chert (Tabor et al, , ). Sandstones from the WMB have a mix of Mesozoic and Proterozoic dates (Brown, ; Dragovich et al, ; Sauer et al, ), with the relative abundance of Proterozoic detrital zircons increasing with younger maximum depositional ages (Sauer et al, ).…”

“…The presence of Archean zircons in the Nanaimo Group has been previously used to argue for (Housen & Beck, ) and against (i.e., Mahoney et al, ) large‐scale margin‐parallel translation. In addition, recent studies link the presence of two distinct Proterozoic age peaks from forearc and accretionary wedge rocks from Washington to Alaska (i.e., the WMB, Yakutat Terrane, and Nanaimo Group) to sedimentary sources located in Southern California and southwestern Laurentia (Garver & Davidson, ; Matthews et al, ; Sauer et al, ) or southern Idaho and northwestern Laurentia (Dumitru et al, ). The same pair of Proterozoic age peaks and similar detrital zircon signatures are also observed in coeval sedimentary units in the accretionary wedge (Franciscan Complex), forearc (Great Valley), and underplated trench sediments (Pelona‐Orocopia‐Rand schist) near the southern end of the Sierra Nevada (Chapman et al, ; Dumitru et al, ; Jacobson et al, ; Sharman et al, ).…”