Style, controls, and timing of fold-and-thrust deformation of the Jago stock, northeastern Brooks Range, Alaska

Abstract

Involvement of the Devonian Jago stock in Cenozoic fold-and-thrust deformation of the northeastern Brooks Range illustrates the influence of a relatively small, isolated crystalline body on the mechanical stratigraphy and subsequent deformational behavior of an otherwise layered sedimentary package. The small size of the stock allowed it and the structurally coupled overlying Mississippian Kekiktuk Conglomerate to deform nonpenetratively as a horse in a regional duplex, in contrast to the semiductile behavior of the nearby but much larger Okpilak batholith. Shear was localized in the upper part of the stock and the conglomerate due to partial detachment of the overlying Carboniferous Lisburne Group. North-vergent thrust-related folds formed in the mechanically layered Lisburne Group carbonates instead of the symmetrical, unfaulted detachment folds more typical of the region because an underlying regional detachment horizon in the Mississippian Kayak Shale is depositionally absent over the stock. Unusually competent contact-metamorphosed pre-Mississippian metasedimentary rocks were thrust over the stock and its cover because a ramp formed at the edge of the stock and cut upsection through the Lisburne Group due to the absence of Kayak Shale. A 40Ar/39Ar age of foliated white mica indicates thrusting of the stock by 61 Ma; fission-track ages indicate cooling at ~44 and ~28 Ma. These ages indicate a cooling history that implies ~11 km of unroofing since ~61 Ma, only ~1.5 km of which can be explained by the inferred duplex structure. The remaining ~9.5 km of unroofing is most likely due to subduplex structural thickening above a deep regional detachment.

Cenozoic Exhumation History of the Eastern Margin of the Northern Canadian Cordillera

“…Our AHe results indicate another exhumation phase occurred in the northern Richardson Mountains during late Eocene-early Oligocene time (40-30 Ma). These findings are generally consistent with AFT and ZFT data from the northeastern Brooks Range that indicate out-of-sequence deformation during discrete phases at ∼45, ∼35, and ∼25 Ma (Murphy et al, 1994;O'Sullivan, 1996;O'Sullivan & Wallace, 2002;O'Sullivan et al, 1995Peapples et al, 1997;Potter et al, 2004). Deformation at ∼45 and ∼25 Ma is evident across much of the Brooks Range and North Slope (Craddock et al, 2018;Moore et al, 2004;O'Sullivan, 1996;O'Sullivan & Wallace, 2002;O'Sullivan, Moore, et al, 1998), whereas deformation at ∼35 Ma is recognized only in the northeastern Brooks Range, primarily along the western and northwestern mountain fronts (O'Sullivan, 1992;O'Sullivan & Decker, 1990;O'Sullivan et al, 1993;O'Sullivan, Wallace, et al, 1998).…”

Late Cretaceous‐Cenozoic Exhumation of the Western Brooks Range, Alaska, Revealed From Apatite and Zircon Fission Track Data

“…exhumation of the east central Brooks Range hinterland and much of the frontal part of the orogen east of 156°W is recorded by AFT and ZFT ages (Figure 1; Blythe et al, 1996;Green & Duddy, 2010;Moore et al, 2004;O'Sullivan, 1996;O'Sullivan et al, 1993Parris et al, 2003;Peapples et al, 1997). The reactivation also apparently extended into the northern part of the foreland, north of the limit of observed folding and thrusting, over a broad portion of coastal, northwestern Alaska (Houseknecht et al, 2011).…”

“…Recognition of the superimposed, Cenozoic contraction was facilitated by thermochronologic analysis of the eastern (e.g., Blythe et al, 1996Blythe et al, , 1997O'Sullivan, 1996;O'Sullivan et al, 1993O'Sullivan et al, , 1995O'Sullivan et al, , 1998aO'Sullivan & Lane, 1997;Parris et al, 2003;Peapples et al, 1997) and central (Cole et al, 1997;Moore et al, 2004) part of the Brooks Range. Broadly, thermochronologic studies reveal widespread exhumation of the range and the southern part of the Jura-Cretaceous foreland that began at~60 Ma (Blythe et al, 1996;Green & Duddy, 2010;Moore et al, 2004;O'Sullivan, 1996;O'Sullivan et al, 1993Parris et al, 2003;Peapples et al, 1997) as well as additional episodes of exhumation that affected portions of the range at 46 Ma,~35 Ma, and~25 Ma (e.g., O'Sullivan et al, 1997). Whereas Jura-Cretaceous collisional orogenesis Tectonics RESEARCH ARTICLE involved hundreds of kilometers of shortening of the upper crust accommodated by the imbrication of thin-skinned allochthons consisting of oceanic rocks and Paleozoic and Mesozoic sedimentary cover of the southern Arctic Alaska terrane (Mayfield et al, 1988;Moore et al, 1994), Cenozoic deformation involved relatively less contraction, accommodated by steeply dipping structures that extended into pre-Mississippian basement rocks of the region (Fuis et al, 1995(Fuis et al, , 2008Levander et al, 1994;Moore et al, 2004).…”

Detrital geochronology of pre-Mississippian strata in the northeastern Brooks Range, Alaska: Insights into the tectonic evolution of northern Laurentia

“…In our cross section through the eastern portion of the NE Brooks Range (Fig. 4), we illustrate the Brookian style of deformation as a series of large imbricated fault-bend folds that culminate to a detachment between 8 and 15 km below the surface, which is derived from previous structural studies in the region (Hanks, 1989;Peapples et al, 1997;Cole et al, 1999;Moore, 1999;O'Sullivan and Wallace, 2002). The ramps of the fault-bend folds are often represented by major thrust faults that are as much as 100 km in length, including the Whale Mountain and Romanzof Mountain thrusts (WMT and RMT; Fig.…”

“…2). Apatite fission-track and 40 Ar/ 39 Ar thermochronological data indicate that many of these basement-involved thrusts underwent significant Cenozoic displacement (Hanks, 1993;O'Sullivan, 1994;Peapples et al, 1997;O'Sullivan and Wallace, 2002).…”