Mechanisms and timing of exhumation of collision‐related metamorphic rocks, southern Brooks Range, Alaska: Insights from ⁴⁰Ar/³⁹Ar thermochronology

Abstract

[1] New 40 Ar/ 39 Ar thermochronologic data document the spatial distribution of cooling ages and rates across a metamorphic culmination in the south central Brooks Range, providing important constraints on the timing and processes responsible for exhumation of deep-seated metamorphic rocks. The data indicate widespread episodic exhumation-related cooling, with short-lived events in the mid-Albian to early Cenomanian and Paleocene to Eocene. Hornblende ages indicate that cooling from the metamorphic peak began at 105 -103 Ma across the epidoteamphibolite facies core of the culmination. Mica ages, however, increase southward from $90 Ma to $100 Ma, indicating that average cooling rates increased southward. Thermal modeling suggests that Albian cooling is the result of a short-lived exhumation event that ended by the early Cenomanian, and that exhumation rates were between $1 and 8 mm/yr, with higher rates toward the south. The southward increase in cooling/exhumation rates is attributed to a southward increase in the component of tectonic exhumation by normal faulting on the south flank of the range. Rapid erosion, which is recorded by thick molasse deposits in flanking basins that are coeval with rapid cooling, may have been aided by surface uplift in the footwall of these faults. The northward younging of mica ages may not be explained by a simple core complex model involving metamorphic rocks being progressively dragged out from beneath an extensional fault. The overall spatial distribution of metamorphic zones and cooling ages/rates is best explained by a two-step process: (1) a brief episode of extensional faulting producing a regional northward tilt with superimposed doming during the AlbianCenomanian and (2) relative uplift of the core during renewed Tertiary contraction, which enhanced the domal geometry. Diffusion-domain modeling of K-feldspar data indicates a period of little or no cooling that spanned much of the Late Cretaceous followed by rapid increase in cooling rates in the Paleocene to Eocene. Increased Tertiary cooling rates are attributed to renewed contraction leading to surface uplift and erosion.

Use of multiple oxygen isotope proxies for elucidating Arctic Cretaceous palaeo-hydrology

“…Precipitation estimates for the PCF ranged between 500 to 1500mm/yr (Spicer & Parrish 1990). Globally, there is evidence for two warming events in the Maastrichtian (Li & Keller 1998;Barrera & Savin 1999;Nordt et al 2003;Wilf et al 2003) that may overlap with the timing of the PCF deposition. The older of these two warming events occurred between 69.5 and 68.5 Ma and is associated with increases in atmospheric pCO 2 concentrations of 1000-1200 ppmv (Nordt et al 2003).…”

“…The palaeo-latitude of Cretaceous rocks in this area is thought to be between 828 and 858N (Witte et al 1987;Besse & Courtillot 1991;Rich et al 2002;. Ash layers in the vicinity of the Liscomb Bonebed returned weighted mean K - Ar and 40 Ar/ 39 Ar ages of 69.1 + 0.3 Ma (Conrad et al 1990) and 69.2 + 0.5 Ma (Flaig 2010), which is consistent with an Early Maastrichtian age (Maastrichtian ¼ 70.6-65.5 Ma, ICS 2010 Geologic Time Scale) suggested by palynological data (Frederiksen 1990;Fiorillo et al 2010a, b;Flaig et al 2013).…”

Greenschist Facies Metamorphic Zircon Overgrowths as a Constraint on Exhumation of the Brooks Range Metamorphic Core, Alaska

“…Just north of the Schist Belt lies the Central Belt, which contains rocks of similar age and affinity but which are less penetratively deformed and which appear to have experienced lower pressures during metamorphism (Till et al, ; Figure ). The boundary between the Schist and Central Belt is poorly defined in most places and has been interpreted as a zone of top‐to‐the‐north thrust faulting (e.g., Moore et al, ; Patrick et al, ; Till, ; Till et al, ; Till & Snee, ), a zone of top‐to‐the‐south back thrusting (e.g., Vogl et al, ), or as a strain gradient north of which the younger extensional foliation is no longer dominant (Christiansen & Snee, ; Law et al, ; Little et al, ). To its south, the Schist Belt is structurally overlain by less resistant, lower grade rocks of the Greywacke‐Phyllite belt , which are in turn structurally overlain by normal fault‐bound fragments of the Devonian‐Jurassic Angayucham oceanic terrane and/or Cretaceous Yukon‐Koyukuk Basin sediments (e.g., Box, ; Carlson, ; Gottschalk & Oldow, ; Miller & Hudson, ; Moore et al, , ; Oldow et al, ; Patton Jr. et al, , ; Figure ).…”

“…These yield mostly Cretaceous ages (~140–100 Ma) but are often characterized by complex 40 Ar/ 39 Ar step‐heating spectra. Such complexity may reflect variable thermally induced diffusive argon loss, a mixture of mica ages or phases, and/or inclusion of excess 40 Ar, thereby rendering geological interpretations uncertain and leading to a wide diversity of tectonic models (Blythe et al, , ; Box, ; Brosge & Reiser, ; Christiansen & Snee, ; Dillon et al, ; Dinklage, ; Gottschalk, ; Gottschalk & Oldow, ; Gottschalk & Snee, ; Little et al, ; Nelson & Grybeck, ; Patton Jr. et al, ; Silberman et al, ; Till, , ; Till et al, ; Till & Snee, ; Toro et al, , ; Turner et al, ; Vogl, ; Vogl et al, ). The difficulty interpreting 40 Ar/ 39 Ar white mica ages with respect to the age of fabric development has added to the disagreement about the metamorphic and structural evolution of the Brooks Range and thus to the relative overlap of structural events occurring in the hinterland compared to those documented at higher structural levels.…”

“…U‐Pb ages of igneous rocks within the highest ultramafic allochthons and the fossil and detrital zircon ages of synorogenic deposits provide fairly good constraints on the timing of thrust emplacement of the arc terrane onto the continental margin (e.g., Moore et al, , and references therein). In contrast, there are very few clear constraints on the timing of deformational and metamorphic events in the hinterland of the fold and thrust belt, some of which have been linked to either the later stages of collision and/or to postcollisional extension (e.g., Little et al, ; Miller & Hudson, ; Till, ; Vogl et al, ). Disagreements in interpretation have arisen due to the general difficulty of dating multiple greenschist facies events (e.g., Moore & Box, ), which is traditionally limited to using thermochronological data sets (e.g., 40 Ar/ 39 Ar mica ages).…”