Geology topics

David W. Farris

Publications and source records attributed to David W. Farris.

4 recordsLinked to original sources

USGS2020

Selected geologic maps of the Kodiak batholith and other Paleocene intrusive rocks, Kodiak Island, Alaska

Kodiak Island in southern Alaska is one of the premier examples globally for the study of forearc magmatism. This location contains two Paleocene intrusive belts that formed due to the subduction of a migrating spreading ridge and slab-window: the Kodiak batholith and the trenchward magmatic belt. These magmatic rocks are part of the Sanak-Baranof belt, which extends for greater than 2,100 km along the southern Alaskan margin and vary in age from 61 to 50 Ma west to east. Trenchward-belt rocks, with an 40 Ar/ 39 Ar age of 60.2±0.9 Ma, intrude into the Paleocene Ghost Rocks Formation and are composed of granitoids, basaltic dikes, and small gabbroic plutons that lie along or southward of the Kalsin Bay Fault. Such intrusions were emplaced at shallow levels and have abundant evidence of incomplete intermingling of basaltic and granitic magmas. These textures indicate trenchward-belt intrusions that froze before complete assimilation, leaving behind features such as abundant locally stoped blocks, gabbroic pods within granitic intrusions, and microstructural evidence such as strongly embayed olivine and pyroxene phenocrysts in granitoid bodies. The Kodiak batholith and satellite intrusions extend for over 110 km along the axis of Kodiak Island and vary in width from 2 to 6 km. These intrude into the Late Cretaceous Kodiak Formation. U-Pb ages on zircon from the intrusions range from 59.2±0.2 Ma in the southwest to 58.4±0.2 Ma near its northwest tip. We interpret these ages as tracking the location of a migrating triple junction and associated slab-window. The batholith is composed of granite and granodiorite, with lesser amounts of tonalite and diorite. The center of the Kodiak batholith contains high-inclusion zones with abundant residual host rock fragments that were carried up from 5–10 km below current exposure levels. These high-inclusion zones contain biotite aggregates, pure quartz clots, and large xenocrysts of sillimanite, kyanite, andalusite, and garnet. This is a higher-pressure mineral assemblage than exists in the batholith metamorphic aureole. Gravity observations and modeling are consistent with the high-inclusion zones extending downward for 5–10 km. The Kodiak batholith results from a migrating triple junction and slab-window that led to high degrees of partial melting within the Kodiak accretionary prism.

Alaska↗

USGS2016

Formation of the Isthmus of Panama

The formation of the Isthmus of Panama stands as one of the greatest natural events of the Cenozoic, driving profound biotic transformations on land and in the oceans. Some recent studies suggest that the Isthmus formed many millions of years earlier than the widely recognized age of approximately 3 million years ago (Ma), a result that if true would revolutionize our understanding of environmental, ecological, and evolutionary change across the Americas. To bring clarity to the question of when the Isthmus of Panama formed, we provide an exhaustive review and reanalysis of geological, paleontological, and molecular records. These independent lines of evidence converge upon a cohesive narrative of gradually emerging land and constricting seaways, with formation of the Isthmus of Panama sensu stricto around 2.8 Ma. The evidence used to support an older isthmus is inconclusive, and we caution against the uncritical acceptance of an isthmus before the Pliocene.

Science Advances↗

USGS2009

The role of ridge subduction in determining the geochemistry and Nd–Sr–Pb isotopic evolution of the Kodiak batholith in southern Alaska

The Paleocene Kodiak batholith, part of the Sanak–Baranof belt of Tertiary near-trench intrusive rocks, forms an elongate body (~ 150 km long) that transects Kodiak Island from SW to NE. The batholith consists of three zones (Southern, Central, and Northern) of kyanite-, muscovite-, and garnet-bearing biotite tonalite and granodiorite and less abundant granite that intruded an accretionary prism (Kodiak Formation, and Ghost Rocks Formation). Small and likely coeval bodies (Northern, Western, and Eastern satellite groups) of quartz gabbro, diorite, tonalite, granodiorite, and leucogranite flank the batholith. The batholith is calc-alkalic, has an aluminum saturation index of > 1.1, FeO t /(FeO t + MgO) ~ 0.65 (at SiO 2 = 65 wt.%), and increases in SiO 2 (~ 61 wt.%–73 wt.%) and decreases in TiO 2 (~ 0.9 wt.%–0.3 wt.%) from SW to NE. As a group, the granitic rocks have light REE-enriched chondrite-normalized patterns with small or no negative Eu anomalies, primitive mantle-normalized negative anomalies for Nb and Ti, and positive anomalies for Pb. Small to large negative anomalies for Th are also distinctive. The quartz gabbros and diorites are generally characterized by generally flat to light REE chondrite-normalized patterns (no Eu anomalies), and mantle-normalized negative anomalies for Nb, Ti, and P. Pb isotopic compositions ( 206 Pb/ 204 Pb = 18.850–18.960; 207 Pb/ 204 Pb = 15.575–15.694; 208 Pb/ 204 Pb = 38.350–39.039) are intermediate between depleted mantle and average continental crust. The Southern zone and a portion of the Central zone are characterized by negative ε Nd values of − 3.7 to − 0.3 and T DM ages ranging from ~ 838 Ma to 1011 Ma. Other granitic rocks from the Central and Northern zones have higher ε Nd values of − 0.4 to + 4.7 and younger T DM ages of ~ 450 to 797 Ma. Granitic and mafic plutons from the Eastern satellites show a wide range of ε Nd values of − 2.7 to + 6.4, and T DM ages from 204 Ma to 2124 Ma. 87 Sr/ 86 Sr values of the Southern and Central zones overlap and tend to be slightly more radiogenic ( 87 Sr/ 86 Sr > 0.70426) than the Northern zone ( 87 Sr/ 86 Sr < 0.70472). 206 Pb/ 204 Pb values increase slightly from the Southern and Central zones toward the Northern zone. There is no clear correlation of the major or trace elements with ε Nd , Pb or Sr isotopic values. Kodiak Formation and the Ghost Rocks Formation overlap the isotopic compositions (e.g., 206 Pb/ 204 Pb = 18.978 to 19.165, 87 Sr/ 86 Sr of 0.705715 to 0.707118, and ε Nd of − 6.7 to − 1.5 at 59 Ma) and T DM values (959 to 1489 Ma) of the batholith. Production of large volumes of granitic rocks in the Sanak–Baranof belt, and particularly on Kodiak Island, reflects a sequence of processes that includes underplating of mantle-derived mafic (possibly from the mantle wedge) and intermediate rocks under the accretionary flysch, interlayering of mantle-derived and flyschoid rocks, and partial melting of the mixed lithologic assemblages. Limited degrees of fractional crystallization or assimilation and fractional crystallization influenced compositions of the granitic rocks. The contribution of mantle-derived rocks that resided in the accretionary prism for only a short period of time prior to partial melting likely exceeds 40% (up to 80%). The balance (60 to 20%) is from a recently recycled crustal component represented by the Kodiak Formation. This type of progressive intracrustal melting from mixed sources controlled the geochemical character of the batholith and is most consistent with the hypothesis that the granitic rocks are associated with a slab-window produced by collision of a spreading oceanic center and a subduction zone and migration beneath the accretionary prism.

Gulf of Alaska↗

USGS2006

Emplacement of the Kodiak batholith and slab-window migration

The Kodiak batholith is one of the largest, most elongate intrusive bodies in the forearc Sanak-Baranof plutonic belt located in southern Alaska. This belt is interpreted to have formed during the subduction of an oceanic spreading center and the associated migration of a slab window. Individual plutons of the Kodiak batholith track the location and evolution of the underlying slab window. Six U/Pb zircon ages from the axis of the batholith exhibit a northeastward-decreasing age progression of 59.2 ± 0.2 Ma at the southwest end to 58.4 ± 0.2 Ma at the northeast tip. The trench-parallel rate of age progression is within error of the average slab-window migration rate for the entire Sanak-Baranof belt (~19 cm/yr). Structural relationships, U/Pb ages, and a model of new gravity data indicate that magma from the Kodiak batholith ascended 5-10 km as a northeastward-younging series of 1-8-km-diameter viscoelastic diapirs. Individual plutons ascended by multiple emplacement mechanisms including downward flow, collapse of wall rock, stoping, and diking. Stokes flow xenolith calculations suggest ascent rates of 5-100 m/yr and an effective magmatic viscosity of 107-108 Pa s. Pre-existing structural or lithologic heterogeneities did not dominantly control the location of the main batholith. Instead, its location was determined by migration of the slab window at depth.

Geological Society of America Bulletin↗