Geology topics

Research about Talkeetna Mountains

Source-linked reports with geographic coverage including Talkeetna Mountains.

5 recordsLinked to original sources

USGS2012

Early Tertiary exhumation of the flank of a forearc basin, southwest Talkeetna Mountains, Alaska

New geochronologic and thermochronologic data from rocks near Hatcher Pass, southwest Talkeetna Mountains, Alaska, record earliest Paleocene erosional and structural exhumation on the flank of the active Cook Inlet forearc basin. Cretaceous plutons shed sediments to the south, forming the Paleocene Arkose Ridge Formation. A Paleocene(?)-Eocene detachment fault juxtaposed ~60 Ma metamorphic rocks with the base of the Arkose Ridge Formation. U-Pb (analyzed by Sensitive High Resolution Ion Micro Probe Reverse Geometry (SHRIMP-RG)) zircon ages of the Cretaceous plutons, more diverse than previously documented, are 90.3±0.3 (previously considered a Jurassic unit), 79.1±1.0, 76.1±0.9, 75.8±0.7, 72.5±0.4, 71.9±0.3, 70.5±0.2, and 67.3±0.2 Ma. The cooling of these plutons occurred between 72 and 66 Ma (zircon fission track (FT) closure ~225°C). 40 Ar/ 39 Ar analyses of hornblende, white mica, and biotite fall into this range (Harlan and others, 2003). New apatite FT data collected on a west-to-east transect reveal sequential exhumation of fault blocks at 62.8±2.9, 54±2.5, 52.6±2.8, and 44.4±2.2 Ma. Plutonic clasts accumulated in the Paleocene Arkose Ridge Formation to the south. Detrital zircon (DZ) ages from the formation reflect this provenance: a new sample yielded one grain at 61 Ma, a dominant peak at 76 Ma, and minor peaks at 70, 80, 88, and 92 Ma. The oldest zircon is 181 Ma. Our apatite FT ages range from 35.1 to 50.9 Ma. Greenschist facies rocks now sit structurally between the plutonic rocks and the Arkose Ridge Formation. They are separated from plutonic rocks by the vertical Hatcher Pass fault and from the sedimentary rocks by a detachment fault. Ar cooling ages (Harlan and others, 2003) and new zircon FT ages for these rocks are concordant at 61-57 Ma, synchronous with deposition of the Arkose Ridge Formation. A cooling age of ~46 Ma came from one apatite FT sample. The metamorphic protolith (previously considered Jurassic) was deposited at or after 75 Ma based on new DZ data. The probability curve has a major peak from 76 to 102 Ma, minor peaks at 186, 197, 213, 303, 346, and 1,828, and two discordant grains at ~2,700 Ma. This is similar to DZ populations in the Valdez Group. The short period of time between deposition, metamorphism, and exhumation are consistent with metamorphism in a subduction-zone setting. Ductile and brittle structures in the metamorphic rocks are consistent with exhumation in a transtensional setting.

Alaska↗

USGS2011

Lithogeochemistry of mineralized and altered rock samples from the northern Talkeetna Mountains, south-central Alaska

Mineralized and altered rock samples collected from the northern Talkeetna Mountains, Alaska, were analyzed by two different inductively coupled plasma atomic-emission spectrometry (ICP-AES) methods for as many as 44 elements; by fire assay and either direct-coupled plasma (DCP) or atomic absorption spectrophotometry (AAS) for gold (Au); by cold vapor atomic absorption (CVAA) for mercury (Hg); and by irradiated neutron activation analysis (INAA) for tungsten (W). The analytical results showed that some samples contain high values of multiple elements and may be potential indicators of hydrothermal mineralization in the area.

Alaska↗

USGS2004

A kinematic model for the southern Alaska orocline based on regional fault patterns

Among the most prominent physiographic features of southern Alaska are a series of nested arcuate lineations, including the Denali fault, that parallel the concave-southward southern coastline of the state. These features are generally interpreted as major dextral shear zones that formed in the Late Cretaceous to early Tertiary in response to stresses imposed on the western edge of North America by transcurrent motion and oblique subduction along the North American margin. South-central Alaska consists of a collage of Paleozoic and Mesozoic tectonostratigraphic terranes and overlap assemblages. Following accretion to the continent, these terranes were transported northward along its margin along strike-slip faults such as the ancestral Denali fault that formed by oblique subduction. The terranes would have arrived at about their present position by Eocene time. It is commonly held that southwestern Alaska rotated into its present configuration by the middle Eocene, in response to impingement of northeast Asia against western Alaska, to form the southern Alaska orocline. Subsequent to this rotation during the middle and late Tertiary, southern Alaska terranes were presumably transported through the Alaska orocline by continued dextral movement along faults on the east limb of the orocline, such as the Denali and Tintina. Both initial bending of the crust to form the orocline and subsequent transport of crust through the orocline would result in significant crustal shortening within the bend. A model is suggested herein whereby shortening is accommodated by a system of secondary, northeast-trending thrust faults. The distribution of these faults shows a consistent pattern within the bend: the faults appear to splay off at or near the major dextral shear zones and generally occur west of the orocline’s axis. That these faults occur where deformation would be greatest to crust driven through the bend suggests that the faults are directly related to crustal dynamics within the bend. If this model is correct, one may infer the sense and timing of motion along many faults that otherwise lack or have limited documented histories. The interaction of strike-slip and thrust faults suggested by the model is reflected in the rupture sequence of the November 3, 2002, M7.9 Denali earthquake, which involved both initiation of slip along a previously unknown east-northeast–trending thrust fault and subsequent strike-slip motion along the McKinley strand of the east-west–trending Denali fault. This event is likely due, in part, to stresses imposed by accretion of the Yakutat terrane that is presently working its way into the bend of the orocline and deforming as a result of collision. Faulting along the western margin of the Yakutat terrane resembles that seen in central Alaska within the hinge of the bend. As such, it likely represents a present-day analog for crustal deformation associated with the orocline and may therefore provide clues to earlier stages of crustal deformation in central Alaska.

Alaska↗

USGS2003

Density and magnetic susceptibility values for rocks in the Talkeetna Mountains and adjacent region, south-central Alaska

This report presents a compilation and statistical analysis of 306 density and 706 magnetic susceptibility measurements of rocks from south-central Alaska that were collected by U.S. Geological Survey (USGS) and Alaska Division of Geological and Geophysical Surveys (ADGGS) scientists between the summers of 1999 and 2002. This work is a product of the USGS Talkeetna Mountains Transect Project and was supported by USGS projects in the Talkeetna Mountains and Iron Creek region, and by Bureau of Land Management (BLM) projects in the Delta River Mining District that aim to characterize the subsurface structures of the region. These data were collected to constrain potential field models (i.e., gravity and magnetic) that are combined with other geophysical methods to identify and model major faults, terrane boundaries, and potential mineral resources of the study area. Because gravity and magnetic field anomalies reflect variations in the density and magnetic susceptibility of the underlying lithology, these rock properties are essential components of potential field modeling. In general, the average grain density of rocks in the study region increases from sedimentary, felsic, and intermediate igneous rocks, to mafic igneous and metamorphic rocks. Magnetic susceptibility measurements performed on rock outcrops and hand samples from the study area also reveal lower magnetic susceptibilities for sedimentary and felsic intrusive rocks, moderate susceptibility values for metamorphic, felsic extrusive, and intermediate igneous rocks, and higher susceptibility values for mafic igneous rocks. The density and magnetic properties of rocks in the study area are generally consistent with general trends expected for certain rock types.

Alaska↗

USGS2003

Principal facts for 408 gravity stations in the vicinity of the Talkeetna Mountains, south-central Alaska

Gravity data were collected between 1999 and 2002 along transects in the Talkeetna Mountains of south-central Alaska as part of a geological and geophysical study of the framework geology of the region. The study area lies between 61° 30’ and 63° 45’ N. latitude and 145° and 151° W. longitude. This data set includes 408 gravity stations. These data, combined with the pre-existing 3,286 stations, brings the total data in this area to 3,694 gravity stations. Principal facts for the 408 new gravity stations and the 15 gravity base stations used for control are listed in this report. During the summer of 1999, a gravity survey was conducted in the western Talkeetna Mountains. Measurements at 55 gravity stations were made. One gravity base station was used for control for this survey. This base station, STEP, is located at the Stephan Lake Lodge on Stephan Lake. The observed gravity of this station was calculated based on an indirect tie to base station ANCL in Anchorage. The temporary base used to tie between STEP and ANCL was REGL in Anchorage. During the summer of 2000, a gravity survey was conducted in the western Talkeetna Mountains. Measurements at 56 gravity stations were made. One gravity base station was used for control for this survey. This base station, GRHS, is located at the Gracious House Lodge on the Denali Highway. The observed gravity of this station was calculated based on multiple ties to base stations D87, and D57 along the Denali Highway. During the summer of 2001, a gravity survey was conducted in the western Talkeetna Mountains. Measurements at 90 gravity stations were made. One gravity base station was used for control for this survey. This base station, HLML, is located at the High Lake Lodge. The observed gravity of this station was calculated based on multiple ties to base stations ANCU in Anchorage, PALH in Palmer, WASA in Wasilla, and TLKM in Talkeetna. Also during the summer of 2001, a gravity survey was conducted in the vicinity of Tangle Lakes. Measurements at 86 gravity stations were made. The Tangle Lakes area is located about 25 km west of Paxson and north of the Denali Highway. One gravity base station was used for control for this survey. This base station, TLIN, is located at the Tangle Lakes Inn. The observed gravity of this station was calculated based on multiple ties to base stations ANCU in Anchorage, PALH in Palmer, BD27 in Gulkana, B-07 on the Richardson Highway, and base stations D42, and D57 along the Denali Highway. During the summer of 2002, measurements at an additional 107 gravity stations were made in the vicinity of Tangle Lakes. Base station TLIN at the Tangle Lakes Inn was again used for control. Additional ties to base stations ANCU and B-07 were made.

Alaska↗