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#           Three-dimensional shape and structure of the Susitna basin, south-central Alaska, from geophysical data      

      August 1, 2020    

We use gravity, magnetic, seismic reflection, well, and outcrop data to determine the three-dimensional shape and structural features of south-central Alaska’s Susitna basin. This basin is located within the Aleutian-Alaskan convergent margin region and is expected to show effects of regional subduction zone processes. Aeromagnetic data, when filtered to highlight anomalies associated with sources within the upper few kilometers, show numerous linear northeast-trending highs and some linear north-trending highs. Comparisons to seismic reflection and well data show that these highs correspond to areas where late Paleocene to early Eocene volcanic layers have been locally uplifted due to folding and/or faulting. The combined magnetic and seismic reflection data suggest that the linear highs represent northeast-trending folds and north-striking faults. Several lines of evidence suggest that the northeast-trending folds formed during the middle Eocene to early Miocene and may have continued to be active in the Pliocene. The north-striking faults, which in some areas appear to cut the northeast-trending folds, show evidence of Neogene and probable modern movement. Gravity data facilitate estimates of the shape and depth of the basin. This was accomplished by separating the observed gravity anomaly into two components—one representing low-density sedimentary fill within the basin and one representing density heterogeneities within the underlying crystalline basement. We then used the basin anomaly, seismic reflection data, and well data to estimate the depth of the basin. Together, the magnetic, gravity, and reflection seismic analyses reveal an asymmetric basin comprising sedimentary rock over 4 km thick with steep, fault-bounded sides to the southwest, west, and north and a mostly gentle rise toward the east. Relations to the broader tectonic regime are suggested by fold axis orientations within the Susitna basin and neighboring Cook Inlet basin, which are roughly parallel to the easternmost part of the Alaska-Aleutian trench and associated Wadati-Benioff zone as it trends from northeast to north-northeast to northeast. An alignment between forearc basin folds and the subduction zone trench has been observed at other convergent margins, attributed to strain partitioning generated by regional rheologic variations that are associated with the subducting plate and arc magmatism. The asymmetric shape of the basin, especially its gentle rise to the east, may reflect uplift associated with flat-slab subduction of the Yakutat microplate, consistent with previous work that suggested Yakutat influence on the nearby Talkeetna Mountains and western Alaska Range. Yakutat subduction may also have contributed to Neogene and later reverse slip along north-striking faults within the Susitna basin.

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#### Anjana K Shah

##### Research Geophysicist

Geology, Geophysics, and Geochemistry Science Center

ashah@usgs.gov

303-236-1218

#### Jeffrey Phillips

##### Scientist Emeritus

Geology, Geophysics, and Geochemistry Science Center

jeff@usgs.gov

303-236-1206

#### Kristen Lewis, PhD

##### Supervisory Geophysicist/USGS Geomagnetism Program Group Leader

Geomagnetism Program

klewis@usgs.gov

303-273-8471

#### Richard G Stanley

##### Scientist Emeritus

Geology, Minerals, Energy, and Geophysics Science Center

rstanley@usgs.gov

650-329-4918

#### Peter J Haeussler, Ph.D.

##### Research Geologist

Alaska Science Center

pheuslr@usgs.gov

907-786-7447

####             Chris Potter (Former Employee)          

##### Research Geologist

Central Energy Resources Science Center

## Related Content

#### Anjana K Shah

##### Research Geophysicist

Geology, Geophysics, and Geochemistry Science Center

ashah@usgs.gov

303-236-1218

#### Jeffrey Phillips

##### Scientist Emeritus

Geology, Geophysics, and Geochemistry Science Center

jeff@usgs.gov

303-236-1206

#### Kristen Lewis, PhD

##### Supervisory Geophysicist/USGS Geomagnetism Program Group Leader

Geomagnetism Program

klewis@usgs.gov

303-273-8471

#### Richard G Stanley

##### Scientist Emeritus

Geology, Minerals, Energy, and Geophysics Science Center

rstanley@usgs.gov

650-329-4918

#### Peter J Haeussler, Ph.D.

##### Research Geologist

Alaska Science Center

pheuslr@usgs.gov

907-786-7447

####             Chris Potter (Former Employee)          

##### Research Geologist

Central Energy Resources Science Center

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#### Geology, Geophysics, and Geochemistry Science Center

West 6th Avenue and Kipling StreetDenver Federal Center Building 20Denver, CO80225United States

303-236-1800

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