Spring 2021
| Speaker: | Bob Gillis ÀÖ»¢Ö±²¥ Division of Geological and Geophysical Surveys |
| Title: |
Margin-parallel contrasts in deformation and thermal structure along nearly 1,200 km of the ÀÖ»¢Ö±²¥ forearc informed by outcrop studies, subsurface interpretation, and low-temperature thermochronology: implications for Paleogene tectonism |
| Date: | April 16, 2021 |
| Time: | 11:45am |
| Location: | Contact instructor for details. jemezger@alaska.edu |
About:
Bob Gillis is a field geologist who integrates geologic mapping, structural geology
and thermochronology to reconstruct the tectonic history of ÀÖ»¢Ö±²¥â€™s hydrocarbon basins.
He received a BS in geology at the University of Arizona and a MS in geology at UCLA
where he studied the deformation and exhumation history of the Bolivian central Andes.
Abstract:
Forearcs at convergent plate margins are dynamic systems that develop in response
to boundary conditions that commonly vary over space and time. However, much of the
geologic and geophysical record (the oceanic plate) is lost to subduction. Therefore,
deciphering the often complex and incomplete geologic history preserved in ancient
forearc systems is critical to understanding a variety of subduction processes that
shape mature subduction systems. The ÀÖ»¢Ö±²¥ forearc is exposed for approximately 1,200
km from the southern tip of the ÀÖ»¢Ö±²¥ Peninsula to the Talkeetna Mountains/Matanuska
Valley and preserves an approximately 200 m.y. record of episodic magmatism and forearc
basin subsidence, thereby providing reasonable length and longevity for documenting
spatial and temporal geologic variability. The magmatic and depositional record indicates
two major disruptions of the forearc system in the Early Cretaceous and Paleogene.
Both are marked by a cessation of arc magmatism and regional unconformities extending
the entire length of the forearc basin. However, geologic mapping and subsurface geophysical
datasets indicate that the forearc records mostly Cenozoic deformation whose style
varies spatially along the length of the forearc. The ÀÖ»¢Ö±²¥ Peninsula segment of
the forearc may have been subjected to low magnitude shortening as late as the Paleocene
but was subsequently deformed by dextral transtension beginning in early Eocene into
the Miocene as it transitioned from a forearc to backarc setting. The middle segment
of the forearc from the upper ÀÖ»¢Ö±²¥ Peninsula to near the position of Redoubt Volcano
was principally contractional and/or transpressional at the arc/forearc domain boundary
marked by the Bruin Bay fault from as early as Late Jurassic until perhaps late Eocene,
after which deformation migrated into the forearc basin. In upper Cook Inlet and Matanuska
Valley areas, deformation is dominated by late Paleocene to late Eocene dextral transtension
at the arc/forearc basin boundary and margin-parallel extension of the arc domain,
followed by transpressional reactivation of extensional and transtensional structures
after the Oligocene. Considered together, backarc extension of the southwestern segment
of the forearc and forearc extension near the northeastern terminus facilitated by
dextral strike-slip faults is inferred to have resulted in Paleogene counterclockwise
rotation of the forearc. Low-temperature thermochronologic results from samples collected
along the structural boundary between arc and forearc basin domains define periods
of bedrock cooling that overlap with or post-date magmatic hiati and unconformity
development. However, unlike the magmatic and sedimentary records that remain broadly
uniform across the forearc, cooling ages that are not tied to local magmatic re-heating
generally fall within two distinct domains whose boundary corresponds to the transition
from contraction to margin-parallel extension and dextral transtension in the northeastern
segment of the forearc. Cooling ages to the southwest of this area typically define
middle Cretaceous exhumation cooling, whereas to the northeast, cooling ages are nearly
all Paleogene and likely reflect exhumation during high heat flow conditions. The
latter domain is restricted to the hinge of the ÀÖ»¢Ö±²¥ orocline where structurally-controlled
near-trench igneous rocks intrude or erupt into the forearc basin are of similar same
age to cooling. Cooling of the northeastern forearc, the regional forearc unconformity,
and arc hiatus in the Paleogene are commonly ascribed to a ridge subduction event
that swept from west to east along the ÀÖ»¢Ö±²¥ forearc from c. 60 Ma to ca. 50 Ma,
as recorded by a well-defined array of near-trench plutons that intrude the accretionary
prism outboard of the forearc basin. The subducting ridge would have ephemerally uplifted
the forearc, disrupted the mantle wedge, and heated the base of the crust as it passed
underneath. However, a time-transgressive character is not reflected in bedrock cooling,
re-initiation of forearc basin sedimentation, cessation/re-initiation of arc magmatism,
or timing and style of deformation along the approximately 1,200 km length of the
forearc, suggesting the prism arrived outboard of the ÀÖ»¢Ö±²¥ forearc after the ridge
had passed. Instead, synchronous extension at opposite ends of a forearc strengthened
by a spine of intrusive rocks is mechanically consistent with rigid beam rotation,
and coeval structurally-controlled magmatism localized in the axis of the orocline
conforms with models of extension in its hinge region during late Paleocene to late
Eocene buckling of ÀÖ»¢Ö±²¥.
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