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Articles for Geosphere posted online in March

Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Locations studied this month include the
western Himalaya, the boundary between the southern Coast Ranges and
western Transverse Ranges in California, the northern Sierra Nevada, and
northwest Nepal.


Marine sedimentary records of chemical weathering evolution in the
western Himalaya since 17 Ma


Peng Zhou; Thomas Ireland; Richard W. Murray; Peter D. Clift

Abstract:
The Indus Fan derives sediment from the western Himalaya and Karakoram.
Sediment from International Ocean Discovery Program drill sites in the
eastern part of the fan coupled with data from an industrial well near the
river mouth allow the weathering history of the region since ca. 16 Ma to
be reconstructed. Clay minerals, bulk sediment geochemistry, and magnetic
susceptibility were used to constrain degrees of chemical alteration.
Diffuse reflectance spectroscopy was used to measure the abundance of
moisture-sensitive minerals hematite and goethite. Indus Fan sediment is
more weathered than Bengal Fan material, probably reflecting slow
transport, despite the drier climate, which slows chemical weathering
rates. Some chemical weathering proxies, such as K/Si or kaolinite/(illite
+ chlorite), show no temporal evolution, but illite crystallinity and the
chemical index of alteration do have statistically measurable decreases
over long time periods. Using these proxies, we suggest that sediment
alteration was moderate and then increased from 13 to 11 Ma, remained high
until 9 Ma, and then reduced from that time until 6 Ma in the context of
reduced physical erosion during a time of increasing aridity as tracked by
hematite/goethite values. The poorly defined reducing trend in weathering
intensity is not clearly linked to global cooling and at least partly
reflects regional climate change. Since 6 Ma, weathering has been weak but
variable since a final reduction in alteration state after 3.5 Ma that
correlates with the onset of Northern Hemispheric glaciation. Reduced or
stable chemical weathering at a time of falling sedimentation rates is not
consistent with models for Cenozoic global climate change that invoke
greater Himalayan weathering fluxes drawing down atmospheric CO 2 but are in accord with the idea of greater surface reactivity
to weathering.

View article:

https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02211.1/595660/Marine-sedimentary-records-of-chemical-weathering


Late Pleistocene rates of rock uplift and faulting at the boundary
between the southern Coast Ranges and the western Transverse Ranges in
California from reconstruction and luminescence dating of the Orcutt
Formation


Ian S. McGregor; Nathan W. Onderdonk

Abstract:
The western Transverse Ranges and southern Coast Ranges of California are
lithologically similar but have very different styles and rates of
Quaternary deformation. The western Transverse Ranges are deformed by
west-trending folds and reverse faults with fast rates of Quaternary fault
slip (1-11 mm/yr) and uplift (1-7 mm/yr). The southern Coast Ranges,
however, are primarily deformed by northwest-trending folds and
right-lateral strike-slip faults with much slower slip rates (3 mm/yr or
less) and uplift rates (1 mm/yr). Faults and folds at the boundary
between these two structural domains exhibit geometric and kinematic
characteristics of both domains, but little is known about the rate of
Quaternary deformation along the boundary. We used a late Pleistocene
sedimentary deposit, the Orcutt Formation, as a marker to characterize
deformation within the boundary zone over the past 120 k.y. The Orcutt
Formation is a fluvial deposit in the Santa Maria Basin that formed during
regional planation by a broad fluvial system that graded into a shoreline
platform at the coast. We used post-infrared-infrared-stimulated
luminescence (pIR-IRSL) dating to determine that the Orcutt Formation was
deposited between 119 ± 8 and 85 ± 6 ka, coincident with oxygen isotope
stages 5e-a paleo-sea-level highstands and regional depositional events.
The deformed Orcutt basal surface closely follows the present-day
topography of the Santa Maria Basin and is folded by northwest-trending
anticlines that are a combination of fault-propagation and
fault-bend-folding controlled by deeper thrust faults. Reconstructions of
the Orcutt basal surface and forward modeling of balanced cross sections
across the study area allowed us to mea­sure rock uplift rates and fault
slip rates. Rock uplift rates at the crests of two major anticlinoria are
0.9-4.9 mm/yr, and the dip-slip rate along the blind fault system that
underlies these folds is 5.6-6.7 mm/yr. These rates are similar to those
reported from the Ventura area to the southeast and indicate that the
relatively high rates of deformation in the western Transverse Ranges are
also present along the northern boundary zone. The deformation style and
rates are consistent with models that attribute shortening across the Santa
Maria Basin to accommodation of clockwise rotation of the western
Transverse Ranges and suggest that rotation has continued into late
Quaternary time.

View article:

https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02274.1/595661/Late-Pleistocene-rates-of-rock-uplift-and-faulting


Influence of pre-existing structure on pluton emplacement and
geomorphology: The Merrimac plutons, northern Sierra Nevada,
California, USA


V.E. Langenheim; J.A. Vazquez; K.M. Schmidt; G. Guglielmo, Jr.; D.S.
Sweetkind

Abstract:
In much of the western Cordillera of North America, the geologic frame­work
of crustal structure generated in the Mesozoic leaves an imprint on later
plutonic emplacement, subsequent structural setting, and present landscape
morphology. The Merrimac plutons in the northern Sierra Nevada (California,
USA) are a good example of the influence of pre-existing structure at a
larger scale. This paper updates and refines earlier studies of the
Merrimac plutons, with the addition of analysis of gravity and magnetic
data and new 206Pb/238U zircon dates. The gravity and
magnetic data not only confirm the presence of two different neighboring
plutons, but also (1) support the presence of a third pluton, (2) refine
the nature of the contact between the Merrimac plutons as being
structurally controlled, and (3) estimate the depth extent of the plutons
to be ~4-5 km. The zircon 206Pb/238U dates indicate
that the two main plutons have statistically different crystallization ages
nearly 4 m.y. apart. Geomorphic analyses, including estimates of relief,
roughness and drainage density and generation of chi plots, indicate that
the two main plutons are characterized by different elevations with large
longitudinal channel knickpoints that we speculatively attribute to
possible reactivation of pre-existing structure in addition to lithologic
variations influencing relative erosion susceptibility in response to prior
accelerated surface uplift.

View article:

https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02281.1/594115/Influence-of-pre-existing-structure-on-pluton


Protolith affiliation and tectonometamorphic evolution of the Gurla
Mandhata core complex, NW Nepal Himalaya


Laurent Godin; Mark Ahenda; Djordje Grujic; Ross Stevenson; John Cottle

Abstract:
Assigning correct protolith to high metamorphic-grade core zone rocks of
large hot orogens is a particularly important challenge to overcome when
attempting to constrain the early stages of orogenic evolution and
paleogeography of lithotectonic units from these orogens. The Gurla
Mandhata core complex in NW Nepal exposes the Himalayan metamorphic core
(HMC), a sequence of high metamorphic-grade gneiss, migmatite, and granite,
in the hinterland of the Himalayan orogen. Sm-Nd isotopic analyses indicate
that the HMC comprises Greater Himalayan sequence (GHS) and Lesser
Himalayan sequence (LHS) rocks. Conventional interpretation of such
provenance data would require the Main Central thrust (MCT) to be also
outcropping within the core complex. However, new in situ U-Th/Pb monazite
petrochronology coupled with petrographic, structural, and microstructural
observations reveal that the core complex is composed solely of rocks in
the hanging wall of the MCT. Rocks from the core complex record Eocene and
late Oligocene to early Miocene monazite (re-)crystallization periods
(monazite age peaks of 40 Ma, 25-19 Ma, and 19-16 Ma) overprinting pre-
Himalayan Ordovician Bhimphedian metamorphism and magmatism (ca. 470 Ma).
The combination of Sm-Nd isotopic analysis and U-Th/ Pb monazite
petrochronology demonstrates that both GHS and LHS protolith rocks were
captured in the hanging wall of the MCT and experienced Cenozoic Himalayan
metamorphism during south-directed extrusion. Monazite ages do not record
metamorphism coeval with late Miocene extensional core complex exhumation,
suggesting that peak metamorphism and generation of anatectic melt in the
core complex had ceased prior to the onset of orogen-parallel hinterland
extension at ca. 15-13 Ma. The geometry of the Gurla Mandhata core complex
requires significant hinterland crustal thickening prior to 16 Ma, which is
attributed to ductile HMC thickening and footwall accretion of LHS
protolith associated with a Main Himalayan thrust ramp below the core
complex. We demonstrate that isotopic signatures such as Sm-Nd should be
used to characterize rock units and structures across the Himalaya only in
conjunction with supporting petrochronological and structural data.

View article:

https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02326.1/595237/Protolith-affiliation-and-tectonometamorphic

Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Locations studied this month include the
western Himalaya, the boundary between the southern Coast Ranges and
western Transverse Ranges in California, the northern Sierra Nevada, and
northwest Nepal.


Marine sedimentary records of chemical weathering evolution in the
western Himalaya since 17 Ma


Peng Zhou; Thomas Ireland; Richard W. Murray; Peter D. Clift

Abstract:
The Indus Fan derives sediment from the western Himalaya and Karakoram.
Sediment from International Ocean Discovery Program drill sites in the
eastern part of the fan coupled with data from an industrial well near the
river mouth allow the weathering history of the region since ca. 16 Ma to
be reconstructed. Clay minerals, bulk sediment geochemistry, and magnetic
susceptibility were used to constrain degrees of chemical alteration.
Diffuse reflectance spectroscopy was used to measure the abundance of
moisture-sensitive minerals hematite and goethite. Indus Fan sediment is
more weathered than Bengal Fan material, probably reflecting slow
transport, despite the drier climate, which slows chemical weathering
rates. Some chemical weathering proxies, such as K/Si or kaolinite/(illite
+ chlorite), show no temporal evolution, but illite crystallinity and the
chemical index of alteration do have statistically measurable decreases
over long time periods. Using these proxies, we suggest that sediment
alteration was moderate and then increased from 13 to 11 Ma, remained high
until 9 Ma, and then reduced from that time until 6 Ma in the context of
reduced physical erosion during a time of increasing aridity as tracked by
hematite/goethite values. The poorly defined reducing trend in weathering
intensity is not clearly linked to global cooling and at least partly
reflects regional climate change. Since 6 Ma, weathering has been weak but
variable since a final reduction in alteration state after 3.5 Ma that
correlates with the onset of Northern Hemispheric glaciation. Reduced or
stable chemical weathering at a time of falling sedimentation rates is not
consistent with models for Cenozoic global climate change that invoke
greater Himalayan weathering fluxes drawing down atmospheric CO 2 but are in accord with the idea of greater surface reactivity
to weathering.

View article:

https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02211.1/595660/Marine-sedimentary-records-of-chemical-weathering


Late Pleistocene rates of rock uplift and faulting at the boundary
between the southern Coast Ranges and the western Transverse Ranges in
California from reconstruction and luminescence dating of the Orcutt
Formation


Ian S. McGregor; Nathan W. Onderdonk

Abstract:
The western Transverse Ranges and southern Coast Ranges of California are
lithologically similar but have very different styles and rates of
Quaternary deformation. The western Transverse Ranges are deformed by
west-trending folds and reverse faults with fast rates of Quaternary fault
slip (1-11 mm/yr) and uplift (1-7 mm/yr). The southern Coast Ranges,
however, are primarily deformed by northwest-trending folds and
right-lateral strike-slip faults with much slower slip rates (3 mm/yr or
less) and uplift rates (1 mm/yr). Faults and folds at the boundary
between these two structural domains exhibit geometric and kinematic
characteristics of both domains, but little is known about the rate of
Quaternary deformation along the boundary. We used a late Pleistocene
sedimentary deposit, the Orcutt Formation, as a marker to characterize
deformation within the boundary zone over the past 120 k.y. The Orcutt
Formation is a fluvial deposit in the Santa Maria Basin that formed during
regional planation by a broad fluvial system that graded into a shoreline
platform at the coast. We used post-infrared-infrared-stimulated
luminescence (pIR-IRSL) dating to determine that the Orcutt Formation was
deposited between 119 ± 8 and 85 ± 6 ka, coincident with oxygen isotope
stages 5e-a paleo-sea-level highstands and regional depositional events.
The deformed Orcutt basal surface closely follows the present-day
topography of the Santa Maria Basin and is folded by northwest-trending
anticlines that are a combination of fault-propagation and
fault-bend-folding controlled by deeper thrust faults. Reconstructions of
the Orcutt basal surface and forward modeling of balanced cross sections
across the study area allowed us to mea­sure rock uplift rates and fault
slip rates. Rock uplift rates at the crests of two major anticlinoria are
0.9-4.9 mm/yr, and the dip-slip rate along the blind fault system that
underlies these folds is 5.6-6.7 mm/yr. These rates are similar to those
reported from the Ventura area to the southeast and indicate that the
relatively high rates of deformation in the western Transverse Ranges are
also present along the northern boundary zone. The deformation style and
rates are consistent with models that attribute shortening across the Santa
Maria Basin to accommodation of clockwise rotation of the western
Transverse Ranges and suggest that rotation has continued into late
Quaternary time.

View article:

https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02274.1/595661/Late-Pleistocene-rates-of-rock-uplift-and-faulting


Influence of pre-existing structure on pluton emplacement and
geomorphology: The Merrimac plutons, northern Sierra Nevada,
California, USA


V.E. Langenheim; J.A. Vazquez; K.M. Schmidt; G. Guglielmo, Jr.; D.S.
Sweetkind

Abstract:
In much of the western Cordillera of North America, the geologic frame­work
of crustal structure generated in the Mesozoic leaves an imprint on later
plutonic emplacement, subsequent structural setting, and present landscape
morphology. The Merrimac plutons in the northern Sierra Nevada (California,
USA) are a good example of the influence of pre-existing structure at a
larger scale. This paper updates and refines earlier studies of the
Merrimac plutons, with the addition of analysis of gravity and magnetic
data and new 206Pb/238U zircon dates. The gravity and
magnetic data not only confirm the presence of two different neighboring
plutons, but also (1) support the presence of a third pluton, (2) refine
the nature of the contact between the Merrimac plutons as being
structurally controlled, and (3) estimate the depth extent of the plutons
to be ~4-5 km. The zircon 206Pb/238U dates indicate
that the two main plutons have statistically different crystallization ages
nearly 4 m.y. apart. Geomorphic analyses, including estimates of relief,
roughness and drainage density and generation of chi plots, indicate that
the two main plutons are characterized by different elevations with large
longitudinal channel knickpoints that we speculatively attribute to
possible reactivation of pre-existing structure in addition to lithologic
variations influencing relative erosion susceptibility in response to prior
accelerated surface uplift.

View article:

https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02281.1/594115/Influence-of-pre-existing-structure-on-pluton


Protolith affiliation and tectonometamorphic evolution of the Gurla
Mandhata core complex, NW Nepal Himalaya


Laurent Godin; Mark Ahenda; Djordje Grujic; Ross Stevenson; John Cottle

Abstract:
Assigning correct protolith to high metamorphic-grade core zone rocks of
large hot orogens is a particularly important challenge to overcome when
attempting to constrain the early stages of orogenic evolution and
paleogeography of lithotectonic units from these orogens. The Gurla
Mandhata core complex in NW Nepal exposes the Himalayan metamorphic core
(HMC), a sequence of high metamorphic-grade gneiss, migmatite, and granite,
in the hinterland of the Himalayan orogen. Sm-Nd isotopic analyses indicate
that the HMC comprises Greater Himalayan sequence (GHS) and Lesser
Himalayan sequence (LHS) rocks. Conventional interpretation of such
provenance data would require the Main Central thrust (MCT) to be also
outcropping within the core complex. However, new in situ U-Th/Pb monazite
petrochronology coupled with petrographic, structural, and microstructural
observations reveal that the core complex is composed solely of rocks in
the hanging wall of the MCT. Rocks from the core complex record Eocene and
late Oligocene to early Miocene monazite (re-)crystallization periods
(monazite age peaks of 40 Ma, 25-19 Ma, and 19-16 Ma) overprinting pre-
Himalayan Ordovician Bhimphedian metamorphism and magmatism (ca. 470 Ma).
The combination of Sm-Nd isotopic analysis and U-Th/ Pb monazite
petrochronology demonstrates that both GHS and LHS protolith rocks were
captured in the hanging wall of the MCT and experienced Cenozoic Himalayan
metamorphism during south-directed extrusion. Monazite ages do not record
metamorphism coeval with late Miocene extensional core complex exhumation,
suggesting that peak metamorphism and generation of anatectic melt in the
core complex had ceased prior to the onset of orogen-parallel hinterland
extension at ca. 15-13 Ma. The geometry of the Gurla Mandhata core complex
requires significant hinterland crustal thickening prior to 16 Ma, which is
attributed to ductile HMC thickening and footwall accretion of LHS
protolith associated with a Main Himalayan thrust ramp below the core
complex. We demonstrate that isotopic signatures such as Sm-Nd should be
used to characterize rock units and structures across the Himalaya only in
conjunction with supporting petrochronological and structural data.

View article:

https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02326.1/595237/Protolith-affiliation-and-tectonometamorphic

###

GEOSPHERE articles are available at

https://geosphere.geoscienceworld.org/content/early/recent

. Representatives of the media may obtain complimentary copies of GEOSPHERE
articles by contacting Kea Giles at the address above. Please discuss
articles of interest with the authors before publishing stories on their
work, and please make reference to GEOSPHERE in articles published.
Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

https://www.geosociety.org/

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