Structure and neotectonics of the northwestern Nanga Parbat-Haramosh massif

The Nanga Parbat-Haramosh massif (NPHM) is a unique structural and topographic high in the northwestern corner of the Himalayan convergence zone. Previously, the NPHM was thought to be bounded by the MMT, a thrust along which the Kohistan-Ladakh island arc was obducted onto the northern margin of fridia. This study presents field evidence for the existence of the Raikot fault, an active dextral reverse fault which has truncated the MMT and forms the western boundary of the NPHM. The Raikot fault separates medium-grade Mesozoic to mid-Cenozoic mafic metasediments and intrusive rocks of the Kohistan sequence from high-grade Proterozoic metasediments and orthogneiss of the Nanga Parbat group. The Kohistan sequence rocks have experienced one tight to isoclinal folding event probably associated with the obduction of the island arc, and a second folding event associated with movement on the Raikot fault. The Nanga Parbat group rocks were transposed by a pre-Himalayan isoclinal folding event and have subsequently been folded around east-trending axes in the early Cenozoic by the obduction of Kohistan, then around north-trending axes in the late Cenozoic, associated with the uplift of the NPHM and the initiation of the Raikot fault. The Raikot fault consists of both mylonite zones and numerous major and minor faults. Slickensides and mylonitic lineations both indicate dextral reverse slip. The Raikot fault and associated folding appear to have accommodated 15 to 25 km of uplift in the late Cenozoic. The concentration of the uplift and involvement of the Moho suggests that the Raikot fault follows a major crustal structure, possibly a pre-collision Indian plate boundary. If this is the case, rotational underthrusting of greater India along the MMT would require dextral slip along the Raikot fault. It is proposed that the Raikot fault is a terminal tear fault on the MCT. Four ages of glaciation are recognized in the area, and correlated with regional glacial chronologies. Numerous Holocene offsets of glacial deposits and alluvial fans occur. A till surface attributed to the most recent major advance has been offset approximately 160 m vertically by strands of the Raikot fault. Analysis of the offsets in combination with thermoluminessence dating yield a maximum Holocene uplift rate of 3.9 mm /yr. This uplift rate is compatible with published values derived from fission-track cooling rates

The Portland Basin; a (big) river runs through it

ABSTRACT Metropolitan Portland, Oregon, USA, lies within a small Neogene to Holocene basin in the forearc of the Cascadia subduction system. Although the basin owes its existence and structural development to its convergent-margin tectonic setting, the stratigraphic architecture of basin-fill deposits chiefly reflects its physiographic position along the lower reaches of the continental-scale Columbia River system. As a result of this globally unique setting, the basin preserves a complex record of aggradation and incision in response to distant as well as local tectonic, volcanic, and climatic events. Voluminous flood basalts, continental and locally derived sediment and volcanic debris, and catastrophic flood deposits all accumulated in an area influenced by contemporaneous tectonic deformation and variations in regional and local base level

By

for selected urban areas in western Orego

plate1.jpg

The Nanga Parbat-Haramosh massif (NPHM) is a unique structural and topographic high in the northwestern corner of the Himalayan convergence zone. Previously, the NPHM was thought to be bounded by the MMT, a thrust along which the Kohistan-Ladakh island arc was obducted onto the northern margin of fridia. This study presents field evidence for the existence of the Raikot fault, an active dextral reverse fault which has truncated the MMT and forms the western boundary of the NPHM. The Raikot fault separates medium-grade Mesozoic to mid-Cenozoic mafic metasediments and intrusive rocks of the Kohistan sequence from high-grade Proterozoic metasediments and orthogneiss of the Nanga Parbat group. The Kohistan sequence rocks have experienced one tight to isoclinal folding event probably associated with the obduction of the island arc, and a second folding event associated with movement on the Raikot fault. The Nanga Parbat group rocks were transposed by a pre-Himalayan isoclinal folding event and have subsequently been folded around east-trending axes in the early Cenozoic by the obduction of Kohistan, then around north-trending axes in the late Cenozoic, associated with the uplift of the NPHM and the initiation of the Raikot fault. The Raikot fault consists of both mylonite zones and numerous major and minor faults. Slickensides and mylonitic lineations both indicate dextral reverse slip. The Raikot fault and associated folding appear to have accommodated 15 to 25 km of uplift in the late Cenozoic. The concentration of the uplift and involvement of the Moho suggests that the Raikot fault follows a major crustal structure, possibly a pre-collision Indian plate boundary. If this is the case, rotational underthrusting of greater India along the MMT would require dextral slip along the Raikot fault. It is proposed that the Raikot fault is a terminal tear fault on the MCT. Four ages of glaciation are recognized in the area, and correlated with regional glacial chronologies. Numerous Holocene offsets of glacial deposits and alluvial fans occur. A till surface attributed to the most recent major advance has been offset approximately 160 m vertically by strands of the Raikot fault. Analysis of the offsets in combination with thermoluminessence dating yield a maximum Holocene uplift rate of 3.9 mm /yr. This uplift rate is compatible with published values derived from fission-track cooling rates

MadinIanP1987.pdf

The Nanga Parbat-Haramosh massif (NPHM) is a unique structural and topographic high in the northwestern corner of the Himalayan convergence zone. Previously, the NPHM was thought to be bounded by the MMT, a thrust along which the Kohistan-Ladakh island arc was obducted onto the northern margin of fridia. This study presents field evidence for the existence of the Raikot fault, an active dextral reverse fault which has truncated the MMT and forms the western boundary of the NPHM. The Raikot fault separates medium-grade Mesozoic to mid-Cenozoic mafic metasediments and intrusive rocks of the Kohistan sequence from high-grade Proterozoic metasediments and orthogneiss of the Nanga Parbat group. The Kohistan sequence rocks have experienced one tight to isoclinal folding event probably associated with the obduction of the island arc, and a second folding event associated with movement on the Raikot fault. The Nanga Parbat group rocks were transposed by a pre-Himalayan isoclinal folding event and have subsequently been folded around east-trending axes in the early Cenozoic by the obduction of Kohistan, then around north-trending axes in the late Cenozoic, associated with the uplift of the NPHM and the initiation of the Raikot fault. The Raikot fault consists of both mylonite zones and numerous major and minor faults. Slickensides and mylonitic lineations both indicate dextral reverse slip. The Raikot fault and associated folding appear to have accommodated 15 to 25 km of uplift in the late Cenozoic. The concentration of the uplift and involvement of the Moho suggests that the Raikot fault follows a major crustal structure, possibly a pre-collision Indian plate boundary. If this is the case, rotational underthrusting of greater India along the MMT would require dextral slip along the Raikot fault. It is proposed that the Raikot fault is a terminal tear fault on the MCT. Four ages of glaciation are recognized in the area, and correlated with regional glacial chronologies. Numerous Holocene offsets of glacial deposits and alluvial fans occur. A till surface attributed to the most recent major advance has been offset approximately 160 m vertically by strands of the Raikot fault. Analysis of the offsets in combination with thermoluminessence dating yield a maximum Holocene uplift rate of 3.9 mm /yr. This uplift rate is compatible with published values derived from fission-track cooling rates

plate2.jpg

The Nanga Parbat-Haramosh massif (NPHM) is a unique structural and topographic high in the northwestern corner of the Himalayan convergence zone. Previously, the NPHM was thought to be bounded by the MMT, a thrust along which the Kohistan-Ladakh island arc was obducted onto the northern margin of fridia. This study presents field evidence for the existence of the Raikot fault, an active dextral reverse fault which has truncated the MMT and forms the western boundary of the NPHM. The Raikot fault separates medium-grade Mesozoic to mid-Cenozoic mafic metasediments and intrusive rocks of the Kohistan sequence from high-grade Proterozoic metasediments and orthogneiss of the Nanga Parbat group. The Kohistan sequence rocks have experienced one tight to isoclinal folding event probably associated with the obduction of the island arc, and a second folding event associated with movement on the Raikot fault. The Nanga Parbat group rocks were transposed by a pre-Himalayan isoclinal folding event and have subsequently been folded around east-trending axes in the early Cenozoic by the obduction of Kohistan, then around north-trending axes in the late Cenozoic, associated with the uplift of the NPHM and the initiation of the Raikot fault. The Raikot fault consists of both mylonite zones and numerous major and minor faults. Slickensides and mylonitic lineations both indicate dextral reverse slip. The Raikot fault and associated folding appear to have accommodated 15 to 25 km of uplift in the late Cenozoic. The concentration of the uplift and involvement of the Moho suggests that the Raikot fault follows a major crustal structure, possibly a pre-collision Indian plate boundary. If this is the case, rotational underthrusting of greater India along the MMT would require dextral slip along the Raikot fault. It is proposed that the Raikot fault is a terminal tear fault on the MCT. Four ages of glaciation are recognized in the area, and correlated with regional glacial chronologies. Numerous Holocene offsets of glacial deposits and alluvial fans occur. A till surface attributed to the most recent major advance has been offset approximately 160 m vertically by strands of the Raikot fault. Analysis of the offsets in combination with thermoluminessence dating yield a maximum Holocene uplift rate of 3.9 mm /yr. This uplift rate is compatible with published values derived from fission-track cooling rates

plate3.jpg

The Nanga Parbat-Haramosh massif (NPHM) is a unique structural and topographic high in the northwestern corner of the Himalayan convergence zone. Previously, the NPHM was thought to be bounded by the MMT, a thrust along which the Kohistan-Ladakh island arc was obducted onto the northern margin of fridia. This study presents field evidence for the existence of the Raikot fault, an active dextral reverse fault which has truncated the MMT and forms the western boundary of the NPHM. The Raikot fault separates medium-grade Mesozoic to mid-Cenozoic mafic metasediments and intrusive rocks of the Kohistan sequence from high-grade Proterozoic metasediments and orthogneiss of the Nanga Parbat group. The Kohistan sequence rocks have experienced one tight to isoclinal folding event probably associated with the obduction of the island arc, and a second folding event associated with movement on the Raikot fault. The Nanga Parbat group rocks were transposed by a pre-Himalayan isoclinal folding event and have subsequently been folded around east-trending axes in the early Cenozoic by the obduction of Kohistan, then around north-trending axes in the late Cenozoic, associated with the uplift of the NPHM and the initiation of the Raikot fault. The Raikot fault consists of both mylonite zones and numerous major and minor faults. Slickensides and mylonitic lineations both indicate dextral reverse slip. The Raikot fault and associated folding appear to have accommodated 15 to 25 km of uplift in the late Cenozoic. The concentration of the uplift and involvement of the Moho suggests that the Raikot fault follows a major crustal structure, possibly a pre-collision Indian plate boundary. If this is the case, rotational underthrusting of greater India along the MMT would require dextral slip along the Raikot fault. It is proposed that the Raikot fault is a terminal tear fault on the MCT. Four ages of glaciation are recognized in the area, and correlated with regional glacial chronologies. Numerous Holocene offsets of glacial deposits and alluvial fans occur. A till surface attributed to the most recent major advance has been offset approximately 160 m vertically by strands of the Raikot fault. Analysis of the offsets in combination with thermoluminessence dating yield a maximum Holocene uplift rate of 3.9 mm /yr. This uplift rate is compatible with published values derived from fission-track cooling rates

The Portland Hills Fault: Uncovering a Hidden Fault in Portland, Oregon Using High-Resolution Geophysical Methods

The Portland metropolitan area historically is the most seismically active region in Oregon. At least three potentially active faults are located in the immediate vicinity of downtown Portland, with the Portland Hills Fault (PHF) extending directly beneath downtown Portland. The faults are poorly understood, and the surface geologic record does not provide the information required to assess the seismic hazards associated with them. The limited geologic information stems from a surface topography that has not maintained a cumulative geologic record of faulting, in part, due to rapid erosion and deposition from late Pleistocene catastrophic flood events and a possible strike-slip component of the faults. We integrated multiple high-resolution geophysical techniques, including seismic reflection, ground penetrating radar (GPR), and magnetic methods, with regional geological and geophysical surveys to determine that the Portland Hills Fault is presently active with a zone of deformation that extends at least 400 m. The style of deformation is consistent with at least two major earthquakes in the last 12–15 ka, as confirmed by a sidehill excavation trench. High-resolution geophysical methods provide detailed images of the upper 100 m across the active fault zone. The geophysical images are critical to characterizing the structural style within the zone of deformation, and when integrated with a paleoseismic trench, can accurately record the seismic history of a region with little surface geologic exposure

Neotectonic Analysis of Upper Klamath Lake, Oregon: New Insights from Seismic Reflection Data

We present marine high-resolution seismic reflection data from Upper Klamath Lake, Oregon, to discern the underlying structure and estimate Quaternary slip rates in this actively extending Basin and Range system. The sediment patterns and structures imaged on our seismic profiles reveal a complex geologic system that reflects a changing climate record, shallow water conditions, growth faulting, contrasting sediment sources, and high slip rates. We observe that Upper Klamath Lake is a sediment-saturated environment, and sediment accumulation rates are therefore controlled by basin subsidence rather than sediment supply. Published slip rates for Holocene extension are greater than our determined late Quaternary slip rates, assuming reasonable rates of deposition. The apparent increased Holocene fault-slip rates may be in part an artifact of long recurrence intervals between major earthquakes, with recent seismicity accommodating long-term strain. The quantity of observed faults below the lake is at least an order of magnitude greater than those mapped outside the lake, suggesting that many hidden faults throughout the region may be unaccounted for when estimating Basin and Range extension rates