Led by Prof.
Alexander E. Gates
Department of Earth and Environmental Sciences
Rutgers University
Newark, NJ 07102
The following descriptions of the stops are from the "Field Trip to the Western Hudson Highlands, New York"
We did not have time to visit Stops 5 and 7.
The following photos were taken by Gil Hanson unless otherwise noted. If you have better photos or photos that show features not shown here please e-mail them to me at gilbert.hanson@sunysb.edu
Photos courtesy of Gates et al, 2003 are from the Guide "Field Trip to the Western Hudson Highlands, New York"
The rocks in Harriman and Bear Mountain State Parks record a long and complex history that began 1.3 billion years ago and continues today. Most of the rocks were deposited as sediments and volcanics about 1.3 billion years ago in a setting similar to Japan today. About 1.0 to 1.1 billion years ago, a huge mountain building event called the Grenville Orogeny resulted from a continental collision similar to that which built the Himalayas. This event turned all of the rocks into the gneisses that we see today. The rocks were buried to about 25-30 km depth during this event and heated to over 700°C. A second phase of the Grenville orogeny was the formation of large strike-slip faults similar to the San Andreas Fault, CA of today. Magnetite (iron ore) in veins several miles in length was mined for iron for two centuries in early American history. The Grenville Orogeny was one of several that built the supercontinent Rodinia. There was a period of tectonic quiescence for over 200 million years until Rodinia broke-up in a worldwide rifting event. |
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These rocks are representative of the low energy
deposits of the sequence. They are interpreted to have formed in a restricted
marine basin that was likely euxinic and with a significant volcanic input. In
other areas, these rocks can contain biotite gneiss with 55% garnets, thin
marble lenses, and layers of pyroxene-plagioclase gneiss that are interpreted
to be of volcanic origin.
The following photos are thumbnail. Click on them to get larger images.
The rock is interpreted to be metamorphosed
sedimentary to volcaniclastic rock that was deposited in a high energy
environment. The hornblende-rich rock reflects volcanic input. The apparent
fining upward sequence might reflect a prograding fan or delta or shifting
channel sequence. The folds were generated during westward directed fold nappe
emplacement. These recumbent folds are observed at all scales. They appear to
accompany peak metamorphic conditions. This tectonic event is interpreted to
have been a Himalayan type continental collision.
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Subsequent
to the first tectonic event which included the nappe emplacement and granulite
facies metamorphism, there was a period of intermediate plutonism. The
xenolith was deformed and metamorphosed prior to intrusion. The xenolith
became more ductile as a result of the heat of the pluton. Thus drag folds
formed along its edges as it fell into the magma. The magma was hot enough to
cause partial melting of the rim of the xenolith, producing a granitic melt.
The diorite crystallized at higher temperature than the granitic melt.
Fractures opened in the newly crystallized rock and the remaining granitic
melt squeezed into them forming the veins. Later deformation produced the
mylonitic fabric in the diorite. This outcrop is at the eastern edge of a
large dextral strike-slip shear zone with similar orientation.
The interlayered mafic-intermediate gneiss are interpreted as metavolcanics of island arc affinity. During the nappe emplacement event, metamorphism achieved granulite facies. Locally, the gneiss underwent anatexis and formed migmatite. Note that this rock still preserves the evidence of the first tectonic event with no overprinting. Contrast this rock with Stop 4b.
Lozenges of mafic gneiss contained within contorted layered biotite and
hornblende quartzofeldspathic gneiss. The mafic gneiss is the same as in Stop
4a but it contains magnetite veins and contorted folds. The quartzofeldspathic
gneiss is layered as defined by variations in biotite content and locally
hornblende content. The layering is also contorted and wraps around the
lozenges. The fold axes and long axes of the lozenges are parallel and plunge
shallowly to the northeast.
Stops 4a and 4b are grouped together because the compositions are
similar and interpreted to be part of the same sequence. There is a large
dextral strike-slip shear zone to the northwest. This deformation postdates
the nappe emplacement phase. The rocks in stop 4a are considered to have been
unaffected or only mildly affected by this later deformation. Deformation
progressively increases towards the northwest as tracked by the progressive
increase in linear fabric and steepening of the foliation.
The veins are interpreted to have formed in dilational joints and fractures during the waning stages of dextral strike-slip shearing. Metamorphic fluids flushed through these fractures and reacted with the wall rock. The fluids mobilized elements from the reactions with the wall rock. These reactions buffered the composition of the fluids. When these fluids encountered the right conditions either physically or chemically, they deposited the ore and gaunge minerals. With the banding of different assemblages and compositions reflects the changing chemistries of the fluids. These changes may reflect changes in flux, fluid source, or physical conditions. The pegmatites may have intruded along the same pathways as the fluids.
Stop 6 Vertical iron mine shaft. Water at bottom.
STOP
8: Mylonite Zone
(Rt 17A near Sloatsburg, outcrop is on the north side
of the westbound lane just before the first left turn lane to the west of Rt.
17)
Because
this shear zone developed in a biotite-rich gneiss, it displays kinematic
indicators better than most zones. It is another in the series of anastomosing
dextral shear zones that were produced during the second event. The gneiss is
interpreted to have a volcaniclastic origin.
Stop 8 Alec Gates is showing the direction of shearing |
Stop 8 Close up of mylonite. Handle of hammer is parallel to axes of broad folds in mylonite. Direction of compressional stress which causes the dextral shearing in the mylonite is perpendicular to handle. |
![]() Stop 8 Kinematic indicators. Courtesy of Vesna Kundic |
Stop 8 Late undeformed hornblende-rich intrusion in mylonitic country rock. Hammer head is near contact. Hornblende crystals appear to be growing away from contact into the intrusive body. |