Wednesday, September 19, 2007

Mammoth Lakes Pleistocene Movements

An Alpine Meadow (Sierras, from Berkeley, 1967)
A splash of yellow on a field of green
Caused the bees to stir-
Raising hopes of winter honey,
Where snapdragons previously were…

Sunlit slopes and fragrant air
Keep the bees a-hum
Raising petal fetishes,
Where the bees are from.

What is your secret of activity
All the summer long?
I hope to break the meadow code
Of your flower song.

Harold L. Overton

Mammoth Lakes Elderhostel (Sept. 16-21, 07)

A fisher named Fisher fished a fissure
For Brown Trout, a notable delight- a fish, sure!

But the fish, being fissile,
To the depths, went like a missile-
Pulling the fisher quite deep in the fissure.

The fish and his plan, readily outsmarted supercilious Man;
Now other fishers are fishing the fissure for ‘ol Fisher.
Harold L. Overton

Glenn Wasson and I made yet another Elderhostel, in the eastern part of California, to fit the Bishop fault and the vulcanism along it into my scheme of continent-wide fracturing and faulting.

Pleistocene Fissure trends North to South, N-S

The Long Valley (Mammoth Lake) caldera is in the general trend of young violent extrusions, such as Monocraters and others along the border of the Sierras adjoining the western edge of the Basin and Range province (found in Eastern California, Nevada, and Utah- as well as parts of AZ, ID, and NM).
The town of Mammoth Lake (a ski resort) lies on California state hiway 203, which is a dead-end road proceeding westward from Federal 395, near the town of Lee Vining. It lies south of Monocraters and Mono Lake, which are closer to Lee Vining in Eastern California.
The vulcanism in the Basin and Range province interior is mostly basaltic and slow-flowing, as opposed to that at the western boundary- such as the Mammoth Lake caldera, which is silicic and explosive (a caldera is larger than a crater, being classified as one with a diameter much larger than its height). This relates that the phenomenon is basically different on the edge of B&R compared to that in the interior. Even on the Colorado Plateau, most of the vulcanism on the side adjoining the B&R is basaltic in composition-indicating that the flows are high iron content, quiet, and slow compared to that on the California border. Mammoth and Monocraters contain light rocks (in both color and weight), such as tuffs and ash, and were drastic, violent, and sudden, in comparison. This empathizes that the mechanism is altogether different at the border of B&R relative to that in the interior of both the Colorado plateau (CP) and the B&R. What is the reason (or better, the fundamental mechanism) for the divergence of rates and composition? Of course, the initial reason for the two varying types of vulcanism is that of silica content- a high glass content, as in rhyolitic and andesitic magmas, produces a violent endpoint (similarly to glass-blowing), compared to the high iron content of basalt, which flows rather than explodes. This analysis hopes to advance a mechanism, and relate how it fits in with the understanding of the mechanism creating the CP.

Surface Observations about the Mammoth Lakes Caldera
A surface geographical map is shown below, as prepared by the Park Service, which indicates that the creeks and valleys orient either N-S or NW-SE mainly. We will test again whether the north to south orientations are created by stresses which are younger (as postulated in all of my other work around the CP).

The youngest craters and major faults do orient N-S, and these are shown in photos for Panum (estimated at 700 years), Mammoth craters (some 760k ybp), and the Bishop fault and its spurs. Silver Lake lies outside of the caldera, but exhibits a N-S fracture on its south side.

Silver Lake, outside the caldera, exhibits a N-S fracture

Even inside the multi-mile caldera, the hot creeks tend to orient similarly. However, Inyo Crater Lake orients in a NW direction, and we should test whether the fracture which produced it is older than 2 million years- the time of the reorientation of crustal stresses as shown in my previous work in this Blog.

INYO Crater exhibits a NW-SE trend, indicating a previous fracture orientation

There is also some regularity to the age of the various eruptions. For the latest cones and craters, there is a tendency for the sites to become more northerly with time- Panum near the south coast of Mono Lake being the latest. These are in the N-S trend of craters probably associated with the magma under the Long Valley series.

Mono Lake and Paoha Island show N-S trend of extrusions (Basalt Crater)
Outside of this trend, the older volcanoes appear more random, which could be because of the postulated change of the crustal stresses from NW to N-S at about the 2 mybp time. In this area, as well as in the Hurricane Fault region shown earlier in this Blog, there occur both NW and N-S faults, which may have bifurcated within this Pleistocene time.
1. The dominant trends of surface features orient either N-S or NW-SE, and the northerly orientation appears to be the younger.
2. The extrusions appear to move northerly with time, indicating an advancing fracturing and faulting trend for the Long Valley caldera;
3. The rocks are mainly more siliceous for this border of the Basin and Range- Sierra Uplift Province; and
4. The whole feature is actively heating, rising, and shearing now.