SDAG Monthly Meeting
Wednesday, January 18, 2017
Catalina Room (southern end of MVCC)
Marina Village Conference Center
1936 Quivira Way
San Diego, CA 92109
FROM INTERSTATE 5: Take the SEA WORLD DRIVE exit. From SEA WORLD DRIVE,
take WEST MISSION BAY DRIVE on your right. When you see the large green sign that
says QUIVIRA ROAD, get in the farthest left of the two left turn lanes. Turn left, go one
very short block and turn left again. Drive about one half mile and MARINA VILLAGE will
be on your right.
FROM INTERSTATE 8:Exit at SPORTS ARENA BLVD., then take WEST MISSION BAY DRIVE exit to the right.
You will be on INGRAHAM STREET for a short distance from which you will take the next exit
marked WEST MISSION BAY DRIVE on your right. When you see the large green sign
that says QUIVIRA ROAD, get in the farthest left of the two left turn lanes. Turn left, go
one very short block and turn left again. Drive about 1/2 mile and MARINA VILLAGE will be on your right.
Menu: Traditional Buffet - Roast Beef/Chicken/Veg. Cash bar (Walawender Tavern).
Cost: $30.00 for non-members, $25.00 for members, $15.00 for students and professors.
if pre-registered by the deadline, $5 extra if you did not make a reservation.
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Speaker: Monte Marshall - Professor Emeritus SDSU
"The Human and Geologic History of Patagonia - A Beautiful Place at the Bottom of the World"
Patagonia, which comprises the southern five provinces of Argentina, and even less of Chile, is at the bottom of the world, the continent closest to Antarctica. It's geographically asymmetrical, with vast, arid, wind-swept plains on the Atlantic side and a relatively narrow chain of jagged, snow-covered mountains and rain forests on the Pacific. But its remoteness didn't stop the Native Americans from settling there possibly less than a thousand years after the first Americans, from East Asia and Eurasia, made their way across the Bering Straits to Alaska more than 15,000 years ago. During the last glacial maximum at 20 Ka, sea level was 120 m lower than today, and that opened a land path for them that stretched almost from the Aleutian Islands to the Arctic Ocean. The oldest archeological
sites are in North America and are between 15 and 16 Ka. The oldest site in South America found so far is in southern Chile and is dated at 14 Ka. To migrate those thousands of miles so quickly suggests that they came down the west coast, down what has been called the "kelp highway". They had the Americas for themselves until one day a strange vessel commanded by Ferdinand Magellan sailed into their harbors in 1520. Thus began the "Columbian Exchange" - but a very unbalanced one, since the brutality and greed of the Spanish conquistadors and the smallpox and other diseases brought by them decimated the Native Americans. Almost exactly 300 years later, a young Charles Darwin sailed up the same waters on the HMS Beagle and made some very astute observations about the biology, geology, and people of Patagonia.
Like all continents, most of the basement of South America consists of plutonic and metamorphic rocks of Archean and Proterozoic age. The ancient cratonal shields are in the north, especially in Brazil, and the only exposures of pre-Cambrian rocks in Patagonia are on the westernmost islands of southern Chile. The plains in Argentina comprise large areas of Jurassic to Cenozoic volcanic rocks and deep basins of petroliferous Mesozoic to Neogene sedimentary and volcanic rocks. The geology of the southern/Patagonian Andes is considerably less complicated than that to the north. The majority of the exposed rocks are Juro-Cretaceous and range from plutonic and metamorphic to volcanic and sedimentary. These Mesozoic rocks are bordered along the Chilean coast in northern Patagonia, and divided in southern Patagonia, by elongate regions of Paleozoic plutonic and metamorphic rocks. Whereas many of the mountains in southern South America are high enough to have glaciers, the cold moist air from the south Pacific has formed several ice fields that feed glaciers that pour down the high valleys on both sides of the Patagonian Andes. Together they form the third largest ice field/cap in the world, after Antarctica and Greenland.
The andesitic volcanoes are one of the most famous features of the Andes and their cause and distribution bring us to the main plate tectonic feature of western South America - active subduction along the entire 7,000 km length of the continent. There are three pieces of the southeast Pacific seafloor diving beneath South America - a fraction of the Cocos Plate on the north, the entire Nazca Plate in the center, and a fraction of the Antarctic Plate on the south. But there are three intervals where there are no currently active volcanoes. The study of earthquake hypocenters shows that the dip of the subducting oceanic lithosphere in these three stretches is only about 5 degrees. Under the active volcanoes it ranges from 20 to 30 degrees. The regions of shallow dip are associated with unusually thick oceanic crust, like the Nazca Ridge and Chile Rise. The central Patagonian Andes has one of these volcanic gaps, as does most of southern Patagonia, including Tierra del Fuego.
Probably the most famous tourist attraction in Patagonia is the row of 3,000 m high granite towers in Torres del Paine Park. They are the erosional remnants of a 13 Ma, dome-shaped intrusion (laccolith) of granitic magma into black, Cretaceous flysch (shales and turbidites), that was more than 2,000m thick and was 2-3 km beneath the surface. Erosion has removed all the flysch overlying some of the spires, but some have brilliant white bases with black caps. The laccolith extends for almost 20 km in a N-S direction, and 10 km E-W.
This talk will have many beautiful pictures of the Torres, whales mating, calving glaciers, sheep-herding, wild llamas, Ushuaia (the southernmost city in the world), fantastically folded strata, and penguins in a pear tree!
When I was born in 1939 at Mercy Hospital, San Diego was a pretty small town. Mission Valley was full of dairy farms, not shopping centers. I grew up in University Heights on a canyon that was full of eucalyptus trees - at least until I got my first Boy Scout axe. I loved that canyon! I built tree houses/forts, dug tunnels in the SD formation, hunted for scorpions and trap-door spiders, and got so sick from hiking through the poison oak that I could miss days of school. My canyon led to another that led to where Hwy. 163 is now, and from there we could descend into Mission Valley to catch pollywogs in the SD River. On our first hike we had to pass over a long strip of dirt that giant machines with steel sheep's feet were compacting - that turned out to be the foundation for I-8! At St. Augustine's High School, my priest/physics teacher became my role model, and so off I went to Villanova U to major in philosophy, with a minor in math and physics. After two years of theology, I decided to return to SD and study physics and astronomy. But my uncle, who was the Professor of Meteorology at State, introduced me to Baylor Brooks, founder of the geology department. I took his intro geo class, went on field trips, and quickly found that geologists have more fun than astrophysicists - and was hooked for the rest of my life!
After getting my PhD in geology and geophysics at Stanford and working at the USGS in Menlo Park, I returned to SDSU in 1975. My main courses were geophysics, structure, petroleum geology, and paleomagnetism and plate tectonics. My main research projects were applying paleomagnetism to SOCAL tectonics and detailed gravity studies of metropolitan SD faults. I theoretically retired in 2005, but still loving learning and teaching, I have been immersed in geologic community service ever since.
Upcoming SDAG meetings - 2017
February 15: Geoff Cromwell, Scott Rugh, Wes Danskin (USGS) -
Using Well Data to create 3D Geologic Model of SD-Tijuana Area
March 15: Student Presentations
April 19: John Minch - 1995 Kobe Earthquake
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