Teacher Key Page


Keys to Stop Questions and Analysis:

STOP 1: Look at the pictures from STOP 1.  

STOP 2: Look at the pictures from STOP 2.  

STOP 3: Look at the pictures from STOP 3.  

STOP 4: Look at the pictures from STOP 4.  

STOP 5: Look at the pictures from STOP 5.  

Analysis:
How might all of these rocks gotten from where they were deposited or formed, to where they are now?  prior to the San Andreas system, the Pacific boundary was a subduction zone, with the oceanic plate being subducted under the North American continent.  As the sea floor subducted, surface material was scraped off and accreted on to the continent- including small volcanic sea mounts and guyots, pillow basalts, and sea floor sediments.

What type of tectonics do you think were prevalent on the Pacific coast around 175 my to about 65 mya, prior to the San Andreas Fault taking over as plate boundary?  subduction of a ocean plate under a continental plate.

Does this type of tectonics explain the features seen in the Marin Headlands today? yes.  subduction zones are known to scrape material off of the subducting plate and accrete this material onto the continent.  the scraping action of subduction adds "exotic terranes" to the continent, such as the pillow basalts, sheeted dikes, deep marine sediments and more.  All of these get deformed and folded as they are accreted (hence the folding of the cherts), and faulted by the compressional action of accretion.


All STOP location rock and other interpretations and analysis are from notes taken during the DiLeonardo 2011 Field Trip taken by the author on March 10, 2011