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Featured Stream channels as a indication of fault movement

  1. Jan 3, 2017 #1

    davenn

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    I was zooming around the NE region of the South Island of New Zealand in Google Earth today and discovered a classic example of offset stream channels across a major fault, the Awatere Fault.

    The Awatere Fault which is one of the major faults that the Alpine fault splinters into in the upper South Island. It's companions are the Wairau Fault -- the direct continuation of the Alpine Fault; The Clarence Fault and the Hope Fault. These 4 major faults all run parallel to each other and between them release the tectonic stresses that build up along this section of the Australian and Pacific plate boundary.

    The image below shows offsets of streams and ridgelines

    Awatere Fault stream channels.JPG

    The yellow arrows clearly mark the direction of the fault trace
    Two obvious ridge offsets are marked A--A and B--B
    The upper side B ends in a steep bank at the fault trace.
    1, 2, 3 lines mark a stream offset from the blue line on the upper side of the trace.
    I suspect some of the other stream channels to the left of the # 1, are quite possibly even earlier offsets

    For anyone interested in looking at the bigger/clearer image in Google earth. The coordinates are
    41.779398 S 173.785685 E


    Dave
     
    Last edited: Jan 4, 2017
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  3. Jan 6, 2017 #2

    Bystander

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    Is there actually a difference in composition/soil make-up? Or, to paraphrase, what's the actual offset (miles, or kilometers)?
     
  4. Jan 6, 2017 #3

    davenn

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    The sets of offsets shown above are in the order of 10's of meters and not necessarily all in the one event
    Using the roughly , approximately accurate :wink: measuring in google earth is showing about 200 metres between each of 1 -->2 -->3

    now, I know of no quake that has produced a 200m offset, so these would have been a progress of several large events over a very long period of time

    10 - 15m offset is typical for very large quakes, > M 8

    I haven't studies the specific makeup of the rocks of this region other than knowing they would mainly be metamorphosed seafloor sediments that have been uplifted by the tectonics in the region.


    Dave
     
  5. Jan 6, 2017 #4

    fresh_42

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    I had difficulties to interpret the offsets in A--A and B--B compared to other ones (1,2,3), because of their significant difference, while at the same time the shape of the mountains below and above the fault looks like there is missing something big on the upper part. But maybe I didn't understand the termini well enough.
     
  6. Jan 6, 2017 #5

    Bystander

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    Just trying to match the "geological character" in terms of "general morphology" and compare it to the more recent "track." The light/shadow boundary is really pronounced.
     
  7. Jan 6, 2017 #6

    davenn

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    OK I will zoom in on that area a bit ... the resolution of GE is pretty crappy when zoomed in too far .....

    Fig.1

    trace and ridge offsets.JPG

    Fig. 2
    a drawing view looking along the fault from lower left to upper right
    with these ridges and with the streams, it is important to consider the erosion process as they smooth off the ridge and water channel features.

    Ridge separation and erosion.GIF


    hope that helps with the visualisation of what is happening ?
    NOTE: the slope of the fault is correct for what is happening in the region. The slope will be close to correct to within a few degrees. The major faults in this area are reasonably steeply dipping to the east


    Dave
     
    Last edited: Jan 6, 2017
  8. Jan 6, 2017 #7

    fresh_42

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    Thank you for your explanation and pardon me for the trouble I made. What I couldn't get is, why the horizontal displacement at A and B is so much more than those at 1,2,3. Shouldn't there be more rock between B and 1 on the upper part? Or alternatively: there once must have been a giant vertical split that "swallowed" a large amount of mountain. But I'm not sure whether it can be seen this way.
     
  9. Jan 6, 2017 #8

    davenn

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    no probs. :smile: ask away, I will soon say if it's something I cannot answer

    Consider that stream evolution/creation is a much faster process than that of the formation of the mountain ridges so as a result
    there will be more stream channels cut during the same period of time.
    With the ridge, pick either of the two marked ones ( there are others also), the ridge doesn't reform after it has been separated. The two sections just continue to move further apart with each successive quake. Where as the stream section on the uphill side of the fault is continuing to rebuild its channels across the fault.
    And with every fault motion, the channel gets rebuilt

    did that clear the observation ?

    Dave
     
  10. Jan 6, 2017 #9

    fresh_42

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    Yes indeed, and pretty impressive. The only other fault I've seen so far (on TV) has been the one through Island between the North-American and the Eurasian plate. I wonder whether there are long-term prognoses considering NZ (before the entire landmass will build a next super-continent once more). Will the southern island be drawn apart or will only the mountains get higher?
     
  11. Jan 6, 2017 #10

    davenn

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    ahhhh Iceland, now that is a different whole ball game. Iceland is on a spreading ridge, namely the Mid-Atlantic Ridge.
    The MAR consists of primarily of spreading ridges interspersed with transform ( strike-slip ) fault zones.

    Fig.1
    overall tectonic motion of Iceland
    red dotted line is the approx. location of the spreading ridge
    blue arrows indicate the relative motion of the land away from the ridge axis
    This results in Iceland slowly getting wider and wider

    iceland.jpg


    Fig.2
    spreading and transform fault zones of the Mid-Atlantic Ridge
    A section of the ridge roughly due east of Bermuda

    Transform and spreading zones MAR.JPG

    the thin yellow arrows indicate the transform (strike-slip) faults
    the thick half arrows indicate the relative motion on each sine of those faults
    the blue arrows indicate the overall motion of the spreading sections

    Overall tectonics of New Zealand in the next post......
     
    Last edited: Jan 6, 2017
  12. Jan 7, 2017 #11

    davenn

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    Mountain building in the South Is's Southern Alps is still an active and ongoing process. The movement along the main section of the Alpine Fault is an approx. 3:1 ratio, that is, for every 3 metres of horizontal movement, there is 1 metre of vertical movement. These figures immediately tell us that AP isn't a pure strike-slip nor is it a pure thrust type of fault, rather it is an oblique slip fault with the main part of the motion being a strike-slip.

    Not knowing the level of geology knowledge of those reading this, just a little diversion on fault motions/types .....

    Fig.1
    Note the angle/direction of the slope of the fault plane arbitrary as they can vary greatly

    Fault types1.GIF


    OK, lets continue..... This will be a general overview of tectonics in New Zealand unless more detail is asked for

    Even on a large scale, the tectonics of the Pacific and Australian plate boundary, that New Zealand sits astride, is quite complex. Referring to Fig.2 below and starting with the North Island, it can be seen that all of the Nth Is. sits on the Australian Plate and that the Pacific Plate is subducting be low it. Plate motions are quite significant in this region at around 50 - 60mm / year and since this isn't a continuous movement at the main fault interface, it means that there is an ongoing build-up of stress that is released in periodic large quakes.
    As noted on the diagram, the volcanic plateau is a region of extensional faulting as the region is stretched. This thins the crust significantly and accounts for the large amount of volcanic activity of the region. The volcanoes are fed by the melted oceanic floor of the Pacific Plate as it sub-ducts to the 300 - 600km depth.
    The southern section of the North Island is noted for its long parallel faults which are an "assumed" continuation of the same series of parallel faults of the NE South Island.
    The last major fault in this region to have moved was the West Waiararapa Fault which is the eastern most of the faults. That event occurred in 1855 and had an estimated magnitude of M8.1. This was calculated from the damage and the offsets on the fault (12m horizontal and 7m vertical)

    Fig.2
    courtesy of GNS
    upload_2017-1-8_16-8-26.png

    Fig.3
    Cross-section, West to East, across the centre of the Nth Is.

    Nth Is X-section.GIF


    As we move down into the South Is., the complexity of the tectonics increases somewhat.
    The NE corner of the island is dominated by a series of major parallel faults. From west to east, the Wairau Fault, the Awatere Fault, the Clarence Fault and the Hope Fault. These are all splay faults of the main Alpine Fault to the south-west and runs down the backbone of the South Is..
    These NE faults are all oblique thrust faults produced by the steep subduction of the Pacific Plate under the region, see Fig.4

    Fig.4
    courtesy of GNS
    Subduction nthrn Sth Is.GIF

    The major subduction boundary is along the eastern most fault, the Hope Fault and its smaller associated ones. These were the primary source of the recent M 7.8 event in Nov. 2016. All land/crust to the west of the Hope Fault is part of the Australian Plate. As shown in the offset streams and ridges in the first post of this thread, it is not a pure thrust regime as there is also horizontal offset.

    Moving SW into the main Alpine Fault Zone, the thrust direction completely reverses and now the Pacific Plate is riding up and over the Australian Plate. This is an oblique slip system, with the mentioned earlier, 3 : 1 ratio of strike slip to reverse thrust. Here is a cross-section across the central South Island

    Fig.5

    Sth Is X-section.GIF


    Finally down into the SW corner of the South Island. The tectonics again reverse to a subduction zone. The Alpine Fault has now left land and is a km or two offshore and parallels the shore till it leaves the New Zealand region and continues down towards Macquarie Island. The offshore plate boundary is primarily a steeply dipping reverse thrust where the Australian Plate is subducting under the SW corner of the South Island.

    Fig.6
    Shows earthquakes on that steeply dipping fault zone
    courtesy of GNS

    19930810 M72 Fiord2.jpg


    As can be seen, the active subduction zone goes down to around 150 km, substantially less than the 600 km under the North Island.


    I will leave it at that for this post. It has covered a fair bit of information

    feel free to ask more questions or for any clarifications



    Dave

    PS .... @fresh_42 ... out of curiosity, if you don't mind telling .... where are you located ? just country ?
     
    Last edited: Jan 8, 2017
  13. Jan 7, 2017 #12

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    :wideeyed::)) Epiphany. Thank you. Thank you. Thank you. It's obvious, but I never thought of it in that way ... seen it in "slickensides" in slumps all the time when out "fossiling" in my spare/hobby/volunteer/conscript time, but never put two and two together.
     
  14. Jan 8, 2017 #13

    davenn

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    no problem :smile:

    I wonder if you have read the finished post ?

    and @fresh_42


    Dave
     
  15. Jan 8, 2017 #14

    fresh_42

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    Now I have. Many thanks, that's been a marvelous lesson on the tectonic situation there. I never would have thought it to be so complex. I always thought it to be like the Himalayan or the Alps here, but it seems to be more of a lab situation for geologists where many of different phenomena can be studied in a relatively small area. :ok::thumbup:
     
  16. Jan 8, 2017 #15

    davenn

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    indeed, that's why I enjoy doing geology in NZ :smile:
     
  17. Jan 10, 2017 #16

    Sanborn Chase

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    Dave, Is there a craton somewhere in NZ's formation, or is it all of volcanic formation?
     
  18. Jan 10, 2017 #17

    davenn

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    I have never heard the word craton used when describing NZ geology
    Cratons are usually more associated with the very old and stable cores of continents and usually well away from the plate boundaries

    NZ's history on the other hand, has been very tumultuous with several in-dispersed periods of sedimentation and mountain building.

    you may find this of interest to read as a general overview .......
    http://sci.waikato.ac.nz/evolution/geologicalHistory.shtml


    cheers
    Dave
     
  19. Jan 10, 2017 #18

    1oldman2

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  20. Jan 10, 2017 #19

    davenn

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  21. Jan 11, 2017 #20

    Sanborn Chase

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