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EarthCache

Gold Hill: A Good Fault

A cache by frograil Send Message to Owner Message this owner
Hidden : 1/15/2010
In North Carolina, United States
Difficulty:
1 out of 5
Terrain:
1 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

In this EarthCache, we will use observation and common sense to discover the ancient Gold Hill Fault zone.


NOTE WELL:  Even though this is a very minor road, it is a public road, and you should be alert for traffic.  Also, the fencing in the area carries a significant electrical charge, and you should stay well away from any fences.  Please do not go down into the creek area itself.


"Streams have a way of seeking out major junctions in the earth's surface, picking out old weaknesses."                                                                        Richard Fortey


You are on Old Mine Road, standing at the edge of a bridge over Little Buffalo Creek near the village of Gold Hill.  If you look both north and south from the bridge, you might notice something odd about this little creek.  Unlike most such rivulets that meander all around, exploiting the softest rocks available, this stream seems to go almost in a straight line to the northeast and southwest.  In fact, if you were a bird, and flew southwest over the creek, you'd see Buffalo Creek, Adams Creek and Rocky River all coming from the northwest, into what started out as Little Buffalo, and turning almost 90º to the southwest to flow in the same direction as the creek you're observing.  You can draw a straight line from where you are standing to the far southern border of Stanly County, and that straight line will almost exactly trace the course of the creek/river.

Adding to the weirdness, if you could fly a short distance northeast to Abbotts Creek, you'd find the same phenomenon in reverse:  It goes straight to the northeast, along the same trend line that Little Buffalo follows to the southwest.

In 1910, Francis Baker Laney wrote a 137-page Bulletin for the North Carolina Geological Survey entitled "The Gold Hill Mining District". Dr. Laney didn't have the benefit of a helicopter or airplane to fly over this area, nor did he have many of the tools and techniques available to the field geologist today, but he did have two very important tools:  He was smart and he was observant.  He recognized that Abbotts Creek, Reedy Branch, Little Buffalo Creek, Buffalo Creek and Rocky River flowed in basically a straight line for "at least 75 miles".

Geologists preceding Dr. Laney had suggested that there must be a fault in the Gold Hill area, and he offered the following evidence:

1.  Pretty much following the line of the streams, the rocks to the west were different from those to the east. To the west were granites and medium grade metamorphic rocks, while the eastern rocks were mostly sedimentary and low grade metamorphic in nature.

2.  There were numerous springs along the "line of contact" between the two types of rock, but few to the east or west beyond it.

3.  Numerous minor joints and faults were seen, and they were generally parallel to the line of contact.

4.  Numerous granite dikes intruded the rocks to the west, but did not exist in the eastern rocks.

5.  There were ore bodies and mines along the area of the line of contact, a few or none to the east and west.

Modern geologists are as intrigued by this fault as was Dr. Laney.  In fact, it turns out that there are two actual faults, the Gold Hill on the west and Silver Hill fault on the east.  They are parallel to each other, and the distance between them is generally a few miles.  The area between these faults is important, and that area today is referred to as the Gold Hill Fault zone (GHFz).  Today, researchers led by Dr. Jim Hibbard of North Carolina State University believe that the GHFz is a very important clue to the ancient history of today's Piedmont.  The rocks west of the zone have been radioactively dated to have formed some 613 million years ago (Ma), while those to the east date back only to about 540 Ma.

The Time Line*

To help understand what happened to create such different rocks, the Gold Hill Fault zone, and the interaction between them, it's helpful to see the time line of several important events.  Please understand that there is plenty of controversy over some of these dates and events, but they represent the results of a considerable body of research.

    ~630-610 Ma.  As an ancient ocean seafloor dove under (subducted) under another ancient seafloor, molten rock (magma) was created. A chain of volcanic islands was formed, spewing out billions of tons of volcanic debris.  Eventually, the source of magma dried up, the volcanos became extinct, and the island chain sank under the ocean.

    ~579-554 Ma.  The volcanic island arc (and all of its volcanic sediments) collided with either another volcanic island arc or a continent (geologists really don't know which), resulting in deformed rock layers of the island arc.  The layers were arched up and down in miles-long structures, and the rocks within were lightly changed (metamorphosed).

    ~550-530 Ma.  Another period of volcanism began on top of the old volcanic island arc deposits. Evidence of this volcanism is today very visible in the rocks and old mountains of the Uwharrie Mountains, from Asheville to just south of Morrow Mountain State Park.

    ~450 Ma.  The "new" volcanic island arc (with the old one underneath) collided with the ancient North American proto-continent.

For those standing at Little Buffalo Creek, this last event holds the key to the straightness of the creek.  Once again, the layers of rock were arched up and down in miles-long structures, and there was more light metamorphism.  The shove from the southeast and the resistance to the northwest created an "irresistable force vs. immovable object" situation, and something had to give.  The Gold Hill Fault zone formed and thrust the older rocks (the 613 Ma ones) up over the basement rocks of the proto-continent, and out from under and next to the 540 Ma rocks.  

A terrane is a region of the earth's crust, bounded by faults, that is clearly unrelated to the rocks around it, indicating that it has moved as a unit from elsewhere.  The region west of the GHFz is referred to as the Charlotte Terrane; those rocks east of the zone comprise the Carolina Terrane.  The fault zone is the boundary between them, and must have been the scene of intense heat, pressure, and shearing/breaking of rocks.  So... Where's the fault?

There is no visible "crack in the earth" like the San Andreas Fault in California, nor is there a dramatic, obvious change in topography like that of the Teton Range's Teton Fault in Wyoming. Like Dr. Laney, we must rely on our brains and powers of observation to figure out that there really was a major, active fault here, and that the rocks within the GHFz were extensively broken, stretched and fractured. Those streams, the springs, and the gold ore all still point to the presence of a fault zone.

So, why is the Gold Hill Fault zone a "good fault?"  The defining characteristic of virtually any type of fault is that it creates a chaotic place for rocks. They get broken, ground up, boiled, squeezed, etc, etc, all of which make easy routes of escape for hot liquids (called volatiles) from the heat and pressure in the zone.  Liquid water is a different critter within an active fault zone than what we're familiar with at the surface. Super-heated water dissolves and carries lots of minerals with it, including abundant quartz, plus some other minerals and elements, including some gold and copper (depending on the chemistry of the rocks in the zone). As the volatiles reach cooler areas under reduced pressures, the quartz and other minerals begin to precipitate out.  Without the mineral-concentrating activity of the fault zone hundreds of millions of years ago, there would be little or no gold here, and no village of Gold Hill today.

"For wherever the earth moves, metals are concentrated"
                                            Richard Fortey

Note: For other EarthCaches in the Gold Hill mining district, go here.


Logging Requirements:

To log this EarthCache, send me an e-mail (Not part of your log entry!) with the following:

1.  Make the first line of your e-mail:  "Gold Hill:  A Good Fault"

2.  Tell me how many are in your party.

3.  Send me answers to the following questions:

     a.  The GHFz has been described as "extinct", but some scientists think it was reactivated when South America/Africa slammed into North America, about 300 Ma.  This is a toughie, but try to answer it for me, and remember the straight creek/river, and the quote at the top of the write-up: Describe a scenario whereby the fault zone might once again become active.

     b.  You are a geologist in the field, and you have noticed that the rocks east of the fault zone and west of the fault zone are different from each other.  Between the two faults (that is, in the fault zone itself), would those rocks be different or the same as either those to the east or west?  Why?

4.  Please post a photo with your party on the bridge (watch for traffic).


Bibliography:

*The author thanks Phil Bradley, Senior Geologist, North Carolina Geological Survey, for his valuable corrections, additions and suggestions relative to this write-up.  Any mistakes herein, however, are solely the responsibility of the author.

The author thanks Vivian Hopkins, Vice President, The Historic Gold Hill and Mines Foundation, Inc., and Chair of the Foundation's History Committee.  She has been a tour guide, source of knowledge, and careful fact checker for the author.

Fortey, Richard. Earth, An Intimate History. Alfred A. Knopf, New York.  2004

Hibbard, J., et al.  "The Heart of Carolinia:  Stratigraphic and Tectonic Studies of the Carolina Terrane of Central North Carolina",  Geological Society of America, Southeastern Section Field Trip Guide, 2008.

Laney, F. B.  The Gold Hill Mining District.  Bulletin 21, North Carolina Geological and Economic Survey, Raleigh. 1910.

Meldahl, K. H.  Hard Road West.  University of Chicago Press, 2007.

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Last Updated: on 8/19/2017 7:31:57 PM Pacific Daylight Time (2:31 AM GMT)
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Coordinates are in the WGS84 datum

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