1. The main objective of this talk is to present evidence for a high-strain zone along the so-called Northey Hill Line in western New Hampshire, and briefly to speculate on the implications for structure in the underlying basement.
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2. The regional geology involves a sequence of thrust nappes that transported rocks westward and thickened the crust during the Acadian Orogeny. Bronson Hill rocks, in purple, were displaced west over the Vermont sequence, in blue; above that the Monadnock sequence and Bethlehem gneiss were displaced westward along the Brennan Hill thrust, Fall Mtn nappe, and Piermont allochthon. And finally the Central Maine sequence and Kinsman granite were emplaced above all that. This stacking of hot rocks onto cooler rocks resulted in an inverted metamorphic sequence.
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3. All of that was arched by the movement of pre-existing Ordovician domes (shown in black) in the latest Acadian, with attendant folds and foliation superposed on the nappe-stage structures. This is the Bronson Hill anticlinorium.
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4. Mesozoic normal faults further disrupted the map pattern.
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5. What can we say about the Northey Hill line?
* It is younger than the domes, and older than the Mesozoic faults. Perhaps Northfieldian?
* Its type locality is here in the north.
* It was mapped out to the south along the Silurian unconformity, and shown
* on Billings’ 1955 state map going all the way to the CT River.
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6. * Yet Lyons et al’s 1997 state map shows it only in the north.
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7. So let’s look first at the Northey Hill area, south of Littleton, NH. Here is the fault, bisecting an area of Littleton Formation (in blue).
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8. Here is Billings’ 1935 map, showing the Northey Hill fault displacing a stratigraphic syncline, with Littleton Formation in the center. Rocks are older both to the west and to the east. The Garnet Hill structure continues at least 70 km SW, and the Salmon Hill Brook structure plunges SW. * If these are truly synclines, they should plunge NE toward the younger rocks. * Let’s look at Billings’ line of cross section.
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9. We’ll zoom in on the cross section through Northey Hill. *
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10. Billings shows the fold as a syncline, which means the west side of the NHL had to have moved up. Note that he shows Dl on top of older units on the west limb. In fact, if the folds plunge SW, older units must be above Dl in the overturned limb of an early fold. The stratigraphic syncline must be antiformal. * This also means that the west side of the NHL moved down, not upwards as Billings showed it. *
11. * Here is my interpretation of his map, showing the antiformal synclines plunging SW. At this latitude rocks on both sides of the fault are at staurolite grade.
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12. Here is a typical sample of the Littleton from Northey Hill.
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13. Prior to displacement on the NHL the Garnet Hill structure had been intruded by the Bethlehem gneiss, perhaps in conjunction with the Fall Mountain nappe, bringing hotter rocks in from the east.
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14. Back in 1954, Baker had proposed the idea of the overturned limb of an early fold in his Masters thesis at Dartmouth. * The east-verging transport sense is at odds with Jim Thompson’s west-verging nappes in southern New Hampshire. I’m proposing that the Garnet Hill syncline lies below a west-verging nappe, * with the syncline opening downwards toward the west. The core of the anticline can be traced to the Cornish nappe farther south. Note that in this new interpretation, both faults - - the Mesozoic Ammonoosuc fault and the NHL - - are W side down.
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15. Now we’ll follow the NHL south. Here is the Garnet Hill antiform, cored by Littleton Formation and Bethlehem gneiss. The NHL was mapped by Billings and Hadley more or less along the western contact of the Littleton, the Silurian unconformity, which is highly attenuated, I think because we are on the overturned limb of the Cornish nappe - - and as Don Wise taught us to say, “overturned limbs catch hell”. But a zone of late, vertical foliation * up to a mile or more wide parallels the line. Rumble called this the Sunday Mountain cleavage belt. It overprints nappe-stage and dome-stage foliations.
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16. The late foliation is typically a pervasive, fine-grained cleavage that strikes NNE, with very steep to vertical dips. It transposes and locally obliterates older foliations.
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17. We move now a bit south to the area of Orfordville, still between the Cornish nappe and the Bronson Hill anticlinorium. The next slide shows a map by Dan Orange of the area inside this box.
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18. Orange made a detailed petrologic study of mineral assemblages across this part of the orogen for his Master’s thesis at MIT, working with Frank Spear. The NHL follows along the contact between Partridge and Littleton Formations, with a few lenses of Fitch or Clough along the way. The Sunday Mountain cleavage belt is shown again in yellow, where dominant foliation is nearly vertical.
19. Orange mapped details of the NHL where it is well exposed near Orfordville. * Here is an overlay showing just the map units. Note the repetition of units in an east-facing stratigraphy: Partridge, Clough, Fitch; Clough, Fitch, Littleton; Partridge, Fitch, Littleton. So we have repetitions of stratigraphy, and discrete fault surfaces within this zone - - it’s not simply a matter of an attenuated Silurian unconformity. Rumble noted rotated garnets in the Littleton at this locality, which indicate west side down shear.
20. Now back to Orange’s bigger map. In the cleavage belt * pelitic rocks may contain garnet, although where the cleavage is best developed they are very fine-grained, almost slaty rocks. * On either side the rocks also have staurolite. * Farther east they are at sillimanite grade. Orange made detailed petrologic studies of samples from six locations across the area, shown here as red squares, one right from the NHL, 3 locations to the W and 2 to the E. *
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21. Here he has plotted each sample location with a corresponding PTt path diagram on a cross section. (More about the cross section in a moment.) Note that PTt paths in the west are clockwise, initially moving toward higher P and moving toward hotter T, presumably due to loading of hot rocks above, and then relaxing toward lower pressures and temperatures. Those to the E are also clockwise, but lack the initial P increase. In contrast, the NHL sample shows only decompression. Orange concluded that the NHL “may be a zone of intense ductile deformation with a large normal component”.
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22. Now consider the cross section. Note the Garnet Hill syncline shown opening upwards. I think this dragon’s head for the Cornish nappe is incorrect * and I think the Littleton must connect through here *.
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23. A more reasonable cross section would have the Cornish nappe rooted above the Garnet Hill syncline, not below. It is exposed at the earth’s surface because the west side of the NHL dropped down.
24. Still farther south, this is the area of Lyons’ map that I have been remapping in recent years. Here is my map at the same scale* - - thanks to Greg Barker at the NH Survey for digitizing the results. Here is the Lebanon dome, draped by the overturned limb of the Cornish nappe, which may be allochthonous above the Monroe thrust. The upper limb here continues into Vermont and truncates against the Ammonoosuc fault, near one of Orange’s sample sites. The staurolite isograd trends diagonally across the area, staying west of the Lebanon dome. It does not encircle the dome, as shown on previous maps, and has nothing to do with contact metamorphism against the older igneous rocks of the dome. Sinistral shear along the Northey Hill line is indicated by en echelon stepping of faults, for example here south across Hardy Hill.
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25. Here is a blow-up of that area, and my drafted map * with yet another color scheme - - please bear with me. Littleton is in gray. The Clough Quartzite, in red, outlines a nappe-stage isoclinal fold, deformed by dextral dome-stage folds, and offset by a series of sinistral faults. Rocks on the west moved relatively down and toward the south.
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26. Next we’ll zoom in to a small area in Lyme.
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27. On the west limb of the Garnet Hill syncline, we see that foliation is steeper * than overturned bedding, whereas on the east limb bedding is steeper * than foliation. In fact, this is where I first got the idea that the syncline closes upwards,
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28. . . . because if you look at the cross section view, if the foliation formed axial planar to the fold, the fold has to be an antiform. * This is consistent with the rotation sense of the Jacobs Brook syncline that Rumble mapped farther north.
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29. We’ll zoom in now to the area of that fold near Smarts Mountain. The Littleton-Clough contact is folded in a minor recumbent fold. In cross-section view we see that it appears to be a minor fold on the lower limb of an antiform.
30. Back to the bigger picture. * It seems likely that the NHL continues SW somehow along the dome-stage Meriden antiform, but the details need to be reconciled with recent mapping in that area. *
31. Does the Northey Hill line then continue into southern Vermont as the Westminster West fault system? * I think it may. Both are sinistral, with late, steep foliation. Late muscovite from the foliation in Vermont has been dated at about 300 Ma. Maybe it’s time for some Ar/Ar work along the Northey Hill line in NH? *
32. And finally, what do these structures suggest about the suture between Laurentia and Gander? Although the Red Indian line may well be at the earth’s surface west of the Moretown slice, I think we need to pay attention to the concentration of late Paleozoic and Mesozoic faults along the Connecticut River as an indication of a deep-seated suture.
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33. The poster for this meeting portrays this idea very nicely!
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34. The Red Indian Line at depth would be here, east of the Chester dome. * Its position at the surface is likely farther west in Vermont, but that’s a topic for others to address. Thank you.