Aseismic Ridge Subduction As a Driver for the Ordovician Taconic Orogeny and Utica Foreland Basin in New England and New York State

OLD G

The Geological Society of America Special Paper 540 OPEN ACCESS

Aseismic ridge subduction as a driver for the Ordovician Taconic orogeny and Utica foreland basin in New England and New York State

Robert D. Jacobi EQT Production, EQT Plaza, 625 Liberty Avenue, Suite 1700, Pittsburgh, Pennsylvania 15222, USA, and Department of Geology, University at Buffalo, SUNY, 126 Cooke Hall, Buffalo, New York 14260, USA

Charles Mitchell Department of Geology, University at Buffalo, SUNY, 126 Cooke Hall, Buffalo, New York 14260, USA

ABSTRACT

Aseismic ridge subduction is common along modern convergent margins. We enumerate six criteria that can be used to recognize aseismic ridge subduction in orogens, including a magmatic gap with uplift followed by bimodal volcanism, which commonly includes explosive, voluminous rhyodacitic volcanism that erupts far from the trench. Features temporally linked with the explosive volcanism include retroarc thrusts and consequent thrust-loaded retroarc foreland basin development. Using these criteria to examine features of the Taconic orogen, together with new stratigraphic and structural data from the Utica basin that constrain the basin subsidence architecture and thrust timing, we propose that at least the older units of the 456–435 Ma Oliverian Plutonic Suite in New England were generated during steepening of the downgoing slab after passage of a subducting aseismic ridge. Weak- ened crust from delamination and decompression melting promoted westerly directed thrusts (present-day coordinates) that loaded the Taconic retroarc foreland. The resulting Utica basin subsided rapidly and nearly synchronously over an ~150-km- wide region and contains interbedded 453–451 Ma ash layers from the Oliverian Plu- tonic Suite or coeval plutons to the south. This history of basin subsidence indicates that the major thrust loads that drove development of the Utica basin were emplaced over a similarly brief interval beginning ca. 455 Ma. Thus, the Taconic thrusts, the Utica basin, the volcanic ashes, and the early Oliverian felsic magmatic units could all be related to an aseismic ridge subduction event. Because of the ubiquity of seamount chains, we expect that aseismic ridge subduction affected other segments of the Taconic orogen.

Jacobi, R.D., and Mitchell, C., 2018, Aseismic ridge subduction as a driver for the Ordovician Taconic orogeny and Utica foreland basin in New England and New York State, in Ingersoll, R.V., Lawton, T.F., and Graham, S.A., eds., Tectonics, Sedimentary Basins, and Provenance: A Celebration of William R. Dickinson’s Career: Geological Society of America Special Paper 540, p. 617–659, https://doi.org/10.1130/2018.2540(27).

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INTRODUCTION metasedimentary unit of the Shelburne Falls arc (Macdonald et al., 2014, 2017; Karabinos et al., 2017). The Rowe belt micro- Subduction of aseismic and seismic ridges is a natural result continent was separated from Laurentia by a relatively narrow of closure of an oceanic basin. As used here, an aseismic ridge oceanic tract, the “Taconic seaway.” refers to a seamount chain, ridge, or oceanic plateau located away Closure of the Iapetus Ocean at ca. 475 Ma led to the sub- from a spreading center, and a seismic ridge refers to a seafl oor- sequent closure of the Taconic seaway, and early Taconic thrusts spreading center (e.g., Vogt, 1973; McCann and Sykes, 1984). began forming (Macdonald et al., 2014, 2017; Karabinos et al., The consequent dynamics of aseismic ridge subduction (which 2017) in response to crustal shortening or subduction or both. include fl at-slab subduction, arc jumps, and, late in the process, Following these events, breakoff of the eastwardly subducted continental melting from slab steepening with asthenospheric oceanic slab at ca. 466 Ma and attendant asthenospheric upwell- upwelling) can be found, for example, along the western margin ing resulted in (1) late-stage mafi c rocks with suprasubduction- of South America, where aseismic ridge subduction is currently zone signatures that intruded the remnant forearc of the Shel- taking place and has likely affected much of the margin at some burne Falls arc (Kim et al., 2003; Coish et al., 2015), and (2) the point during the Cenozoic (e.g., Ramos and Folguera, 2009). It is 466.0 ± 0.1 Ma felsic units in the Barnard Volcanic Member of probable that, locally, ocean basin closure can involve strike-slip the Missisquoi Formation (a correlative of the Hawley Formation tectonics (e.g., Waldron et al., 2014), but ultimately ridge subduc- in the Shelburne Falls arc) and time-equivalent volcanic ashes in tion must occur during complete closure of an ocean basin. the Indian River Formation located in the Taconic thrust slices The northern Appalachian orogen of the United States and (Karabinos et al., 2017; Macdonald et al., 2017). At about the Canada has been a laboratory for the development and revision of time of slab breakoff, the bimodal Ammonoosuc Volcanics devel- plate-tectonic models, beginning with the seminal papers of Bird oped (ca. 469–461 Ma) in the Bronson Hill arc, which lies east of and Dewey (1970) and Dewey and Bird (1971), and continuing the Shelburne Falls arc (Fig. 1A). The origin of the Ammonoosuc to the present (e.g., Macdonald et al., 2014, 2017; van Staal et al., Volcanics is unclear in these models, but may have been related 2016; Karabinos et al., 2017). to slab breakoff or subduction polarity reversal. This paper reviews tectonic models for the Taconic orogeny After closure of the Taconic seaway, a new, west-dipping and elements of aseismic ridge subduction. For the effects of subduction zone (ca. 456–442 Ma) developed beneath the com- seismic ridge subduction, see, for example, Bradley et al. (1993), posite Laurentian margin. A second phase of magmatism in the Bourgois et al. (1996), Santosh and Kusky (2010), Tang et al. Bronson Hill arc, the Quimby sequence (which includes the (2010), Seton et al. (2015), and van Staal et al. (2016). We pres- Quimby Formation and the ca. 456–435 Ma Oliverian and High- ent new data from the Taconic foreland basin bearing on potential landcroft plutonic suites), was generated over the west-dipping elements of aseismic ridge subduction. These new stratigraphic subduction zone. The Quimby Formation includes bimodal vol- and structural data are from the Utica black shale and strati- canics with a 443 ± 4 Ma age (Moench and Aleinikoff, 2003) graphically higher, coarser clastic units in the Mohawk Valley or ca. 455 Ma age (Karabinos et al., 2017). The black shales of of New York State. We also examine other potential elements of the Utica Group, which are dated at ca. 453–450 Ma, based on Taconic ridge subduction such as the alkalic/rhyodacitic units in U-Pb zircon geochronology (Sell et al., 2013, 2015; Macdonald the Bronson Hill arc. Finally, we modify Taconic tectonic models et al., 2017), were deposited during this orogenic phase. The fi nal for the Appalachian orogen in New England and New York State emplacement of the Taconic thrusts also occurred at this time. by incorporating these aseismic ridge subduction elements. The resulting thrust loads and dynamic effects associated with the downgoing slab caused subsidence of the Utica foreland Recent Taconic Plate-Tectonic Models basin (e.g., Macdonald et al., 2014, 2017). This tectonic model, which involves (1) Middle Ordovician The most recent Taconic tectonic models for western New collision of a Gondwanan arc above an east-dipping slab, (2) lim- England rely heavily on advances in detrital zircon data and ited or no eastward subduction beneath a Laurentian microcon- radiometric dates (Macdonald et al., 2014, 2017; Karabinos et tinent, and (3) younger westward subduction beneath a compos- al., 2017; for details of these and older models, see Appendix A). ite Laurentian margin, is consistent with (1) proposals for Late In these recent models, the Shelburne Falls arc initiated on the Ordovician westward subduction at ca. 450 Ma farther south peri-Gondwanan Moretown terrane at ca. 502 Ma above an east- in Connecticut (Sevigny and Hanson, 1993, 1995; Walsh et al., dipping subduction zone on the east side of the Iapetus Ocean 2004; Aleinikoff et al., 2007; Chu et al., 2016), (2) the discovery (present-day coordinates), away from the infl uence of Laurentia of juvenile input to arc detritus in the Utica retroarc foreland from ε (Fig. 1A; Macdonald et al., 2014, 2017; Karabinos et al., 2017). Nd studies (Macdonald et al., 2017), and (3) tilting of Laurentia As Iapetus closed, the arc migrated toward a microcontinent (Coakley and Gurnis, 1995). of Laurentian affi nity that lay on the west side of Iapetus. This In the west-following-east subduction models, the age of microcontinent, represented by the Rowe belt and possibly the fi nal emplacement of the Taconic allochthon and the age of sub- Green Mountain massif, provided zircon-bearing detritus to the sidence of the Utica basin demand that these events occurred in Hawley Formation, which is interpreted to be a metavolcanic and a retroarc position, continent-ward of the Bronson Hill arc. The

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questions we address here are the following: (1) What driving This rate of convergence is consistent with modern convergence mechanism caused these events to occur in a retroarc location? rates. Bradley and Kusky (1986) and Bradley and Kidd (1991) (2) What is the relationship among the retroarc thrusts, Utica pointed out that the cumulative throw on the Utica basin nor- basin subsidence, K-bentonites, and the Oliverian Plutonic Suite? mal faults in the Mohawk Valley and the St. Lawrence Lowlands We suggest that aseismic ridge subduction and its effects should of Quebec increases eastward toward the Taconic thrust front, be considered as a causal mechanism for all four of these ele- and they proposed that this increase was similar in magnitude to ments. Further, we suggest that aseismic ridge subduction should those in modern convergent margins. The Utica basin, thus, was be tested and incorporated into other models and phases of the thought to be consistent with eastward subduction and to repre- Taconic orogeny that are reviewed in Appendix A. Subduction sent a trench fi ll that spilled out over the Laurentian margin dur- of both seismic and aseismic ridges has been incorporated pre- ing fi nal collision. We provide new evidence below that indicates viously into Taconic tectonic models for the Canadian Appa- the Utica basin is not an usual convergent-trench, pro-foreland lachians and Maine, albeit without a fl at-slab component (see basin. These new data are consistent, however, with retroarc fore- Appendix A for details). land basin tectonics resulting from ridge subduction.

Utica Foreland Basin Elements of Aseismic Ridge Subduction and Flat-Slab Subduction The Utica foreland basin (Vermontian phase; sensu Rodgers, 1971), which was the site of deposition of the Utica Group, is Our review of the tectonic features and geological history best exposed in the Mohawk Valley of New York State (Fig. 2). of the Caribbean Antilles, Middle America Trench, and Andean There, the 453–450 Ma black shales overstep westerly the Cam- margin (Appendix B) indicates that aseismic ridge subduction brian–Ordovician “great American carbonate bank” of the Lau- beneath island arcs and convergent continental margins typi- rentian margin (e.g., Landing, 2012; see also Macdonald et al., cally results in some or all of the following spatial and tempo- 2017, their fi gure 2). In models in which Laurentia was part of ral characteristics: the subducting lower plate, as its Iapetan margin approached and (1a) uplift of the accretionary prism (with consequent uncon- entered an east-dipping subduction zone during Utica time, the formities) as the buoyant aseismic ridge on the down- Utica basin was essentially the trench, or more specifi cally, the going oceanic slab passes beneath the upper plate, and outer wall of the trench, i.e., a proforeland basin (Jacobi, 1981; (1b) signifi cantly more deformation in the accretionary prism Rowley and Kidd, 1981; Bradley and Kidd, 1991; following the where the aseismic ridge intersects the prism, with a pos- classifi cation of Ingersoll, 2012). In contrast, in models with a sible trail of ocean-island basalt fragments ripped from west-dipping subduction zone east of the Bronson Hill arc, the the aseismic ridge and incorporated into the prism; Utica basin was a retroarc foreland basin that developed on the (2) fl at-slab to moderately dipping subduction of the buoy- overriding plate (e.g., Macdonald et al., 2014, 2017; following ant, aseismic ridge–bearing plate segment; the classifi cation of Ingersoll, 1988, 2012). Both sets of models (3) signifi cantly reduced volcanic activity in the original arc are compatible with observed NNE/NE-striking normal faults (“magmatic gap” or “volcanic gap”) above the fl at slab; west of the Taconic thrusts (Fig. 2) that were active both before (4) signifi cant uplift of parts of the upper plate (during initial and during deposition of the Utica Shale; these faults record steepening of the downgoing slab after the ridge passes, 1.2 km of down-to-the-east subsidence, based on cumulative as well as during fl at-slab subduction in some areas); throw (Bradley and Kidd, 1991), or ~0.5 km, based on subsid- (5a) continental delamination with alkalic/rhyolitic volcanism ence curves (Macdonald et al., 2017). For details concerning that displays continental partial-melt signatures during the reactivation history of these faults, see Jacobi and Mitchell slab steepening after fl at-slab subduction, and (2002) and Jacobi (2010, 2011). (5b) alkalic/rhyolitic volcanism associated with large calderas In the tectonic models with an east-dipping subduction zone, that can be relatively far-removed (~550–800 km) from the westward advance of the Utica black shale over the Ordovi- the trench; and cian carbonate bank should record the passage of the Laurentian (6) increased retroarc foreland thrusting of both base- continental margin over the peripheral bulge and into the trench, ment and sedimentary cover resulting from an interplay since the Utica black shale was thought to represent trench depo- between convergence rate and crustal weakening from sition (e.g., Rowley and Kidd, 1981; Bradley and Kidd, 1991). heating and delamination (Fig. 4). Indeed, Bradley and Kusky (1986) proposed that the relative convergence rate could be calculated from the black shale overlap. POTENTIAL TACONIC FLAT-SLAB On the basis of limited graptolite localities in the black shale, SUBDUCTION ELEMENTS Bradley and Kusky (1986) calculated an approximate convergence rate of 2–3 cm/yr, assuming a steady convergence rate Several Taconic structural, stratigraphic, and magmatic fea- marked by the apparent constant rate at which the Utica black tures in New England and New York State potentially represent shale onlapped the subsiding Laurentian carbonate bank (Fig. 3). some of the aseismic ridge elements listed here (and described in

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46 ° 48 ° + + + ° + + 47 + + + 45 + ° 72 ° 70 + W ° + ° N + + + + W + + 767 ° + ONT 4 + QUE 74

V V V V QUE V + V V V NY + V V LOGAN’S LINE + + V + V + + V BAIE VERTE-BROMPTON LINE + + + V + + QU QUE + E VT ME R V BAIE VERTE OCEANIC ADIRONDACK DOME NDA, Pr B + ASCOT A + V Po + + Bc + + QUE + Cb Lf V NH H B’ + ROWE FM + Hr + + + CL H POP A V + HM BALM + + + + + H + + + H + + MORETOWN FM + + + + + + V + + O + + Sm + + B’’ CH A’ V GM + + + + V + + + + TACONIC ALLOCHTHON + + + + + + + + + POP ARC (VS) + + + + + + + MEDUCTIC + ME + NB + + + + BK + + + + + NH + + MA + PN + + + + + G + + + + + + + MA GANDERIA SHELBURNE FALLS ARC CT N + + BRONSON HILL ARC AVALONIA

RI + MA +

+ 100 km

Figure 1 (Continued on following pages). Taconic lithotectonic units involved in the Taconic orogeny and other selected units. (A) New England and Parts of Maritime Canada. Background base for the entire map is after Hibbard et al. (2006). Individual unit tectonic assignments for the Maine and Maritime Canada region generally are after van Staal et al. (2016). Individual unit tectonic assignments for New England generally are after Kara- binos et al. (2017) and locally Moench and Aleinikoff (2003). Black circles with annotations indicate approximate waypoints on cross sections in Figure 3 (A, A′) and Figure 5 (B, B′, B″). Bc—Buttermilk Creek fault, BK—Berkshire massif, Cb—City Brook fault, CH—Chickwolnepy intrusions, CL—Chain Lakes massif, CT—Connecticut, E. BDY-BH ARC (K. ET AL)—eastern approximate boundary of the Bronson Hill arc (Karabinos et al., 2017), G—Neoproterozoic Ganderian basement, GM—Green Mountain massif, H—Highlandcroft Plutonic Suite, HM—Hurricane Mountain mélange, Hr—Herkimer fault, Lf—Little Falls fault, MA—Massachusetts, ME—Maine, NB—New Brunswick, NDA—Notre Dame arc, NH— New Hampshire, NY—New York, O—Oliverian Plutonic Suite, ONT—Ontario, PN—Penobscot arc, POP ARC (VS) BALMORAL—Popelogan arc, Balmoral phase (van Staal et al., 2016), POP ARC (VS) MEDUCTIC—Popelogan arc, Meductic phase (van Staal et al., 2016), Po—Poland fault, Pr—Prospect fault, QUE—Quebec, Red Indian Line (E.G., VS)—location of Red Indian Line (e.g., van Staal et al., 2016), RI—Rhode Island, Sm—Saratoga-McGregor fault, VT—Vermont. (B) Newfoundland. Background base for the entire map is after Hibbard et al. (2006). Individual unit tectonic assignments generally are after van Staal et al. (1998) and Zagorevski et al. (2008, 2012). Regional tectonic divisions are after van Staal et al. (1998) and Karabinos et al. (2017). ANNIE AC—Annieopsquotch accretionary tract, BBL—Baie Verte–Brompton Line, GRUB—Gander River ultrabasic (or ultramaﬁ c) belt; NDA—Notre Dame arc, PN—Penobscot arc, PEN ARC—Penobscot arc, VIC ARC SED—Victoria arc–related sediments, VIC ARC/BACKARC—Victoria arc and backarc.

° + + + + + + 50 + 70 ° LAURENTIA/PERI-LAURENTIA + 49 N + + + ° 9° W + + LOGAN’S LINE ° 68 Siluro-Devonian sediments + + 66 Taconic Allochthon

Ordovician Foreland Basin (e.g., Utica) BAIE VERTE OCEANIC

NOTRE DAME ARC Lower-Middle Ordovician QUE Hurricane Mtn melange NB Cambro-Ordovician slope and rise RED INDIAN LINE (E.G., VS) Cambro-Ordovician (?) Rowe Fm.

G + + Cambro-Ordovician Shelf TETAGOUCHE ENSIMATIC + OP ARC (VS) ALMORAL ~ E. BDY-BH ARC + + Precambrian + (K. ET AL) + + + + + + + 48 + + + + + + ° + N + + SHELBURNE FALLS & BRONSON HILL ARCS + + TETAGOUCHE ENSIALIC H Highlandcroft Suite + + + + + + + + Oliverian Plutonic Suite + + + O + + + + + + Bronson Hill Arc + + + 47 Popelogan Arc (Balmoral Phase) + B ° N + + + + PENOBSCOT ARC + Notre Dame Arc + + + + + + + + + + + GANDERIA Shelburne Falls Arc (Hawley Fm) + G + G G AVALONIA NB Moretown Fm NS 46 + + ° N Baie Verte oceanic tract

+ + GANDERIA MARGIN “Popelogan” Arc (Meductic Phase) + MEGUMA 45 ° N Tetagouche Arc/Back-arc: ensimatic

Tetagouche Arc/Back-arc: ensialic

Ganderia margin sediments 44 ° N PN Penobscot Arc

G Ganderia basement

MISCELLANEOUS

Normal fault

V Thrust fault

+ Upper Paleozoic plutons

Avalonia

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58° W

57° W V

55° W 51° N 54° W 51° N B V + N + + + + LAURENTIAN MARGIN + + + 59° W

50° N + + V + BAIE VERTE-BROMPTON LINE (BBL) + V Summerford Volcanics (red) + NDA Dunnage Melange (purple) + RED INDIAN LINE + DASHWOODS GRUB

V V DOG BAY LINE V V

VERTE OCEANIC V V 49° N BAIE ED

V V V DOVER FAULT PEN ARC

VIC ARC S V V

V C

A V V ANNIE EXPLOITS

+ W 53°

NDA V

ARC V + V PEN V PEN ARC

VIC ARC/BACKARC

NOTRE ARC DAME LAURENTIA

BBL NDA

CAPE RAY FAULT GANDERIA 47° N AVALONIA 100 km

LAURENTIA/PERI-LAURENTIA W of RED INDIAN LINE E of RED INDIAN LINE

Overlying black shale Taconic Allochthons Dunnage Melange and graywacke

Cambro-Ordovician slope and rise Notre Dame Arc Victoria Arc/Backarc sediments

Annieopsquotch Cambro-Ordovician Shelf Victoria Arc/Backarc Accretionary Tract

+ Precambrian Baie Verte Oceanic Tract Penobscot Arc/Backarc

Gander margin sediments

Figure 1 (Continued).

Figure 2. General geology of the Mohawk “Mohawk Valley” fault trends (~) Valley region, New York State. Geology Possible fault trends ADIRONDACK is generally after U.S. Geological Survey, Western extent of intense Taconic deformation DOME Mineral Resources, Online Spatial Data, In- m Taconic melange teractive Map for Conterminous U.S. (U.S. Geological Survey, 2016), with contribu- Bc Cb tions after Fisher (1980), Kidd et al. (1995), B aga L. Bradley and Kidd (1991), and Landing et Hk 87 Do Eph cand L al. (2003). Faults are modiﬁ ed from Fisher Man E-S-A G- B’ L-F G. Sa (1980), Bradley and Kidd (1991), Kidd A SS et al. (1995), Hayman and Kidd (2002a, HR M-C LF 2002b), Cross (2004), Cross et al. (2004), ° 43° N 43 N Fo T-H Agle et al. (2006), Jacobi and Agle (2008),

and generally follow those in O’Hara et al.

° AM (2017). Possible faults (indicated by dashed Mo m

FP hawk R. 90 74° 75 Ho B’’ outline with semitransparent ﬁ ll) are modi- CN L R m ﬁ ed from Jacobi (2002) and are based pri-

Sp No marily on lineaments, several of which are SC coincident with known faults to the north. A’ White arrows and white circles with anno- Q Significant cover 88 PL S-M tations refer to approximate waypoints on Devonian cross sections in Figure 3 (A, B′, B″, A′) Silurian and Figure 5 (B, B′, B″). White dotted line Lorraine/ is approximate location of cross section O ALBANY Schenectady m in Figure 7. Red bull’s-eye indicates loca- O Utica Group N 87 tion of the ﬁ eld site with the westernmost carbonates + C-O Galway R. thrust zone (displayed in Figs. 8 and 9). Red C Potsdam and yellow star indicates location of Utica 10 km core 75NY2, discussed in the text. AM— Precambrian Hudson S-R/S-G-H m Amsterdam, Bc—Buttermilk Creek fault, TACONIC ALLOCHTHON Cb—City Brook fault, CN—Canajoharie, Do—Dolgeville fault, Eph—Ephrata fault, E-S-A—East Stone Arabia fault, Fo—Fonda fault, FP—Fort Plain, G-L—Galway Lake fault, G. Sacandaga L.—Great Sacandaga Lake, Hk— Herkimer fault, HR—Herkimer, Ho— Hoffmans fault, LF—Little Falls, L-F—Little Falls fault, Man—Manheim fault, M-C—Mother Creek fault, No—Noses fault, PL—location of Figure 7 cross section based on Plesch (1994), RL—Ruedemann’s Line, Sp—Sprakers fault, SC— Schenectady, S-M—Saratoga-McGregor fault, SS—Saratoga Springs, T-H—Tribes Hill fault. Legend: Q—Quaternary, O—Ordovician, C— Cambrian. Inset of New York State shows location of main map.

detail in Appendix B). However, these elements do not all have relatively local. Next, we review Taconic components that may the same probability of being observed in the regional and local refl ect other elements of fl at-slab subduction. geology of New England and New York State. For example, element 1a (uplift of the accretionary prism as the buoyant aseismic Utica Retroarc Foreland Basin Development ridge passes, with consequent unconformities) will be diffi cult to recognize in New England outcrops since the prism commonly For tectonic models with westward subduction at the time lowers after the ridge has passed (see Appendix B). of Utica deposition (453–450 Ma), west-directed Taconic thrusts Element 1b (signifi cantly more deformation in the accre- and the development of the Utica retroarc foreland basin might tionary prism where the aseismic ridge intersects the prism) was locally represent fl at-slab Element 6 (retroarc thrusting and proposed as an explanation for the Taconic Dunnage Mélange in thrust-loaded basin development). As described in Appendix B, Newfoundland, but more recent geochemistry of intrusions into retroarc thrusting and consequent basin development are gener- the Dunnage unit suggests that the mélange marks the passage of ally thought to be a result of lithosphere weakening combined a seismic ridge during subduction (see Appendix A). The Hur- with a fast relative convergence rate. ricane mélange in Maine also has been ascribed to seismic ridge subduction, rather than aseismic ridge subduction (Schoonmaker Foreland Basin Subsidence: Timing and Geometry and Kidd, 2006; see Appendix A). Element 4 (signifi cant uplift Fast overstep of basal Utica Shale. In the twentieth century, of parts of the upper [overriding] plate) should be recognized Appalachian geologists generally thought that the base of the as unconformities associated with reverse faults and (commonly) Utica Shale gradually overlapped the Laurentian margin carbon- subaerial deposition. Because of the number of tectonic and ate bank (e.g., Ruedemann, 1912; Kay, 1943; Kay and Colbert, metamorphic events overprinted in New England, it will be dif- 1965). Such a scenario was consistent with eastward subduction fi cult to identify this element, especially if the uplifted region was models in which the Laurentian plate margin steadily passed

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WNW ESE

A B′ B″ A′THRUST FRONT Bradley and Kidd (1991) (recalculated) PT/TR 448 Bradley and Kusky (1984) (recalculated) D. com This paper P. man 450 Bradley and Kusky (1984) (original) G. pyg M/R D. spin 452 O. rued

AGE (MA) C. amer SK WH D. mult 454 1126 8 11 VV PawletPaw D P H N S-M GB HK 456 Mt Marinono (C. bicorn)

N. grac 458 PT 140 120 100 80 60 40 20 20 40

CROSS-STRIKE DISTANCE (KM) 90 to BHA-PT

Figure 3. Time-distance plot of the Utica basal contact across the Utica foreland basin in the Mohawk Valley region, New York State. Section is oriented approximately orthogonal to the fault strike. Cross-section end-points A and A′ and intermediate waypoints B′ and B″ are shown on Figures 1 and 2. Figure base is modiﬁ ed from Bradley and Kusky (1986), Rowley and Kidd (1981), and Bradley and Kidd (1991). Radiometric ages for graptolite zone boundaries and new graptolite localities are from Macdonald et al. (2017). In the original interpretation (gray solid circles and gray dashed line), the Utica base appeared to steadily overlap westwardly, based on three outcrops and poorly constrained radiometric ages for graptolite zones (Bradley and Kusky, 1986). The Utica base curves that are recalculated with modern radiometric ages for the graptolite zones are shown for both Bradley and Kusky (1986; gray open circles and gray dotted line) and Bradley and Kidd (1991; gray solid line). The recalculated curves broadly agree with the new Utica basal contact data (black circles with +, and dashed black line) and imply a fast transgression across 100 km west of the Taconic thrust front. Numbers near circles with black outline indicate the section number in Figure 5. Thin vertical lines with boxes indicate the locations of faults: D—Dolgeville fault, H—Hoffmans fault, Hk— Herkimer fault, N—Noses fault, P—Prospect fault, S-M—Saratoga-McGregor fault. Graptolite zones (from base): N. grac— Nemagraptus gracilis, D. mult—Diplograptus multidens, C. bicorn—Climacograptus bicornis, C. amer— Corynoides americanus, O. rued—Orthograptus ruedemanni, D. spin—Diplacanthograptus spiniferus, G. pyg—Geniculograptus pygmaeus, P. man—Paraorthograptus manitoulinensis, D. com—Dicellograptus complanatus. BHA-PT—Bronson Hill arc with exposures of the Partridge Formation; GB—graptolite age of Pawlet and underlying Mount Merino Formations in the Giddings Brook slice of the Taconic allochthon (Berry, 1962; Riva, 1974 in Rowley and Kidd, 1981); MK—graptolite age of assumed matrix shale in the Moordener Kill mélange (Berry, 1962; Berry in Zen, 1967; Berry 1977; Bradley and Kusky, 1986; see Appendix D for discussion of age assignment), and olistoliths in a nearby mélange at Rysedorph Hill of the same age (Zen, 1967; Bradley and Kusky, 1986); PT—graptolite age of the Partridge Formation (Harwood and Berry, 1967; Moench and Aleinikoff, 2003); PT/TR—radiometric date of Partridge metarhyo- lite tuffs (Tucker and Robinson, 1990); SK—possible gastropod age (“Trenton”) associated with pillow lavas at Stark’s Knob (Landing et al., 2003); VV—“Vermont Valley” autochthonous section of Walloomsac and overlying Austin Glen Formation structurally below the Whipstock Hill mélange east of the Taconic allochthon and west of the Green Mountain massif (Thompson, 1967; Potter, 1972—both in Rowley and Kidd, 1981); WH—graptolite age of assumed matrix in the Whipstock Hill mélange (Rickard and Fisher, 1973; Bradley and Kusky, 1986).

over the peripheral bulge and into the trench under a constant margin—covering ~100 km in less than 200,000 yr, which is supply of trench-related muds (e.g., Jacobi, 1981; Rowley and essentially instantaneously within the limits of the chronostrati- Kidd, 1981; Bradley and Kusky, 1986; Bradley and Kidd, 1991). graphic resolution. This rapid transgression took place in latest However, recent work that integrated biostratigraphy (grapto- Climacograptus bicornis Zone time, immediately before the lites), lithostratigraphy, and tephrochronology in measured out- Corynoides americanus Zone (Fig. 3). The age of this transgres- crops and core (Fig. 3) has demonstrated that the depositional sion is ca. 452.7 Ma, bracketed by the Millbrig K-bentonite at front of the black shale raced across this part of the carbonate 452.86 ± 0.29 Ma in the upper part of the underlying Black River

PRINCIPAL CORDILLERA traced through the basal Utica Shale as far west as Canajoha- 20 - 9 Ma rie, New York (Figs. 2 and 5). Across this region, the basal Utica strata and the subjacent Trenton Group exhibit a series of fault- bounded wedges that each thicken to the west. This pattern is 160 km to trench 0.33 m/k.y. 0.17 m/k.y. most clearly evidenced in the basal Utica strata by tracing the FRONTAL CORDILLERA Ridge base of C. americanus Zone, the Sherman Falls K-bentonite bed collision (Fig. 5, ash #1), and the Kuyahoora K-bentonite bed (Fig. 5, ash 9 - 6 Ma #2; both were named by Brett and Baird [2002] but correspond to geochemically fi ngerprinted beds correlated in Mitchell et al. 240 km to trench 0.77 - 0.95 m/k.y. 0.35 m/k.y. [1994]). These westward-thickening wedges are bounded on the west by the Hoffmans, Noses, Little Falls, and Poland faults. PRECORDILLERA The wedges suggest the presence of a back-rotated or listric fault system (accompanied by subsidiary, antithetic step-downs) as 5- 2 Ma the dominant architecture of the Mohawk Valley fault system. The architecture of back-rotated fault blocks is consistent with 300 km to trench 0.58 m/k.y. 0.19 mm/k.y. proposed west-dipping paleoslopes in the Utica Shale based on slump fold orientations (Jacobi and Mitchell, 2002). Figure 4. Development of the Aconcagua fold-and-thrust belt (incorpo- This pattern of listric fault–driven subsidence continues west rating three Cordillera thrust elements) and the resulting thrust- loaded of the Little Falls fault, where the lower Utica Shale grades rap- retroarc foreland basin in the Central Andes at 32°S. The thrusting idly into the dominantly carbonate mudstone strata of the middle phases are related to passage of the Juan Fernandez aseismic ridge un- Trenton Group (Poland and Russia Limestones; see Mitchell der the Andean margin on the downgoing Nazca plate (the “Pampean” or “Chilean” fl at-slab segment). Figure is after Ramos and Folguera et al., 1994; Brett and Baird, 2002; Brett et al., 2004). At Wolf (2009), with modifi cations from Hilley et al. (2004). Crustal thrust un- Hollow Creek (Fig. 5, column 14), in the eastern portion of this der the Precordillera (gray with dotted black outline) was proposed in wedge, the High Falls K-bentonite (ash #9) lies 12.1 m above the Hilley et al. (2004) but not in Ramos and Folguera (2009). Retroarc Sherman Falls K-bentonite (ash #1), whereas in the western part foreland subsidence rates are after Irigoyen et al. (2002) in Ramos and of the wedge at Trenton Falls (column 16), this same ash pair is Folguera (2009). Indicated distances of the various structural elements to the trench are present-day distances. separated by 28.0 m. This geometry suggests a regional-scale, back-rotated, syndepositional, east-facing listric fault block from west of the Poland fault eastward to the City Brook fault (Fig. 5). From the relationships described here, it is evident that rapid Group limestones (Mitchell et al., 2004; Sell et al., 2013, 2015) subsidence took place at ca. 453 Ma, and that this subsidence and the Sherman Falls K-bentonite at 452.62 ± 0.06 Ma in the was accommodated by a series of back-rotated (west-dipping) basal Utica Shale (Macdonald et al., 2017). Recalibration of the half grabens that extended ~150 km into the craton from the cur- radiometric ages of the graptolite zones employed by Bradley rent location of the Taconic thrust front. From the foregoing, it is and Kusky (1986) with the new tephrochronology brings Bradley also evident that the broader, step-wise diachroneity of the shale and Kusky’s (1986) ages of the basal Utica at their three localities overstep shown in the cratonic portion of the revised Bradley and into approximate alignment with our new chronostratigraphy that Kusky (1986) overlap curve (Fig. 3) is still somewhat misleading implies a fast transgression (Fig. 3). vis-à-vis the extent and timing of regional subsidence. Rapid sub- Our new overlap curve suggests that the depositional front sidence took place across the >150-km-wide proximal foreland of the black shale migrated westward across the basin at a rate basin in late C. bicornis Zone time (ca. 453 Ma), but in the west- of ~50 cm/yr or faster. Following Bradley and Kusky’s (1986) ern part of the basin, where both subsidence and clastic supply method of directly equating the rate of shale overstep to the rela- rates were lower, local carbonate supply rates allowed the Trenton tive rate of plate convergence (but for a more nuanced view, see Group lithologies to persist ~2 m.y. longer than they did east of Naylor and Sinclair, 2008), this rate is unrealistically high com- the Little Falls fault, despite being within the domain of tectoni- pared to known relative convergence rates at a trench, and it is cally accelerated subsidence. Differential motions across many of especially high compared to relative convergence rates associ- the faults that cut the Utica Shale (Fig. 5) indicate that tectoni- ated with continental collisions. Consequently, there must have cally driven subsidence continued to affect the Mohawk Valley been a signifi cant dynamic component in the overstep signal not region throughout deposition of the Utica unit sediments and into related directly to plate convergence. the overlying (and laterally equivalent) Schenectady Formation. West-thickening wedges in basal Utica Shale and cor- Utica basin depositional geometry and previous tectonic relatives farther west: Back-rotated fault blocks. Inspection of models. The Utica basin subsidence outlined above suggests that a more comprehensive lithostratigraphic cross section (Fig. 5) the basin likely does not represent the outer wall of a trench at allows us to generate a more complete picture of Taconic fore- a steadily east-subducting Laurentian margin; i.e., this basin was land subsidence. A thin C. bicornis Zone succession can be not a long-lived pro-foreland basin that gradually encroached

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B Bʼ Bʼʼ 12 NW 4 SE

7 Intervals with >50% fine sand Zone

1 Black River 450.7 Ma biozone boundary Valley Silty clay laminites 11 & sandy turbidites 9 D. spiniferus Composite 3 13 Indian Katian Stage Zone Castle Fm G. pygmaeus 8 451.2 Ma

Utica Gr. 2 16 D. 7 10 Hillier spiniferus 451.6 Ma Zone 8 6 Steuben 15 7 5 8 8 17 Zone 5 >50% calc Dolgeville 5 mudstone turbidites Fm 4 Rust 4 O. ruedemanni 4 3 3 3 Deer River K-b 9 Denley 9 9 2 1 451.9 Ma 1 Silty clay laminites 2 Russia 2 Lower 1 B & wackestone turbidites 1 Zone Trenton Flat Glens Falls Ls. 2

Poland B Lowville C. americanus Trenton Group Trenton 1 6 Creek Fm 2 Sugar 14 10 River Beekman- 452.6 Ma Black River Gr. 10 1 alls Ls. Zone Poland town GlensGlens FallsF Ls. Kings Fault Falls 5 Millbrig Kb BR-T Top Beekmantown Amsterdam Ls. Kings Falls Ls Napanee 452.86 Ma Buttermilk Creek ? lower absent Fault

Selby Trenton C. bicornis Watertown 453.74 Ma City Brook Noses Fault Hoffmans Deicke Kb Little Falls 30 Saratoga- Lowville Fault Sprakers Fault

Fault McGregor Sandbian Stone Arabia Fault Fault Herkimer Dolgeville Manheim Fault Fault Fault Fault Pamilia 0 km 10 20 0 m Black River Gr. Beekmantown and older

Figure 5. Lithostratigraphic cross section of Sandbian and Katian units in the Utica foreland basin in the Mohawk Valley region, New York State. Section is oriented approximately orthogonal to the fault strike. Cross-section end-points B and B″ and intermediate waypoint B′ are shown on Figures 1 and 2. BR-T—Black River–Trenton. Orange vertical lines indicate measured sections by Mitchell and students and/or from GC Baird core logs (2015, personal commun.). Measured sections: 1—Smalls Bush–Miller Road–Core 74NY1 composite; 2—NYS Thru- way milepost 212–214; 3—Dolgeville–West Crum Composite; 4—Nowadaga Creek; 5—Ingham Mills–Allen Road composite; 6—East Crum Creek; 7—Core 74NY5; 8—Canajoharie Creek; 9—South Flat Creek; 10—Core 74NY12; 11—Core 75NY11; 12—Core 75NY2; 13—Stony Creek– Countryman–County Home composite; 14—City Brook (AKA Wolf Hollow Creek); 15—Rathbun Brook; 16—Trenton Falls–South Trenton–Remsen composite; 17—Core 74NY10. Subhorizontal red lines correspond to geochemically correlated tephra beds: 1—Sherman Falls K-bentonite (K-b); 2—Kuyahoora II K-b; 3—Deer River K-b; 4—Spring Street K-b; 5—Manheim K-b; 6—Otsquago-Fisher K-b pair; 7—Thruway K-b; 8—Countryman K-b; 9—High Falls K-b; 10—Titus K-b (bed M of Sell et al., 2015). Geochronological ages of dated tephra layers in yellow text are from Macdonald et al. (2017) and Sell et al. (2013). Correlated horizons are confi rmed present where they intersect the measured section lines and are at a projected level where the horizons skip the measured section lines. Colored fi elds correspond to facies as labeled (Ls—Limestone). Subhorizontal yellow lines indicate graptolite zone boundaries; zones are labeled at the right side of the fi gure. Subhorizontal pink line with downward-facing barbs represents the karstic upper surface of the Beekmantown Group (Knox unconformity). Graptolite zones (from base): C. bicorn—Climacograptus bicornis, C. amer—Corynoides americanus, O. rued—Orthograptus ruedemanni, D. spin—Diplacanthograptus spiniferus, G. pyg—Geniculograptus pygmaeus.

westwardly as the Laurentian plate slid into the trench. In particu- outer margin of a trench system. Instead, the pattern of broad, syn- lar, the presence of a persistent thin, latest C. bicornis Zone black chronous basin subsidence documented above suggests the exis- shale interval that extended across the basin, together with the near tence of some mechanism of structural coupling that (1) linked the parallelism between the overlying K-bentonites and the basal con- dominantly extensional regime in the central Mohawk Valley with tact of the Utica Shale are difﬁ cult to reconcile with features we the dominantly compressional regime near the Taconic front and would expect to ﬁ nd had these sediments been deposited on the (2) allowed nearly simultaneous activation of both fault systems

and broadly synchronous subsidence over this region. In the next port the contention that Iapetan-opening faults were reactivated section, we take up the question of what tectonic setting might during Taconic foreland basin subsidence, since the throw on plausibly lead to a history of subsidence of this kind. the basal Cambrian nonconformity is greater than the throw on the Ordovician units (e.g., Séjourné et al., 2002). Similar Foreland Basin Subsidence: Implications for Structure and Taconic fault geometries are observed in seismic refl ection data Tectonic Setting collected to the west-southwest and southwest of the Mohawk Several alternative explanations for the rapid basal black Valley (Jacobi, 2010, 2011, 2012) and are also well documented shale transgression in the Saratoga–McGregor–Little Falls fault across the Rome Trough in Pennsylvania (e.g., Ryder et al., 1992; basin are possible. One is that this fast rate of overlap suggests Ryder, 2014). Thus, it is clear that the crust was already broken that the basin formed in a retroarc foreland setting, possibly when the Taconic thrusts loaded the Laurentian margin. In addi- driven in part by dynamics of fl at-slab subduction. We explore tion to the narrow width of the basin and the rapid shale overstep, this interpretation below. the essentially symmetrical subsidence of the basin at the onset Utica subsidence: Broken crust of a retroarc foreland of Utica Shale deposition is also consistent with some retroarc, basin. The fast transgression and narrow Utica basin are consis- fl at slab–related foreland basins, such as the Bermejo Basin in the tent with characteristics of Andean broken-crust retroarc foreland Andes, which also may have been infl uenced by inherited struc- basins, where the rigidity of the crust has been compromised by tures (see Appendix B; Jordan et al., 2001). preexisting fault systems (Appendix B; e.g., Cardozo and Jordan, Utica subsidence: Pinned detachment. The sharp step in 2001; Jordan et al., 2001). Such is the case with the Mohawk Val- basal Utica ages across the Manheim–Little Falls fault system ley Utica basin, where it has been proposed that Iapetan opening (Figs. 3 and 5), which in turn refl ects the sharp decrease in accu- faults were reactivated as the NNE-trending normal faults active mulation rate and accommodation space growth west of the Little during basin subsidence in Utica time (Bosworth and Putman, Falls fault, suggests that subsidence of the western margin of the 1986; Jacobi, 2002, 2007, 2011; but for a contradictory view, see Saratoga–McGregor–Little Falls fault basin was “pinned” by Bradley and Kidd, 1991). Additionally, some of these same faults the Manheim–Little Falls fault system during basal Utica time also sustained motion at Cambrian-Ordovician boundary time (Fig. 6). This boundary fault zone may represent the western (Jacobi et al., 2006) and Knox unconformity time (e.g., Brad- extent of a main detachment from which the intervening faults ley and Kidd, 1991). To the north in Quebec, Taconic subsidence to the east splay (Fig. 6). Further, since these normal faults might is proposed to have been guided by reactivated Neoproterozoic/ be reactivated Iapetan-opening faults, the proposed detachment Iapetan-opening faults, some of which were also reactivated might originally have been an Iapetan-opening detachment. This around the time of the Cambrian-Ordovician boundary and dur- hypothesis may also explain the observation that NNE-striking ing the hiatus associated with the Knox unconformity (Dix and faults exposed in the Grenvillian crystalline rocks of the Adiron- Rodhan, 2006; Salad Hersi and Dix, 2006; Dix and Al-Dulami, dack dome (north of the Mohawk River valley) and their associ- 2011; Dix and Jolicoeur, 2011; Gbadeyan and Dix, 2013). Fur- ated prominent topographic lineaments generally are not found ther, faults observed on seismic refl ection data in Quebec sup- in abundance farther west than the northerly continuation of the

T2 C-O carbonate T1 intracontinental bank thrust massif T2 T1 Utica Taconic thrusts X X X X X X X X X Laurentia

reverse motion on formerly normal faults Hortonville slices with remnant “parallochthonous” carbonate bank

Figure 6. Conceptual cartoon of the pinned nature of the Laurentian margin controlled by a detachment with splay faults. We propose that the detachment developed during Iapetan opening, was reactivated during the Taconic Utica basin development, and inﬂ uenced that development. The “broken,” thin Laurentian margin promoted fast and synchronous subsidence across a large portion of the margin east of the main detachment ramp. T1 and T2 indicate a possible sequence of increased fault activity. The intracontinental thrust massif could be reactivated Green Mountain and Berk- shire massifs. C-O—Cambro–Ordovician.

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Manheim–Little Falls fault system (e.g., Kay, 1937; Isachsen et al., 2015). Escape tectonics away from the New York Promon- and McKendree, 1977; Bradley and Kidd, 1991; Jacobi, 2011; tory could also have contributed to the proposed oblique slip. Jacobi et al., 2015). The small-throw faults in the carbonate-rich Utica subsidence rates compared to other tectonic settings bank west of the proposed detachment ramp at the Manheim– and potential contribution of eustasy. The amount of subsid- Little Falls fault system (Fig. 5) suggest that a splay of the master ence/throw on the Little Falls fault during the interval between décollement extended farther west and ramped up at the Poland the Kuyahoora and Deer River K-bentonites (~0.65 m.y., ashes #2 and Prospect faults (see Figs. 1 and 2 for locations of faults). and #3, respectively; see Fig. 5) is on the order of 43 m compacted The pinned nature with a detachment is consistent with the (~211 m decompacted), which equates to a sediment accumula- interpretation that the Utica basin is a retroarc foreland basin tion rate of 66 m/m.y. compacted and 0.32 mm/yr (325 m/m.y.) over a west-dipping subduction zone, since detachment/décolle- decompacted. To the limited extent that this accumulation rate can ments are proposed for retroarc foreland basins related to fl at-slab be equated to a basin subsidence rate (see, for example, Naylor subduction (Appendix B; e.g., Ramos and Folguera, 2009). In and Sinclair, 2008), the rate is comparable to the subsidence rate retroarc foreland basins, the prominent faulting is commonly the of the Miocene Bermejo thrust-loaded retroarc foreland basin, basin (foreland)–verging thrust faulting and related folding on the which had a subsidence rate of 0.33 mm/yr in pre-fl at-slab sub- hinterland side of the basin. These retroarc thrusts and folds may duction time and 0.77–0.95 mm/yr during fl at-slab subduction have an analog in the retroarc foreland basin model for the Taconic (see Appendix B; Ramos and Folguera, 2009). A higher Utica Mohawk Valley, i.e., the thrusts of the Taconic allochthon and other accumulation rate (decompacted) of ~0.50 mm/yr (492 m/m.y.) associated thrusts, as well as possible thrusting of the Precambrian in the overlying interval (Deer River K-bentonite to the 450.7 Ma massifs. On the foreland side of the retroarc foreland basin, normal ash; Fig. 5) approaches the subsidence rate of the Bermejo fore- faults typically go unremarked or unrecognized because of their land basin during fl at-slab subduction but the Utica black shale small throw compared to the thrust fault systems on the hinterland accumulation rates may not have accurately refl ected the subsid- side. In fact, Bradley and Kidd (1991) suggested that the normal ence rate, and further, the accumulation rate in the Bermejo Basin faults in the Mohawk Valley were evidence for a convergent mar- was derived from coarser sediments (Appendix B; Ramos and gin (with an east-dipping subduction zone), because at the time Folguera, 2009). For comparison, the sediment accumulation rate of their paper, normal faults in subsiding retroarc foreland basins in the intra-arc Okinawa Trough, an extensional basin that inter- were generally unknown; the subsiding retroarc foreland basin was sects Taiwan and is related to a subduction polarity fl ip (see sec- then viewed as an elastic fl exed beam uncompromised by faults. tion entitled “Tectonic Setting of the Ammonoosuc and Partridge Since that time, however, foreland basin faults have been recog- Volcanics” in Appendix A), is an astounding 3.25 m/1000 yr nized in such retroarc foreland basins as the Acadian and Allegha- (3.25 km/m.y.; Salisbury et al., 2002; Clift et al., 2003). nian Appalachian basin (Jacobi et al., 2013; Jacobi and Starr, 2013) The total amount of subsidence inferred for the Utica fore- and the Cretaceous seaway in Canada (note the unidentifi ed faults land basin is quite limited—~1.2 km down-to-the-east, based in the seismic section in fi gures 5 and 6 of Hadlari et al., 2014), and on the present cumulative fault throw across 14 (+) faults from have been proposed in the fl at slab–related Bermejo Basin in the the western margin of the foreland basin to the Taconic thrust Andes (see Appendix B; Jordan et al., 2001). front (Bradley and Kidd, 1991), and less for the basin from the The detachment model is also consistent with Bradley and Little Falls fault to the Saratoga-McGregor fault—763 m, down- Kidd’s (1991) eastward subduction model in which they proposed to-the-east (Bradley and Kidd, 1991). Based on the subsidence that the Utica basin faults might sole out on a detachment (their models of Macdonald et al. (2017), the subsidence across the fi gure 15, model D), and is consistent with a similar model for basin during Utica time was even less: ~0.5 km. Macdonald et foreland faults farther north (Hayman and Kidd, 2002a, 2002b). al. (2017) noted that their subsidence curve calculated for the In the context of the eastward subduction model, the back-rotated Utica basin has a convex-upward form more similar to subsid- fault blocks and the rapid black shale transgression across the ence in a proforeland basin than a retroarc foreland (Naylor and regional Saratoga–McGregor–Little Falls fault block could be Sinclair, 2008; Xie and Heller, 2009), but they suggested that the viewed as stick-slip dynamics related to fi nal continent-arc colli- time frame is insuffi cient for recognition of a concave-upward sion rather than steady-state convergence (with its implication of curve. The cumulative throw on the faults across the Utica basin gradual subsidence). is only about half that of the Quebec Taconic foreland basin, and Utica subsidence: Strike-slip component. Strike-slip motion less than the Timor Trench (Bradley and Kidd, 1991). These rela- also may have contributed to the broadly synchronous subsid- tively low subsidence rates are in accord with the distal portions ence of the Saratoga-McGregor–Little Falls fault basin. In this of the Bermejo broken, retroarc foreland basin, where total accu- scenario, the NNE-striking normal faults had a strike-slip com- mulation over 2 m.y. was less than 1 km (compacted; Ramos and ponent that produced a series of rhombochasms that underwent Folguera, 2009). If, on the other hand, the Utica basin does rep- rapid and coeval subsidence (Jacobi et al., 2015, 2016). Such resent a proforeland basin that developed during short-lived east- oblique motion has been observed in the Andes, where oblique ward subduction and collision of Laurentia with the composite subduction of aseismic ridges resulted in orogen-parallel transla- arc, then the relatively small subsidence could be related to sev- tion in the arc and foreland (e.g., Morabito et al., 2011; Margirier eral factors, including the following: (1) The continental collision

had already begun, reducing the remnant trench to a shallow considerations for the timing and character of the tectonic driving trough, like the Timor Trough (Bradley and Kidd, 1991), or forces behind these events. (2) prior to fi nal collision, aseismic ridge subduction had reduced The timing of the thrust faulting has long been inferred to the trench to a shallow trough (see Appendix A, element 1a, be Taconic, based on Silurian angular unconformities above “uplift of the accretionary prism”), or (3) the New York Promon- deformed Ordovician units and lack of similar thrusts and folded tory provided extra stability. units along strike in the Silurian–Devonian units (e.g., Kay, 1937; Although it is possible that eustatic sea-level rise (e.g., Brett Vollmer and Bosworth, 1984). However, establishing a precise et al., 2004; Haq and Schutter, 2008) could have been a factor in age for the thrust faulting and associated mélange generation is the basal black shale transgression, even the modest total subsi- a diffi cult proposition. The age of mélange formation has been dence discussed above is an order of magnitude greater than the constrained by graptolite age determinations from blocks within magnitude of the relative sea-level rise observed in the interior of the broken formation units and from the matrix surrounding Laurentia on the Nashville dome. There, the backstripped relative those blocks (Figs. 3 and 7; for reviews, see Vollmer and Bos- sea-level rise was ~10 m/m.y. prior to 452 Ma and 2.6 m/m.y. after worth, 1984; Bosworth, 1989; Plesch, 1994; Kidd et al., 1995). (Holland and Patzkowsky, 1997). Compare these rates to a sedi- We reviewed and revised the biostratigraphic age constraints on ment accumulation rate of 66 m/m.y. compacted (~325 m/m.y. the timing of deformation in Appendix D, and we present a sum- decompacted) for the Kuyahoora–Deer River K-bentonite mary of that effort in Figure 7. These ages, if the graptolite iden- interval east of the Manheim fault, and 100 m/m.y. compacted tifi cations are correct, determine a maximum age of thrusting, as (~492 m/m.y. decompacted) for the Deer River–450.7 Ma do the ages of the transported units in the thrust sheets. However, K-bentonite interval east of the Dolgeville fault. These high accu- these ages do not alone constrain the minimum age, and in fact mulation rates, compared to the coeval “midcontinent” rates that Bosworth (1989) wondered whether some of the phacoidal cleav- have been attributed to eustasy (Holland and Patzkowsky, 1997), age zones record Acadian or younger deformation. Such zones indicate that although a rising eustatic sea-level signal could be could have extensive histories, and although their fi nal motions a part of the initial black shale transgression, the nontectonic must be as young as, or younger than, the youngest included component can account for neither the signifi cant thickening blocks, they might have begun considerably earlier than that. documented by the Utica Shale nor the regional differences in Parautochthonous Cohoes mélange zone. Series of tec- implied subsidence rate. Rather, the thickness must be primarily tonic mélange zones occur between the Taconic allochthon and due to a tectonic component, i.e., basin subsidence, a conclusion ~10 km east of the Saratoga-McGregor fault; together, these arrived at by several workers who have examined the subsidence zones are called the Cohoes mélange (Figs. 2 and 7; Appendix and sedimentation rates of the Utica strata in the Mohawk Valley D; Bosworth and Vollmer, 1981; Vollmer and Bosworth, 1984; (e.g., Kay, 1942, 1955; Cisne et al., 1982; Lehmann et al., 1994; Bradley and Kusky, 1986; Bosworth, 1989; Plesch, 1994; Kidd Brett and Baird, 2002; Mitchell et al., 2004; Brett et al., 2004; et al., 1995; Landing et al., 2003). These mélanges involve Jacobi et al., 2016; Macdonald et al., 2017). phacoidally cleaved, synorogenic fl ysch including turbidites and Utica subsidence: Summary. The new integrated teph- mudrocks (the Austin Glen Graywacke). Exotic clasts within rochronological and graptolite data suggest that the Utica basin the mélanges include rafts of interbedded chert and shale, the probably was not a product of relatively long-lived, eastward Stark’s Knob pillow lavas, and limestone blocks, as well as soft- subduction of the Laurentian margin as it steadily passed over a sediment-deformed graywacke clasts (e.g., Plesch, 1994; Land- peripheral bulge and into the trench. The short-lived Utica basin, ing et al., 2003). Our best estimate for the depositional age of even when combined with the slightly older basin recorded by the deformed sediments within the Cohoes mélange is that they the Pawlet Formation (now in Taconic thrust sheets; Fig. 3; see were deposited synchronously with the deposition of the Utica Appendix D; see also Rowley and Kidd, 1981), did not accom- Shale because the mélange belt includes many blocks that contain modate a long-lived convergence between the composite arcs to C. americanus to Orthograptus ruedemanni zone faunas, as does the east and the oceanic tract to the west. Rather, the basin subsid - the enclosing matrix (Fig. 7; see also Appendix D for a detailed ence is consistent with retroarc thrust and fold belts with yoked review and Fig. A3). Zones of deformed fl ysch with relatively nar- foreland basins (as reviewed in Appendix B) and is most likely row thrust fault systems that display phacoidal cleavage locally a result of geodynamics in a retroarc setting related to aseismic separate the mélange zones (Figs. 2 and 7); the deformed zones ridge subduction in a westward subduction model. grade into broken formation (Bosworth and Vollmer, 1981; Vollmer and Bosworth, 1984; Bradley and Kusky, 1986; Bosworth, Timing of (Retroarc?) Foreland Taconic Thrusts 1989; Plesch, 1994; Kidd et al., 1995; Landing et al., 2003). These Here we take up two related questions: What constraints mélange zones have been interpreted as parautochthonous thrust may be placed on the timing of thrusting that created the mélange zones formed during the emplacement of the Taconic allochthon zones (and perhaps drove the observed history of basin subsid- (e.g., Zen, 1961; Bird and Dewey, 1970; Rowley and Kidd, 1981; ence), and, if the timing of the thrusting can be constrained, what Vollmer and Bosworth, 1984; Plesch, 1994; Kidd et al., 1995). does this timing say about how quickly the basin responded to the Westernmost thrust zone and its timing. The westernmost arrival of the thrust loads? We then examine implications of these system of Taconic thrusts is located ~5 km west of the Cohoes

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458 Ma 456 454 452 450 448

N. grac O. trunc O. bicornis C. grac N. O. quad O. Berry TACONIC N. G. D. Riva ALLOCHTHON gracilis C. amer D. spinif P. manit multidens CHERT BLOCK CHERT LOCALITIES pygmaeus 19 19

18*

EXOTIC RYSEDORPH HILL RYSEDORPH FRONTAL MELANGE

Climacograptus bicornis Climacograptus 17*

18 MOORDENER KILL MOORDENER

Geniculograptus pygmaeus Geniculograptus

16 17 RURAL CEMETERY RURAL 15 16

SHALE ZONE WATERFORD WATERFORD WATERFORD DAM DAM WATERFORD 14 FLYSCH MELANGE FLYSCH

, C. bicornis— “Normans Kill & Hill shales” Snake of Ruedemann TYPE NORMANSKILL TYPE isions based on data from Macdonald et al. 14

among included clasts. Graptolite zones (from

, G. pygmaeus— 13 NEAR E. CONTACT CONTACT E. NEAR

nd east of the westernmost thrust at Vischer Ferry Vischer nd east of the westernmost thrust at BLACK MDST BLOCK BLOCK MDST BLACK discus- ocalities; numbers indicate source and key nits in this cross section follow Ruedemann (1930), nits in this cross section follow 13 EASTERN 12*

EXOTIC MELANGE STARKS KNOB STARKS 11

FALLS LOCALITY FALLS BLOCKS AT COHOES COHOES AT BLOCKS Diplograptus multidens Diplograptus 11

Orthograptus quadrimucronatus. Orthograptus EASTERN LOCALITY EASTERN of Ruedemann Diplacanthograptus spiniferus Diplacanthograptus

9*,10 , D. multidens—

, O. quad— I87 OUTCROP I87 910 HALFMOON GRAYWACKE ZONE HALFMOON GRAYWACKE “Normans Kill shale” “Normans Kill shale” *8

, D. spinif—

Austin Glen Mbr., Normanskill Fm Normanskill Fm Austin Glen Mbr., WESTERN LOCALITY WESTERN 8 7 Nemagraptus gracilis Nemagraptus ruedemanni

WESTERN EXOTIC MELANGE

Orthograptus truncatus intermedius Orthograptus “Snake Hill graptolite facies” Hill graptolite “Snake Orthograptus

OLISTOLITH

6 SNAKE HILL HILL SNAKE

Snake Hill Fm Snake , O. trunc— SARATOGA LAKE LAKE SARATOGA 5* , O. ruedemanni— 56 VISCHER FERRY ZONE VISCHER FERRY

FOLDED AND FAULTED GRAYWACKE AND FAULTED FOLDED Schenectady Fm. ERIE CANAL LOCK 7 LOCK CANAL ERIE

4 THRUST ZONE THRUST VISCHER FERRY FERRY VISCHER y i d Corynoides americanus n

Paraorthograptus manitoulinensis Paraorthograptus a

1,2*,3* 4* n t ROUND LAKE LAKE ROUND a c e m 3 n e e demanni d h e c u 12 intact strata & mélange matrix r SchenectadySchenectadyS clasts

. O. ruedemanni O Depositional age Figure 7. Biostratigraphic control on the age of matrix and clasts in the Taconic mélange belt west of the Taconic allochthon a Taconic mélange belt west of the Taconic Figure 7. Biostratigraphic control on the age of matrix and clasts in (Figs. 8 and 9). Section is oriented approximately orthogonal to the deformation zones (for location, see Fig. 2). Structural u Plesch (1994), Kidd et al. (1995), and Landing (2003); geochronological ages are from Cooper Sadler (2012), with rev Bars indicate biostratigraphic range at particular l or revised. (2017). Biostratigraphic ages designated with asterisk are new on the range indicate additional older ages arrows Appendix C. Dashes on the range indicate uncertain age, and downward sion in 1962, 1963a, 1936b; from base): N. grac— 1969, 1974; Berry, Riva, C. amer— P. manit— P. UNDEFORMED FLYSCH

mélange zone along the Mohawk River near Vischer Ferry and the faults suggests a small component of left-lateral oblique (Fig. 2) and involves fl ysch of the Schenectady Formation with motion on the NNE-striking thrusts. Recumbent folds are com- Diplacanthograptus spiniferus Zone graptolites. The thrusts are mon, and overturned, gently dipping bedding extends eastward slightly west of the original western boundary of the Vischer from the western fault zones, forming the intermediate limb of a Ferry zone and Ruedemann’s Line (Figs. 2, 8, and 9; Plesch, nappe (Fig. 8). 1994; Kidd et al., 1995). The individual fault zones are character- The maximum age of thrusting at this site is unambiguously ized by phacoidally cleaved, interbedded graywacke sandstone/ constrained to be younger than the D. spiniferus Zone sediments siltstones and mudstones. The zones strike north to northeast and that are here deformed; however, the minimum age is more dif- dip moderately to the east (Fig. 8). Asymmetric minor folds in fi cult to determine precisely. We propose that this thrusting most the fault zones strike uniformly north, plunge nearly horizontally, likely occurred relatively soon (geologically speaking) after and indicate top-to-the-west transport (Fig. 8). Floating hinge deposition. The soft-sediment-deformed graywacke clasts that lines of phacoidal cleavage and sediment layering are common are restricted to the thrust zones and the phacoidal cleavage are (Fig. 9). The exception to the shallowly plunging folds are soft- both consistent with thrusting while the sediments were water- sediment-deformed (“wild-folded”) graywacke clasts restricted rich, and with rapid dewatering (although for an alternate view to the mélange (Fig. 9) that have a variety of plunges, some rela- for phacoidal cleavage, see Vollmer, 1981; Bosworth, 1989; Kidd tively steep. The strike divergence between the asymmetric folds et al., 1995). Dewatering during burial leads to increased stiffness and a shift from ductile to brittle behavior over a relatively broad range of depths that vary with sediment composition and strain rate. We estimate from compaction curves and observed brittle behavior in core (e.g., Revil et al., 2002; Bolås et al., 2004; Bjor- lykke et al., 2009) that the observed soft-sediment deformation is most likely to have taken place at burial depths less than about 1 km. Extrapolating the subsidence curve for the Mohawk Valley region provided in Macdonald et al. (2017), 1 km of subsidence suggests an upper age limit of ca. 445 Ma. This minimum age estimate of ca. 445 Ma is consistent with regional structural analysis. Epstein and Lyttle (1993, 2001) argued that autochthonous and parautochthonous deformation zones exposed in the Mohawk Valley (discussed above and see Appendix D) can be traced along strike into northwestern New Jersey and northeastern Pennsylvania. There, the deformation zones are beveled and overstepped by the Lower Silurian Sha- wangunk Conglomerate along a major angular unconformity, the Taconic unconformity. Exposures along the Delaware River cor- ridor show that the basal contact of the Shawangunk Conglomer- ate also truncates the slaty cleavage within the Taconian mélange (Epstein and Lyttle, 2001). The Taconian fl ysch immediately beneath the Taconic unconformity in this region contains a D. spiniferus Zone graptolite fauna (Berry, 1970) coeval with that at the Vischer Ferry thrust site. The age of the overlying Sha- wangunk Conglomerate is more diffi cult to determine precisely. Figure 8. Structure map of the farthest west thrust zone in Regional lithostratigraphic correlations suggest that this unit is the Albany area (for location, see Fig. 2). The deformation laterally equivalent to the Tuscarora Formation farther south, zone on the north bank of Mohawk River is ~1.8 km west where that unit exhibits a gradational contact with the underlying of Ruedemann’s Line (the generally accepted western limit red beds of the Juniata Formation, which are generally consid- of thrust-related deformation; e.g., Kidd et al., 1995), mea- ered Late Ordovician, and with the Medina Group in New York sured approximately cross-strike, and 3.7 km NW of Vischer Ferry; the coordinates of the northwestern site on the map are: (Johnson, 1985; Brett et al., 2006), which also appears to straddle 42°49′20.82″N, 73°51′28.88″W. The thrust zones indicated the Ordovician-Silurian boundary (Bergström et al., 2011). The on this map (red symbols) consist of broken formation along lithostratigraphically equivalent Clinch Mountain Formation in the margins and a central phacoidal (scaly) cleavage zone. Tennessee contains a suite of K-bentonites (the Thorn Hill com- Floating hinge lines are visible in the zone. Easterly dipping plex; Bergström et al., 1998) that are associated with lowermost overturned bedding extends eastwardly out of the map area from the central area of the map. Graptolites collected in the Silurian conodonts (Distomodus kentuckyensis Zone; Manzo, Schenectady Formation are those of the Diplacanthograptus 2002). From the foregoing, the youngest plausible age for the spiniferus Zone (plotted in Fig. 7). Vischer Ferry thrust is about ca. 441 Ma.

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Figure 9. (A) Thrust zone within the interbedded shales, siltstones, and sandstones of the Schenectady Formation (see Figs. 2 and 8 for location). The strata exhibit soft-sediment deformation and developing phacoidal cleavage in the core of the thrust zone. The darker-gray zone in the lower left is shown enlarged in 9B. Arrow B refers to a block of graywacke that is indicated by an arrow in 9B. Arrow C refers to a soft- sediment-deformed block of ﬁ ne sandstone that is shown in 9C. Diplacanthograptus spiniferus Zone graptolites were collected at the site. Photo by Mitchell. (B) Enlargement of the dark-gray, phacoidally cleaved mudstone and thin siltstones at the lower left in Figure 9A. Note the ﬂ oating hinge lines on the left and the soft-sediment-fractured graywacke block in the upper right indicated by arrows. The block apparently went through the ductile-brittle transition during deformation. For scale refer to Figure 9A. Dark-gray mudstone may represent dark-gray/black shales found locally in the Schenectady Formation, in which case this unit is a small exotic block. Photo by Jacobi. (C) Detailed photo of a soft-sediment- deformed graywacke block in the thrust zone that is also shown in the upper right of Figure 9A. For scale refer to Figure 9A. Photo by Jacobi.

Local tectonostratigraphic relationships may permit a in that zone may have developed within the interval of the D. tighter bracketing of the thrust timing. Hanson (2010) and spiniferus Zone (451.6–450.9 Ma). Hanson et al. (2010) documented the presence of fi ve soft- It seems likely that the Snake Hill olistolith (which is domi- sediment-deformation zones in the D. spiniferus Zone rocks nantly composed of relatively coarse clastic sediments that bear within core 75NY-2 (column 12, Fig. 5), which is located on the a shelly fauna deposited during the O. ruedemanni Zone; Eng- upthrown side of the Saratoga-McGregor fault, ~10 km west of lish et al., 2006) came to rest within the D. spiniferus Zone sedi- the Vischer Ferry zone (core location in Fig. 2). The deformed ments of the Vischer Ferry graywacke zone somewhere around zones in the core, given their close proximity in stratigraphic this time as well. Similarly, Riva (1987, p. 931) suggested that position and geographic location, may represent the effects of stratigraphic relations in the Quebec reentrant indicate that “the the Vischer Ferry thrusts. The age of these deformed zones in fi nal emplacement of the frontal units of the Taconic allochthon 75NY-2 are similar to the age of the D. spiniferus Zone–aged took place in late C. spiniferus Zone time or in early C. pygmaeus slump folding episodes and sediment slide scar represented by Zone time at the latest.”1 the Thruway unconformity (Jacobi and Mitchell, 2002). If these distal deformational events correspond to compressional events 1The generic names of the epnonymous species have subsequently been changed recorded in the Vischer Ferry thrust zone, then they suggest to Diplacanthograptus and Geniculograptus, respectively, but the biozones re- that the principal shortening and mélange formation recorded main largely as previously defi ned.

Although we have established that this thrust zone, and WE Knox Unconf. associated folding, are most likely Taconian, the temporal resolu- C-WCS PPOO tion is insuffi cient to distinguish whether the thrusting occurred LLFF SS-M-M before the dominant motion on the normal faults, as might be expected in a simple retroarc foreland basin model where subsidence results from thrust loading. However, if the thrusting occurred soon after deposition in D. spiniferus Zone time, as we suspect it did, then the age does predate fi nal motion on the nor- A mid C. bicornis Zone (455 Ma) mal faults, as determined from growth fault geometries on the far western faults such as the Little Falls fault and the Herkimer TA fault (Fig. 5). Timing of thrusts in the Taconic allochthon and parautochthon and its implications. The Taconic allochthon thrusts must have initiated after deposition of units in the allochtho- nous slices—the Indian River Formation (ca. 466–464 Ma and younger; Macdonald et al., 2017) and the Mount Merino For- B late C. bicornis Zone (454 Ma) mation (ca. 457 Ma based on C. bicornis Zone graptolites; for details, see Appendix D), but the thrusts perhaps initiated during deposition of the Austin Glen and Pawlet formations, which are most likely of late C. bicornis Zone age (e.g., Rowley and Kidd, 1981; Appendix D). In that case, thrust initiation took place between 455 Ma (base of Pawlet Formation in the Taconic allochthon) and perhaps 453 Ma (near the top of the Pawlet Formation; Fig. 10; e.g., Macdonald et al., 2017; Appendix D and Fig. A3). C early C. americanus Zone (453 Ma) In contrast, biostratigraphic consideration of the mélange zones VF immediately west of the Taconic allochthon suggest that the leading edge of the Taconic allochthon, including the frontal thrusts of the Giddings Brook slice, arrived at their present position by late C. bicornis to early C. americanus Zone time (Figs. 2, 7, and 10; ca. 454 Ma to 452.7 Ma; Appendix D and Fig. A3). Farther west, in the parautochthonous belt between the Vischer Ferry thrust zone and the mélanges associated with the Taconic allochthon D mid D. spiniferus Zone (451 Ma) (Figs. 2 and 7), biostratigraphy indicates a D. spiniferus Zone age (ca. 451 Ma) for deposition of the faulted fl ysch of the Austin Figure 10. Conceptual diagram illustrating a stepwise advance of Glen Formation—an age that appears to postdate the proposed basin-forming subsidence. Subsidence resulted from loading by the thrusting phase of the Taconic allochthon (Fig. 10: Appendix D, encroaching thrust stack (orange with piggyback basins in C and D) and clastic wedge (brown: distal sediments; green: proximal sedi- Fig. A3). We suggest that the timing of thrusting marked by the ments). Subsidence moved westward in advance of the load as a conse- discrete mélange zones in the parautochthonous belt was rela- quence of incomplete linkage in extended and broken upper crust. The tively soon after deposition (perhaps as early as ca. 451.6 Ma to a Taconic allochthon incorporated such units as the Pawlet Formation. minimum age of >450.9 Ma). Like the Vischer Ferry thrust zone, See text for further discussion. Abbreviations: C-WCS—Chazy Group we base the older age on soft-sediment-deformed graywacke and equivalent carbonates of the western cover sequence, LF—Little Falls fault, S-M—Saratoga-McGregor fault, PO—Poland fault, TA— clasts that are restricted to the thrust zones, and the younger age Taconic allochthon, VF—Vischer Ferry fault zone. Graptolite zones: on the observation that rocks above the highest deformed zones C. bicornis—Climacograptus bicornis, C. americanus—Corynoides in core 75NY-2 are of the Geniculograptus pygmaeus Zone. The americanus, D. spiniferus—Diplacanthograptus spiniferus. (A) Chazy phacoidal cleavage, characteristic of the mélange zones, is also carbonate deposition on subsiding slab east of the exposed Knox un- consistent with thrusting while the sediments were water-rich conformity (pink band), prior to arrival of Taconic allochthons. (B) Ac- tivation of blocks under western cover sequence and adjacent Cham- and undergoing rapid dewatering. plain Valley basin; deposition of Mt. Merino (brown), and Pawlet and Several conclusions can be drawn from these thrust ages, if Austin Glen (green) clastic sediments during early phases of emplace- the assumptions and biostratigraphy are correct. (1) The Taconic ment of the allochthons. (C) Arrival of the Giddings Brook slice of the thrusts probably had at least two major phases of motion, as Taconic allochthons at approximately its current location and propaga- documented by the western parts of the Taconic allochthon and tion of basin subsidence into the Saratoga-McGregor–Little Falls fault basin during deposition of the lower Utica Shale (brown) and Austin the younger mélange/thrust zones west of the Taconic allochthon Glen Formation (green). (D) Final thrusting through fl ysch apron (e.g., (Fig. 10). (2) Signifi cantly, the thrusting duration of the Taconic Vischer Ferry thrust zone), accompanied by further westward expan- allochthon was relatively short: a maximum length ~4 m.y. sion of the Utica basin into the former region of the Trenton shelf.

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(455–451 Ma) and perhaps less. The duration of thrusting in the of ~60 km of westward motion (Vollmer and Bosworth, 1984) Cohoes mélange zone (parautochthonous belt) is not determin- over ~1 m.y., then the propagation rate for these thrusts was on able, but it is assumed to be of similar duration. The thrusting at the order of 6 cm/yr. The 10 cm/yr propagation rate is an order the far western reaches of the Taconic thrusts (the Vischer Ferry of magnitude higher than the 1.3 cm/yr propagation rate of the thrust zone) also had a short duration (a maximum from 451.6 to retroarc thrust front related to aseismic ridge subduction in the 450.9 Ma). Thus, the interval during which the Taconic alloch- Aconcagua fold-and-thrust belt in the Andes (Ramos and Fol- thon and thrusts to the west were emplaced was extremely lim- guera, 2009). The 10 cm/yr rate is, however, comparable to rela- ited; these thrust events do not document a long-lived tectonic tive plate convergence rates. episode such as would be expected for an accretionary prism resulting from subduction of a large-scale ocean basin. Bronson Hill Alkalic/Rhyodacitic Magmas The timing of the Taconic allochthon thrusts (455 Ma, maximum, to 451 Ma) coincides closely with the timing of subsidence Flat-slab subduction elements 5a (continental delamination in the Saratoga-McGregor–Little Falls fault basin to the west with alkalic/rhyolitic volcanism that displays continental partial- (Fig. 10). The broad zone of nearly synchronous initial subsid- melt signatures during slab steepening after fl at-slab subduc- ence in the Saratoga-McGregor–Little Falls fault basin is dated tion) and 5b (alkalic/rhyolitic volcanism far removed from the at 453 Ma, as recorded by the rapid overstep of the Utica black trench) may apply to selected magmatism in the Bronson Hill shale (Figs. 3 and 5). This subsidence occurred about the time the arc(s). The Bronson Hill magmatism consists of the 467–461 Ma farthest-traveled slices of the Taconic allochthon had advanced Ammonoosuc-Partridge bimodal (but dominantly tholeiitic) vol- half of their total travel distance (if they initiated at 455 Ma and canism and the Quimby sequence, which includes poorly dated were moving relatively steadily) or about a third of their total (443 ± 4 Ma and ca. 455 Ma) bimodal volcanics (including a travel distance if they initiated at 454 Ma. In the second alter- 60-m-thick metatuff in the Quimby Formation), the 456–435 Ma native, the time lag between thrust initiation and subsidence is Oliverian Plutonic Suite with granite, granodiorite, trondhjemite, ~1 m.y., similar to that proposed for the subsidence time lag of and rhyolite porphyry, and the 454–435 Ma Highlandcroft Suite the Andean foreland basin at 32°S (Fig. 4) after thrust loading. with bimodal, but generally granitic and granodioritic compo- The eastern boundary of the ~100-km-wide, Saratoga- sition granites and granodiorites (e.g., Moench and Aleinikoff, McGregor–Little Falls fault basin is located ~10 km west of 2003; Karabinos et al., 2008, 2017; Dorais et al., 2011; Macdon- the fontal thrusts of the Taconic allochthon. Slightly older black ald et al., 2014, 2017). shale/fl ysch basins occur east of the Saratoga-McGregor fault There is little question that, in a general sense, at least some and extend under the present position of the Taconic allochthon of the acidic volcanics/magmas of the Highlandcroft and Oliver- (Fig. 3). These basins also may have subsided relatively synchro- ian plutonic suites have mineralogical and geochemical signa- nously across their width, with sharp steps in basal age across the tures that indicate a contribution from a continental partial-melt master bounding faults, as can be inferred from cross sections source. For example, over 35 years ago, Rowley and Kidd (1981, of Taconic allochthon sections (e.g., Rowley and Kidd, 1981, p. 214) suggested that acidic volcanics in the “upper part of the their fi gure 2). If the basin were on a scale comparable to the Ammonoosuc sequence … may be related to anatectic melting Saratoga-McGregor–Little Falls fault basin (~100 km wide), then of the basement of the volcanic arc due to shortening and thick- at least one more basin should exist east of the Saratoga-McGregor ening of the crust during collision….” Similarly, based on trace fault. The interpretation that two basins probably existed during elements, rare earth elements (REEs), and isotopes (87Sr/86Sr and ε emplacement of the Taconic allochthon implies that the response Nd), Hollocher et al. (2002, p. 38) suggested that the Highland- time from thrust loading is less than 1 m.y., if the Taconic alloch- croft Suite (449 Ma to 440 Ma, his assigned ages) and other late thon thrusted for only 2 m.y. The rapid response to thrust loading plutons such as the Cortland Complex were generated as a result may be a signature of previously “broken” continents, such as in of “decompression melting and heating of the lower crust” that the Andean foreland basins (e.g., Ramos and Folguera, 2009), stemmed from asthenospheric upwelling during slab detach- but other factors can offset the effect of already fractured conti- ment or delamination during trench rollback of an east-dipping nental crust (e.g., Cardozo and Jordan, 2001; Jordan et al., 2001; subduction zone. High initial 87Sr/86Sr ratios (0.7045–0.711) and ε Appendix B). The Laurentian margin was “broken” by faulting Nd values of –5.8 to +1.0 for felsic Bronson Hill magmas in the associated with the Iapetan opening, and, because it had been a Oliverian Plutonic Suite also suggested involvement of an older passive margin until the time of thrust loading, it had not been continental crust (as well as a mantle component) in the genera- “healed” and buttressed by later convergent-margin batholiths tion of these magmas (Samson, 1994; Andersen and Samson, and tectonics. 1995; Samson et al., 1995; Hollocher et al., 2002). Dorais et al. Assuming that the farthest-traveled slices of the Taconic (2008, 2011) suggested that this continental crust was probably 207 204 ε allochthon advanced on the order of 200 km or more (Bradley, Laurentia, based on low Pb/ Pb (15.54–15.6) and Nd values 1989; Kidd et al., 1995), then the propagation rate was on the (–7.9 to –0.5) that lie between inferred mantle compositions and order of 100 km/m.y. (if the thrusts moved for only 2 m.y.). Simi- Laurentian crust. Dorais et al. (2011) proposed that the Lauren- larly, if the thrusts in the parautochthonous belt record a total tian crustal melt component of the Oliverian Plutonic Suite was

acquired when a continental arc developed over a west-dipping The geochemistry of phenocrysts, glass inclusions, and subducting zone that initiated beneath the Laurentian margin xenocrysts in the K-bentonites in both the Trenton Group and the after that margin had obducted the older Ammonoosuc arc (which Utica Shale is also consistent with the Oliverian Plutonic Suite was in turn built on peri-Gondwanan crust during eastward sub- and the Highlandcroft Plutonic Suite as sources (e.g., Jacobi et duction). Modeling suggests the northern granitic plutons (now al., 2016). The Deicke-Millbrig ashes and the volcanic ashes in gneisses), located in central and northern New Hampshire, were the Utica Shale both have elevated 87Sr/86Sr ratios (calculated for derived from melting of garnet-free, intermediate/felsic crust, 450 Ma) that range from 0.710 to 0.712 for the Deicke-Millbrig whereas the southern tonalitic and granodioritic plutons in the ashes and range from 0.706 in C. americanus Zone Utica Shale western part of the Bronson Hill arc from Connecticut to central ashes to 0.709 in C. spiniferus Zone Utica Shale ashes (Sam- New Hampshire were derived from melting of garnet-free mafi c son et al., 1989; Samson, 1996). The Oliverian Plutonic Suite crust, and the high-Sr, low-Y, heavy (H) REE–depleted felsic has similar 87Sr/86Sr ratios, ranging from 0.7045 to 0.711 (Hol- rocks in the eastern part of the southern Bronson Hill arc were locher et al., 2002) and from 0.706 to 0.715 (Dorais et al., 2008). 87 86 ε derived from melting of garnet-bearing mafi c crust (Hollocher Based on Sr/ Sr, Nd values, and xenocrysts, the source of the at al., 2002). Deicke-Millbrig ash layers was neither a typical island-arc tho- All of the examples of crustal melting discussed here could leiite (IAT) nor a mid-ocean-ridge basalt (MORB) setting; rather, result from either downgoing slab detachment, as previously the source was an anatectic melt of an evolved continental crust, proposed for the Highlandcroft Plutonic Suite by Hollocher et or a mantle source followed by signifi cant interaction with an al. (2002), or from steepening of the subducting slab following evolved continental crust (Delano et al., 1990; Samson et al., ε fl at-slab subduction and passage of a subducting aseismic ridge 1989; Samson, 1996). Further, Nd values indicate an Adirondack (Appendix B). The delamination with asthenospheric rise and Grenvillian-like source, and the cooling history of hornblende decompression melting hypotheses are essentially the same basic phenocrysts (determined from 40Ar/39Ar plateaus) is also very model in both cases. It appears that the tectonic framework per- similar to that of the Adirondack Grenville Province (Samson, haps can be used to evaluate and differentiate these alternatives. 1996), suggesting a Laurentian continental source. Zircon grains For example, the 3–5 m.y. volcanic gap represented by the Par- with inherited Grenville ages in Utica ash samples suggest that tridge Formation (Moench and Aleinikoff, 2003) could represent these ashes also erupted through Laurentian crust (Macdonald et the magmatic gap that occurs when aseismic ridge subduction al., 2017), consistent with garnet inherited from a Precambrian fl attens the subducting plate. In that case, the Quimby sequence terrane (Delano et al., 1990). The presence of components appar- volcanics could represent the effects of post-fl at-slab subduction ently derived from a Laurentian source within the Utica ashes is that included asthenospheric upwelling and decompression melt- consistent with the suggestion that the Oliverian Plutonic Suite ing (see Appendix A for further discussion concerning the origin was derived at least in part from partial melting of Laurentian of the Ammonoosuc and Partridge volcanics). continental crust (e.g., Dorais et al., 2011). The geochemistry of biotite phenocrysts in the Deicke and Relation of the Highlandcroft and Oliverian Plutonic Suites Millbrig ash beds also supports a continental arc source (Haynes to the Ash Layers in the Utica Shale and Trenton Group, and et al., 2011). Haynes et al. (2011) pointed out that the geochemis- Ash Layer Origin try of the Deicke biotites closely resembles that of the La Pacana Some of the Highlandcroft Plutonic Suite felsic plutons, ignimbrites and the Cerro Chascun (sic, usually spelled Chas- such as the 452 ± 4 Ma Adamstown pluton (Lyons et al., 1986; con) rhyolites (among others). The ashes resulted from explosive Moench and Aleinikoff, 2003), have ages very similar to the eruptions of hydrous metaluminous to peraluminous magmas ash layers (K-bentonites) in the Utica Shale, such as 452.6 and from a large vent with post-eruption caldera formation (Haynes 451.9 Ma (Fig. 4; e.g., Sell et al., 2013, 2015; Macdonald et al., et al., 2011). The volumes of the Deicke and Millbrig ash beds 2017). This similarity in ages has led to the proposal that felsic are each a minimum of 330 km3 and may have been three times plutons in the Quimby sequence in New Hampshire, or similar that much (Huff et al., 1996; Samson et al., 1989). These vol- plutons in Connecticut, could be a source for (some of) the ash umes compare well to the 5–3 Ma voluminous ignimbrites in layers in the Utica Shale (Moench and Aleinikoff, 2003; Jacobi the northern Puna volcanic fi eld of the La Pacana caldera in the et al., 2016; Macdonald et al., 2017). The Deicke and Millbrig Andean inner arc (see Appendix B), such as the >500 km3 Pujsa ash layers (e.g., Mitchell et al., 2004) are slightly older than the ignimbrite, the >100 km3 Toconao ignimbrite, and the >1600 km3 Utica ashes, based on stratigraphy (they occur in the Trenton Atana ignimbrite (e.g., Kay and Coira, 2009). The La Pacana cal- Group) and radiometric dates of 453.7 ± 0.2 Ma and 452.9 ± dera lies above an anomalously shallow low-velocity zone that 0.2 Ma, respectively (Sell et al., 2013; or 454.5 ± 0.5 Ma and is interpreted to indicate decompression melting of the mantle 453.1 ± 1.3 Ma, respectively, in Tucker and McKerrow, 1995). wedge in a region of asthenospheric upwelling and lithosphere These ages are comparable to the 456–435 Ma Oliverian Plutonic delamination that resulted in crustal partial melting and the pro- Suite, although the Deicke-Millbrig ash layers appear to have a duction of the ignimbrites (e.g., Kay and Coira, 2009; Appen- more southeasterly source (for a review, see Samson et al., 1989; dix B). The asthenospheric upwelling and decompression melt- Kolata et al., 1996). ing occurred during steepening of the fl at slab after passage of

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the Juan Fernandez aseismic ridge (Appendix B). Interestingly, Formation (that overlies the Ammonoosuc Volcanics). All these the second stage of sub-Andean retroarc thrusting commenced models propose that westward subduction resulted in Bronson ca. 4.5 Ma, overlapping the time (4.2–3.8 Ma) of voluminous Hill arc volcanics of the Quimby Formation (456–435 Ma), the ignimbrites, including those at the La Pacana caldera. younger Oliverian and Highlandcroft Plutonic Suites. The geochemistry of biotite phenocrysts, the 87Sr/86Sr and We propose that the Oliverian Plutonic Suite (and the High- ε Nd values, and the cooling history of hornblende phenocrysts that landcroft Plutonic Suite) included volcanism that was related to have a Grenvillian component, and the volume of the ashes are effects of slab steepening after fl at-slab subduction of an aseismic all consistent with partial melting of the Laurentian crust during ridge (Figs. 11A and 11B). In this model, the 452–450 Ma Utica slab steepening after fl at-slab subduction of an aseismic ridge. volcanic ashes (Macdonald et al. 2017) represent ash falls from We suggest a similar origin for the overlying ashes in the Utica the explosive events in the Bronson Hill arc, probably from the Group, since they too have elevated 87Sr/86Sr ratios. An inferred Oliverian Plutonic Suite (and/or similar units to the south). southeasterly source for the Deicke-Millbrig ashes (Kolata et al., Another consequence of fl at-slab subduction (and relatively 1998) suggests that they were not derived from the Oliverian Plu- high convergence rates) is both thick-skinned and thin-skinned tonic Suite, but from some other comparable suite, yet uniden- retroarc thrusting that in turn can cause retroarc foreland basin tifi ed, farther south in the Appalachians (e.g., Samson, 1996). subsidence from loading by the thrusts (Figs. 11A and 11B; Such a source does not, however, preclude an Oliverian source Appendix B). The thrusting results from an interplay between for some of the other ashes in the Deicke-Millbrig suite of ashes. relatively high convergence rates and a lithosphere weakened The possibility of multiple sources related to fl at-slab subduc- by delamination and asthenospheric upwelling. Late motion on tion suggests that multiple episodes of aseismic ridge subduction the thrusts that bound the western extent of the Green Mountain occurred along the western margin of the Taconic seas, compa- massif in southern Vermont (Karabinos, 1988) and the Berkshire rable to the situation along the present Andean margin. massif in Massachusetts (Ratcliffe and Harwood, 1975; Ratcliffe, 1979), as well as the deeper reaches of the Champlain thrust (eg., TACONIC TECTONIC MODELS WITH Stanley and Ratcliffe, 1985), may represent such thick-skinned INCORPORATED FLAT-SLAB thrusting. Mélange zone thrusts west of the Taconic allochthon, SUBDUCTION ELEMENTS as well as the thrust slices within the Taconic allochthon, could correspond to the expected thin-skinned thrusts. Although 470– West-Following-East Subduction Models 460 Ma 40Ar/39Ar cooling ages in western Vermont and Massa- chusetts (Sutter et al., 1985; Tucker and Robinson, 1990) have A subduction polarity fl ip from eastward subduction to been used to suggest uplift at this time (e.g., Macdonald et al., westward subduction is proposed to have occurred after genera- 2014), additional motion on the thick-skinned thrusts could have tion of the Shelburne Falls arc and before the bulk of the Quimby continued into the interval of fi nal Taconic allochthon and parau- sequence (Fig. 11; e.g., Karabinos et al., 1998, 2017; Moench tochthon thrust motion discussed above. and Aleinikoff, 2003; Dorais et al., 2011; Macdonald et al., 2014, The overlap in timing of sub-Andean retroarc thrusting and 2017). In the Karabinos et al. (2017) model, the Moretown terrane explosive volcanism (Appendix B; e.g., Kay and Coira, 2009) and Shelburne Falls arc collided with Laurentian crustal frag- appears to fi t well with the broadly synchronous development of ments ca. 475 Ma, based on such considerations as 471–460 Ma thrusts west of the Taconic allochthon, Utica basin subsidence, 40Ar/39Ar cooling dates in the Laurentian-affi nity Rowe Forma- tephra beds, and the early Oliverian Plutonic Suite. As discussed tion (e.g., Laird et al., 1984; Tremblay et al., 2000; Castonguay in section entitled Parautochthonous Cohoes Mélange zone,” the et al., 2012) and detrital zircon ages and provenance. The pos- initial age of motion on the westernmost, and perhaps youngest, sibly protracted collision set up the dynamics for the subduction thrust in the Utica basin in the Mohawk Valley may have been polarity reversal. The 466.0 ± 0.1 Ma Barnard Volcanic Member syndepositional, i.e., on the order of 451 Ma. The maximum age of the Missisquoi Formation (a correlative of the Hawley Forma- of older thrust systems represented by wider tectonic mélange tion, part of the Shelburne Falls arc) and roughly coeval volcanic zones west of the Taconic allochthon may have a range of ages, ashes in the Indian River Formation in the Taconic allochthon based on matrix and included clasts ages (Fig. 7). The age of (466.2 ± 0.1 Ma and 464.2 ± 0.1 Ma; Macdonald et al., 2017) initial tectonic mélange formation west of the Taconic allochthon are thought to mark either slab breakoff following the collision may date from ca. 453–451 Ma. As Bosworth (1989) noted, the (Fig. 11B) or subduction zone reversal (Karabinos et al., 2017; minimum, perhaps reactivated, age of thrust motion is not well Macdonald et al., 2017). In the Karabinos et al. (2017) model, constrained, and it is less well constrained for the eastern mélange the Ammonoosuc Volcanics straddle the time of slab breakoff zones, in particular. The narrow age range of initial thrusting and subduction polarity reversal. In the Moench and Aleinikoff (453–451 Ma) is consistent with the rapid spread of black shale (2003) and Dorais et al. (2011) models, the polarity fl ip occurs across much of the Utica basin west of the Saratoga-McGregor after the generation of the Ammonoosuc Volcanics. Moench and fault. The relatively narrow Utica basin, and the proposed rapid Aleinikoff (2003) suggested that the subduction reversal occurred response of subsidence to thrust loading are both consistent with during a magmatic gap of some 3–5 m.y. in part of the Partridge a preexisting broken continental crust of anomalously lower

strength, as proposed for the Laurentian margin with preexisting structed during continuous eastward subduction. However, the Iapetan-opening faults (some of which sustained Late Cambrian– short history of Taconic thrusts could be compatible with a lim- Middle Ordovician motion). Those faults were reactivated during ited eastward subduction event (such as the destruction of the basin subsidence as the “Taconic” NNE-striking normal faults Taconic seaway). mapped west of the Taconic thrusts in the Mohawk Valley region. The episodic pulses of shortening events represented by, for DISCUSSION example, mélange formation followed by the late-stage thrusts, and the episodes of rapid spread of black shale deposition might It has been proposed that almost the entire Andean margin signal a non-steady-state contractional framework, similar to that has undergone aseismic ridge subduction at some time in its sub- found in the Canadian Rocky Mountain fold-and-thrust belt (e.g., duction history (e.g., Ramos and Folguera, 2009). Our review of Pană and van der Pluijm, 2015) and in the Andean thrusts where aseismic ridge subduction (see Appendix B) allows us to establish reactivation is common (e.g., Kay and Coira, 2009). a series of six elements that can used to recognize aseismic ridge In these west-following-east subduction models, the origin(s) subduction. Although individual features may not be distinctive of the 469–458 Ma Ammonoosuc Volcanics remains equivocal. of this process, together they do provide an effective means by Slab detachment, aseismic ridge subduction, and subduction which to identify aseismic ridge subduction episodes. Nonethe- polarity reversal models all can involve asthenospheric upwell- less, aseismic events have not been recognized in the southern ing that could result in the bimodal nature of the Ammonoosuc New England Taconic Appalachians in the past. Recognition of Volcanics (see Appendix A for further discussion). some of these fi ve elements can be diffi cult. For example, element 1a (uplift of the accretionary prism with consequent uncon- Continuous Eastward Subduction Models formities) is a transient feature, since the uplift decreases as the aseismic ridge passes from the region. Nevertheless, local uncon- In the continuous eastward subduction model of Hollocher formities should remain, and could be recognized, although it et al. (2002), the Shelburne Falls arc is followed by back-arc may be diffi cult to distinguish this cause of the local unconfor- extension that is recorded by the Ammonoosuc Volcanics. Slab mity from other potential causes. detachment results in the Highlandcroft Plutonic Suite. The Element 1b (signifi cantly more deformation in the accretion- simplicity of this model (and those versions that followed; e.g., ary prism where the aseismic ridge intersects the prism, with a Valley et al., 2015) is attractive, but the new provenance stud- possible trail of ocean island basalt fragments ripped from the ies (Macdonald et al., 2014, 2017; Karabinos et al., 2017) make aseismic ridge and incorporated into the prism) shares a simi- the simple eastern subduction model less attractive than the more lar fate as element 1a. Since exposures of tectonic mélange recent models. For this model, we would again propose an aseis- associated with the Taconic accretionary prisms are relatively mic ridge/fl at-slab subduction model for generation of at least rare, opportunities to map regional differences in the severity some of the Oliverian and Highlandcroft Plutonic Suites. An of deformation that the sediments underwent may be limited at attractive component of the continuous eastward model is that the best. It might be thought that, in the simplest case, if the tectonic boninites in the Shelburne Falls arc could signify that a second mélange zones parallel late-stage faults, piggyback basin fi lls, arc—the Ammonoosuc arc phase of the Bronson Hill arc—was and the inferred trench location, then these mélange zones may developing behind the boninitic arc, as is commonly the case for not be related to aseismic ridge subduction. As documented in a boninitic terrane (e.g., Kim and Jacobi, 1996). In this model, Appendix B, small aseismic ridges will not defl ect the strike of parts of the Ammonoosuc Bronson Hill arc also may record an faults on the walls of the trench. In contrast, if the strike of the aseismic ridge subduction event not related to the younger Oli- tectonic mélange swings obliquely to the regional trench, aseis- verian Plutonic Suite event (see “Discussion”). The Quimby mic ridge subduction may have occurred there, as documented in bimodal volcanics, which follow a 3–5 m.y. “magmatic hiatus” the Middle America Trench (Appendix B). Finally, if the mélange in the Partridge Formation between ca. 461 and 455 Ma (Moench contains blocks of ocean-island basalt (as originally proposed for and Aleinikoff, 2003), could also represent a linkage related to the Dunnage mélange in Newfoundland, Appendix A) or other aseismic ridge subduction. units with enriched MORB (E-MORB) affi nities (e.g., Galapa- In the continuous eastward subduction model, the post-fl at- gos; Appendix B), then the mélange probably records the passage slab magmatism of the Oliverian Plutonic Suite is not linked of an aseismic ridge. to the development of the Utica basin. Rather, the Utica basin Unlike the fi rst two elements of aseismic ridge subduction, subsidence records the approach of the Laurentian margin to the the remaining four elements, arranged in a set of temporal and combined arcs. These arcs could then vent volcanic ash into the spatial linkages, can provide powerful tools for recognition of Utica basin, as a record of aseismic ridge subduction associated aseismic ridge subduction. This sequence involves a temporal with the Oliverian Plutonic Suite. A problem with the continuous and spatial gap in the occurrence of magmatic activity (“mag- eastward subduction model is that the Taconic thrusts we have matic quiescence”) associated with uplift (recognized by a local discussed do not record a long-established contractional event, unconformity) that is followed by volcanism (“volcanic fl are- such as what one would expect for an accretionary prism con- ups”). This volcanism may include voluminous explosive felsic

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502-475 Ma (475 Ma peak activity) Shelburne Falls Laurentia Aseismic Ridge/ Rowe Terrane (?) Arc 510 - 480 Ma A INITIATION Moretown Terrane OF SUBDUCTION AND CONTINUED SUBDUCTION

475 Ma APPROXIMATE TIMING B Low-activity FOR ASEISMIC RIDGE/ARC Low activity Bronson COLLISION Shelburne Hill Falls Arc Arc forearc uplift along faults 470 Ma (Ar/Ar 470 cooling ages) FLAT SLAB C AND CONSEQUENT LOW ARC ACTIVITY + ARC JUMP

? 466 Ma 469-461 Ma Barnard Bronson (Shelburne Falls) Hill Arc) Arc (1) Upper, acidic Ammonoosucs pre-Utica time continental melt-sourced ash Victoria/partial Popelogan Arc Chickwolnepy extension & intrusions Indian River 467 Ma min age Hortonville(?) Ganderia 469-460 Ma SLAB STEEPENING D CONTINENT/MICRO-CONTINENT/ARC COLLISION Delamination, partial melting of microcontinent Possible polarity flip at ~ 450 Ma (Dorais et al. , 2011) or ~465 Ma (Macdonald et al., 2014; retroarc retroarc Shelburne Quimby sequence Karabinos et al., 2017) thrust-loaded thrusting Falls Bronson basin Arc Hill inactive Arc (2) Highlandcroft (452-441 Ma) continental melt-sourced ash Oliverian (456-435 Ma) Victoria/partial Popelogan Arc Utica deposition (453-450) 455-450 Ma Ganderia LITHOSPHERIC PARTIAL MELTING E BRONSON HILL/GANDERIA COLLISION RETROARC THRUSTING crustal (bsmt) thrusts

partial melting of Laurentia/microcontinent from asthenospheric upwelling & delamination A or Taiwan model

Figure 11. (A–B) Plate-tectonic models that incorporate aseismic ridge subduction for the Taconic orogeny. Both models (A and B) generally follow subduction polarity reversal models (e.g., Karabinos et al., 2017). Both models suggest that the volcanic ashes in the Utica black shale were sourced from the Oliverian and Highlandcroft plutonic suites (or equivalent units) that resulted from aseismic ridge subduction. Further, the Utica basin resulted from thrust loading that in turn resulted from a combination of weakened crust and relatively high convergence rates. (A) In this model, the possible Laurentian-afﬁ nity Rowe belt (Karabinos et al., 2017) could actually be the aseismic ridge in the eastward subduction phase of the model (and the ridge basement is dominantly oceanic, while the sediment inﬂ ux is Laurentian in character). The upper, acidic Am- monoosuc volcanics are the result of slab steepening after aseismic ridge subduction (Continued on facing page).

502-475 Ma (475 Ma peak activity) Shelburne Falls Laurentia Rowe Terrane Arc 510 Ma A INITIATION Moretown Terrane OF SUBDUCTION

475 Ma B APPROXIMATE TIMING FOR ROWE TERRANE/ARC COLLISION Victoria/partial Popelogan Arc Upper, acidic Ammonoosucs continental melt-sourced ash Chickwolnepy extension & intrusions pre-Utica ash deposition 467 Ma min age Indian River +/-Hortonville(?) Ganderia 469-466 Ma C1 SLAB BREAK-OFF

slab break-off

Victoria/partial Popelogan Arc Upper, acidic Ammonoosucs continental melt-sourced ash Chickwolnepy extension & intrusions pre-Utica ash deposition 467 Ma min age Indian River +/-Hortonville(?) Ganderia 467-460 Ma SLAB BREAK-OFF C2 SUBDUCTION POLARITY REVERSAL asthenospheric upﬂow

Quimby sequence Bronson Hill Arc (2) Highlandcroft (452-441 Ma) partly continental melt-sourced ash Highlandcroft/Oliverian (456-435(452-441 Ma) Ma)/ Victoria/partial Popelogan Arc Utica deposition Oliverian (456-435 Ma) (453-450) Ganderia 455-450 Ma D LITHOSPHERIC PARTIAL MELTING RETROARC THRUSTING crustal (bsmt) thrusts

partial melting of Laurentia/microcontinent from asthenospheric upwelling & delamination or Taiwan model B

Figure 11 (Continued). (B) In this model, the possible Laurentian-afﬁ nity microcontinent, the Rowe terrane (Karabinos et al., 2017), collides with the Shelburne Falls arc. The upper, acidic Ammonoosuc volcanics are the result of slab breakoff after collision of Laurentia with the composite Shelburne Falls arc and the Rowe terrane, rather than from aseismic ridge subduction as portrayed in model A. Stage C1 portrays the Ammonoosuc Volcanics (and other events such as the Barnard volcanics) developing during slab breakoff, without a coeval subduction polarity reversal. In contrast C2, suggests that slab breakoff and subduction polarity reversal were roughly coeval, perhaps with slab breakoff leading slightly before signiﬁ cant reversed subduction.

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volcanism with partial-crustal-melt signatures that erupted far ~160 km. This crude calculation results in a plausible areal extent removed from the trench, plus bimodal volcanism, followed later for the subducted aseismic ridge. Accordingly, the basic assump- by arc volcanism closer to the trench. If the relative convergent tions of the aseismic ridge subduction model also are reasonable. rate is suffi cient to exert horizontal stress across the delaminated, In the Andes, the along-strike basins are developed as the weakened crust (in the Andes about >4 cm/yr; Appendix B), then aseismic ridge plows past the margin on a very oblique trajec- retroarc thrusting and consequent thrust-loaded basin develop- tory. This oblique trajectory has two implications relevant to the ment will occur in the retroarc foreland during roughly the same present discussion. One is that the retroarc foreland basin that interval as the explosive volcanism. develops as a result of aseismic ridge subduction should be rela- The critical points in this identifi cation stratagem—that a tively restricted along strike if the relative subduction vector is volcanic gap is followed by explosive volcanism with anatectic orthogonal to the trench, and if the long dimension of the aseis- signatures, and that volcanism occurred coeval with the thrust- mic ridge is parallel to the convergence vector. However, as is ing and basin development—are becoming possible to recognize the case in the central Andes, convergence commonly is oblique only because of extensive, high-resolution dating programs, as (Appendix B), and commonly the aseismic ridge was built with well as geochemical research (e.g., Tucker and Robinson, 1990; a long dimension not exactly parallel to the relative conver- Aleinikoff et al., 2007; Sell et al., 2013; van Staal et al., 2016; gence vector. Interplay between these two factors can lead to a Karabinos et al., 2104, 2017; Macdonald et al., 2017). In the past, wider along-strike retroarc foreland basin as the aseismic ridge the large uncertainties in radiometric dates for the Ordovician sweeps along the margin. The resulting retroarc basin(s) will combined with the paucity of reliable dates meant that relatively necessarily be diachronous along strike. In the Andean margin, short periods of local magmatic gaps and fl areups, especially if the retroarc foreland basins are not continuous, and they appear shoshonites, adakites, rhyolites, and dacites were involved, were to refl ect local infl uence of preexisting structure on the general diffi cult to recognize. It was not possible to defi nitely recognize basin form (e.g., Jordan et al., 2001). Alternatively, a signifi cant a 3–5 m.y. magmatic gap, as compared to all the other gaps, or aseismic ridge may locally pin the subduction zone, so that dur- even recognize the coeval nature of the Oliverian Plutonic Suite ing later trench rollback, the trench-seamount chain junction will and the volcanic ash layers preserved in the Utica basin. not migrate along the trench strike (e.g., the Emperor Seamount Using this set of linkages, we propose that at least part Chain in the Pacifi c; Vogt, 1973). of the Oliverian Plutonic Suite represents the explosive volca- In the case of the Utica basin, our mapping of the farthest nism related to slab steepening after fl at-slab subduction that west thrust west of Albany, New York (Figs. 2, 8, and 9), sug- was caused by aseismic ridge subduction. The probable atten- gests that the relative convergence vector was oblique, with a dant delamination weakened the crust, promoting contractional left-lateral component, based on the trend of the encapsulating thrust systems to develop west of the arc. This retroarc thrusting thrust boundaries versus the asymmetric fold axes in the thrust included both thick-skinned thrusting of the crust (represented zone. This oblique motion is consistent with transport directions perhaps by late thrusting along the western boundary of the Green of thrusts and mélange zones farther east (e.g., Vollmer and Bos- Mountain and Berkshire massifs, as well as the deeper reaches of worth, 1984), but it is not consistent with proposed right-lateral the Champlain thrust) and the thin-skinned thrusts of the Taconic motion farther to the west-southwest based on three-dimensional allochthon and parautochthon between the Green Mountains seismic surveys (e.g., Jacobi, 2011, 2012). The divergence in and the normal faults of the Utica basin west of Albany, New sense of motion could be related to escape tectonics around the York. The thrusts loaded the retroarc foreland, and the foreland New York Promontory (e.g., Jacobi, 2011, 2012). In any case, responded almost instantaneously, geologically, partly because an oblique component of motion characterized the Taconic con- the Laurentian margin was already weakened by Iapetan-opening vergence, which would lead to a retroarc foreland basin that faults (and later reactivations of those faults). The narrowness of was laterally extensive and time-transgressive along strike. This the basin, and the relatively short-lived nature of the basin (a few is consistent with what we know generally of the Taconic fore- million years, as opposed to the sort of geologically persistent land basin, which in its broadest sense extends from Alabama to convergence present around the Pacifi c Rim) are compatible with western Newfoundland, but which appears to have consisted of the proposal that the basin was ultimately the result of aseismic several domains with distinct subsidence histories (compare, for ridge subduction. instance, reconstructions of Taconic foreland history in western Can we determine the spatial extent of the aseismic ridge that Newfoundland [Waldron et al., 1993] and Pennsylvania [Ganis was subducted? If we assume that the orthogonal component of and Wise, 2008] with that given herein). These details, however, the relative convergence rate was >4 cm/yr, then we can estimate a are beyond the scope of the present paper. crude dimension of the aseismic ridge, based on that rate being the Given the number of seamounts, seamount chains, seamount lower limit for thrust effects in the Andes (Maloney et al., 2013), provinces, and oceanic plateaus in the present oceans, and the prob- and the observation that the Utica basin was loaded for ~3–5 m.y. ability that almost the entire Andean margin has undergone aseismic Taking an average of 4 m.y. for basin loading, and the convergence ridge subduction at one time or another, it is probable that more than rate, the component of the aseismic ridge dimension that was one aseismic subduction event occurred during the course of the orthogonal to the trench far east of the Mohawk Valley was at least Taconic orogeny. We suggest other sequences should be examined

in light of the aseismic ridge model, including the bimodal Quimby the downgoing slab after passage of a subducting aseismic ridge. volcanism, which follows the 3–5 m.y. magmatic gap of the Par- The continental partial-melt signatures of the Oliverian Plutonic tridge Formation, and the bimodal Ammonoosuc Volcanics. Suite are consistent with this interpretation. In the model with The subduction of aseismic ridges can lead to a segmenta- passage of the subducting aseismic ridge, the probable atten- tion of the arc, both in temporal terms and in structural terms, dant delamination weakened the crust, promoting contractional such as in the Middle America Trench region (Appendix B). thrust systems to develop west of the arc, in the retroarc foreland. Thus, an orogen may exhibit signifi cant cross-strike structures This retroarc thrusting included both thick-skinned thrusting of related to multiple seamounts passing below the upper plate, and the crust (perhaps represented by late thrusting along the west- the resulting magmatic processes may be strongly diachronous ern boundary of the Green Mountain and Berkshire massifs, as along strike. Such a situation may explain the apparent difference well as the deeper reaches of the Champlain thrust), and the thin- in age of the Ammonoosuc Volcanics between Massachusetts and skinned thrusting associated with the Taconic allochthon and par- New Hampshire. The oblique passage of a single aseismic ridge autochthonous mélange zones in the Taconic system between the beneath the margin could also contribute to along-strike diachro- Green Mountains and the normal faults of the Utica basin west neity (and to strike-slip motion in the foreland). of Albany, New York. The thrusts loaded the retroarc foreland, and the foreland responded almost instantaneously, geologically, CONCLUSIONS partly because the Laurentian margin was already weakened by Iapetan-opening faults and reactivations of those faults. The nar- Aseismic ridge subduction is a common occurrence. Almost rowness of the basin, and the short-lived nature of both the basin the entire Andean margin has undergone aseismic ridge subduc- and the dominant motion on the thrusts of the Taconic allochthon tion at some time in its subduction history. Using the Andean and parautochthon (a few million years), as opposed to a typi- margin and the Middle America Trench as models, we developed cal Pacifi c convergence history, are compatible with the proposal a series of six elements that can be used to recognize aseismic that the basin was ultimately the result of aseismic ridge subduc- ridge subduction in an orogen. Four of these elements, arranged tion. Volcanic ash layers in the Utica basin document continental in a set of temporal and spatial linkages, are particularly distinc- partial-melt volcanics similar in age (453–450 Ma) and composi- tive. The four-element sequence involves the following: (1) A tion to the Oliverian Plutonic Suite. Oblique subduction of the spatiotemporal magmatic gap (“magmatic quiescence”) is asso- aseismic ridge will extend the foreland basin along strike, but an ciated with uplift. Flat-slab subduction leads to a shutdown of along-strike transgressive nature should be observed in detail. It magmatism and localized uplift as the aseismic ridge subducts. is possible that the characteristics of the short-lived thrusts, as (2) Volcanism following the magmatic gap (“volcanic fl areup”). well as the yoked short-lived basin subsidence, are also consis- Steepening of the fl at slab after the aseismic ridge passes ini- tent with eastward closure of a narrow ocean basin or extended tiates asthenospheric upwelling and decompression melting, crust with stick-slip contraction during fi nal collision. which can produce partial melting of the overlying lithosphere Because of the ubiquity of seamounts, seamount chains, and a volcanic fl areup. The volcanism can be bimodal and com- and oceanic plateaus, we expect there were other instances of monly includes explosive, voluminous rhyodacitic volcanism aseismic ridge subduction during the Taconic orogeny. We sug- that exhibits a partial-crustal-melt signature and that is erupted at gest other sequences should be examined in light of the aseismic great distances from the trench (up to 850 km). Broadly tempo- ridge model, such as the Partridge Formation 3–5 m.y. magmatic rally linked with the explosive volcanism are (3) retroarc thrust- gap followed by the bimodal Quimby volcanism, and some of the ing, and (4) consequent thrust-loaded basin development that bimodal Ammonoosuc Volcanics, especially the younger 453 Ma occurs in the retroarc foreland. Basinal sediments will contain felsic Ammonoosuc volcanics. Different timings along the length ash layers from the explosive volcanism that have partial-melt of the subduction complex should be expected for the effects of signatures. Retroarc foreland thrusting can include both thick- aseismic ridge subduction, based on different times of impinge- skinned and thin-skinned thrusts. Thrusting and basin subsidence ment for different aseismic ridges, or, the oblique passage of a will develop if the relative convergent rate is suffi cient to exert single aseismic ridge beneath the margin, depending upon the horizontal stress across the delaminated, weakened crust (in the obliquity of subduction of the incoming aseismic ridge. Andes ~4 cm/yr or greater). This critical sequence of aseismic We recognize that not all bimodal volcanic suites are related ridge subduction effects may be recognizable in ancient orogens to aseismic ridge subduction, and, in fact, the same astheno- now that research in those orogens is beginning to benefi t from spheric rise and decompression melting proposed for slab steep- extensive, high-resolution dating programs coupled with geo- ening has been proposed for other convergent settings, including chemical and isotopic analyses, along with the development of a subduction-zone polarity reversals and slab detachment. Never- similarly high-resolution stratigraphy in the foreland basins. theless, we suggest that the sequence of effects of aseismic ridge Using these criteria to examine the Taconic orogen in west- subduction should be in the arsenal for tectonic interpretations ern New England, we propose that at least the older units in the of coeval magmatic, contractional, and basinal elements, and 456–435 Ma Oliverian Plutonic Suite, which consists of mildly that in the Taconic region of western New England, at least one alkalic granites and rhyolite, were generated during steepening of such example exists: the coeval Oliverian Plutonic suite, westerly

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thrusting between the Green Mountain massifs and the normal over a west-dipping subduction zone; this subduction zone developed faults of the Utica basin, and the Utica basin subsidence with along the western margin of Iapetus after the collision of the Shel- anatectic-melt ash layers. burne Falls arc with Laurentia destroyed “Neo-Iapetus.” U-Pb zircon dates on the Shelburne Falls arc rocks peak at about 475 Ma, but older rocks with dates between 502 and 486 Ma also occur (e.g., Karabinos ACKNOWLEDGMENTS et al., 1998; Macdonald et al., 2014, 2017). In these original subduction polarity-fl ip models, the Bronson Hill arc formed approximately We thank Marjorie Gale, Daniel Goldman, Jim Hibbard, Jon 460–442 Ma (e.g., Karabinos et al., 1998). Kim, Paul Karabinos, Steve Leslie, Francis Macdonald, John Modifi cations based on new data have considerably complicated the original subduction polarity-fl ip models. Moench and Aleinikoff Martin, Rich Nyahay, Nick Ratcliffe, Scott Samson, Rich (2003) proposed that the Bronson Hill arc is actually a composite of Schweickert, Bruce Selleck, Cees van Staal, and the Albany two arcs: an older arc (469–458 Ma) and a younger arc (456–443 Ma). crew from the 1970s (Kidd, Dewey, Rowley, Delano, and oth- The older arc is represented by the tholeiitic basalts and thin felsic ers) for the years we have spent discussing Northern Appala- metatuffs of the Ammonoosuc Volcanics, which are overlain by the chian geology—for some (Jim Hibbard for example), it has Partridge Formation, a sequence of predominantly black sulfi dic slate and schist with bimodal volcanics including rhyolites and dacites (e.g., been over 40 years. Jacobi acknowledges Marshall Kay and Bill Moench and Aleinikoff, 2003; Dorais et al. 2011). The Chickwolnepy Dickinson, who were truly inspirational, insightful, and fore- Intrusions (for location, see Fig. 1) are thought to be a part of this older sighted scientists; they were a pleasure to be around. Bill Kidd arc, and they include sheeted dikes that may have been feeders for the guided us over a period of several days to many of the Cohoes Ammonoosuc Volcanics (Fitz, 2002; Moench and Aleinikoff, 2003). mélange sites discussed in the text, and we are indebted to him The reported ages of the Chickwolnepy Intrusions (467 ± 4 Ma— Aleinikoff and Moench, 1992; 458 ± 6 Ma—Aleinikoff et al., 2015) for this valuable assistance. The research presented here has are broadly compatible with the older arc. The age of the Partridge benefi ted from the efforts of several graduate students who con- Formation is determined by C. bicornis Zone graptolites (ca. 457— ducted projects in these strata under our supervision (including 452.5 Ma; Harwood and Berry, 1967), which Karabinos et al. (2017) Paul Agle, Stephanie Amodeo, Gareth Cross, Richard Frieman, noted would belong to Riva’s (1974) interpretation of the N. gracilis Zone (but see Appendix D for a discussion of the overlapping age Stacey Hanson, Anna Hrywnak, Kyle Jones, Todd Marsh, Alex implications of these usages). Additionally, the radiometric age of the O’Hara, Erin Richley, Melissa Roloson, Steve Saboda, and Tay- Partridge Formation was established by a U-Pb zircon date of a vol- ler Schweigel). We thank Francis Macdonald and Bob Hatcher canic unit in Massachusetts of 449 +3/–2 Ma (Tucker and Robinson, for careful, perceptive reviews that considerably improved the 1990) and by detrital zircons, which indicate that part of the Partridge manuscript, and Tim Lawton, the volume editor, for his con- Formation has a maximum age of 452 Ma (Merschat et al., 2016). Recently, an older belt of volcanics was identifi ed in the “Ammo- structive suggestions. This research was supported by various noosuc Volcanics” along the western margin of the Bronson Hill arc in grants to Jacobi from the New York State Energy Research New Hampshire that has dates of 475 ± 9 and 477 ± 7 Ma (Aleinikoff and Development Authority and to Mitchell from the National et al., 2015), which overlap those of the Shelburne Falls arc. Similar Science Foundation (in collaboration with John Delano, Scott dates also have been obtained from intrusions into the Ammonoosuc Samson, Steve Leslie, and Pete Sadler). Volcanics in New Hampshire (475 ± 5 Ma and 466 ± 8 Ma; Valley et al., 2015). These newly obtained dates extend the age range of the Ammonoosuc arc. Younger ages of Ammonoosuc Volcanics in a belt APPENDIX A: PLATE-TECTONIC MODELS immediately east of the older Ammonoosuc Volcanics yielded dates of 457 ± 9 Ma, 452 ± 13 Ma, and 449 ± 7 Ma (Aleinikoff et al., 2015), Taconic Plate-Tectonic Models for New England consistent with the ages of the younger Bronson Hill arc. Correlatives of the Ammonoosuc Volcanics to the north in Quebec include parts Early tectonic models of the Taconic orogeny involved a single of the Ascot complex (Fig. 1A), including 462–460 Ma rhyolites and eastward-dipping subduction zone (present coordinates) under an arc granites (Tremblay et al., 2000; Moench and Aleinikoff, 2003). (Bronson Hill arc; e.g., Jacobi, 1981; Rowley and Kidd, 1981; Stanley The younger arc in the composite Bronson Hill arc (ca. 456 Ma to and Ratcliffe, 1985; Bradley and Kidd, 1991; Ratcliffe et al., 1998, ca. 443 Ma, or possibly as young as 435 Ma) is essentially the Bron- 1999; Hollocher et al., 2002; Schoonmaker et al., 2016). In these mod- son Hill arc of Karabinos et al. (1998). This arc is represented by the els, the west-directed Taconic thrusts represented the Taconic accre- Quim by Formation (443 Ma and 455 Ma, generally graywackes and tionary prism, and the northerly striking normal faults west of the slates with bimodal volcanics), the Oliverian Plutonic Suite (mildly thrusts in eastern New York State represented the Laurentian continent alkalic granites and rhyolite, 456–435 Ma), and the Highlandcroft fl exing and stretching as it passed into the trench. The Upper Ordovi- Plutonic Suite (granitic to mafi c plutons, 454–436 Ma; e.g., Moench cian Utica Group black shales and the overlying coarser clastics were and Aleinikoff, 2003; Karabinos et al., 2017; Macdonald et al., 2014, thought to mark a trench fi ll that spilled over onto the craton. 2017). Based on Nd and Pb isotopic signatures, Dorais at al. (2011) More recently, recognition of Ordovician boninitic volcanics in suggested that the ca. 470 Ma Ammonoosuc portion of the Bronson the Hawley Formation, which lies west of the Bronson Hill arc, and the Hill arc has a Gondwanan-like crustal component, whereas the younger possibility of two ages of Taconic volcanism, led to the concept that Quimby Formation and Oliverian Plutonic Suite have Laurentian Nd two Taconic arcs developed in western New England (e.g., Kim and and Pb isotopic signatures. These data implied to Dorais et al. (2011) Jacobi, 1996, 2002; Karabinos et al., 1998; Kim et al., 2003; Moench that the Oliverian melts rose through the eastward-subducted leading and Aleinikoff, 2003; Dorais et al., 2011; Macdonald et al., 2014, edge of Laurentia (which could include accreted microcontinents of 2017). In the two-arc model, the older Shelburne Falls arc, including Laurentian affi nity such as the proposed Rowe belt of Karabinos et al., the Hawley Formation, was built over an east-subducting oceanic plate 2017; Macdonald et al., 2017). (“Neo-Iapetus” of Karabinos et al., 1998; or Iapetus of Macdonald et Macdonald et al. (2014, 2017) and Karabinos et al. (2017) found al., 2014), whereas the Bronson Hill arc (sensu lato) was constructed that detrital zircon U-Pb age normalized probability density plots

indicated that part of the base into which the Shelburne Falls arc (Moench and Aleinikoff, 2003, their fi gure 7) shows the Chickwolnepy intruded, the Moretown Formation (Fig. 1A, now designated “ter- intrusions (partly feeders for the Ammonoosuc Volcanics; Fitz, 2002; rane”), has a Ganderian (Gondwanan) signature (like the Ammonoosuc Moench and Aleinikoff, 2003) in a backarc position. portion of the Bronson Hill), but surprisingly, the Hawley Formation, More recent models for the Ammonoosuc Volcanics have cen- which includes the Shelburne Falls arc boninites, has both Gondwanan tered on two tectonic associations: subduction polarity fl ip, following and Laurentian detrital zircon signatures. A tectonic model to explain the model of Teng (1996) for Taiwan (e.g., Karabinos et al., 2003), the detrital zircon data and extensive new radiometric dates has been or slab breakoff of an east-dipping oceanic slab. Slab detachment proposed by Macdonald et al. (2014, 2017) and Karabinos et al. (2017) would result in asthenospheric upwelling and decompression melting (see also review in section entitled “Recent Taconic Plate-Tectonic (Fig. 11B), which initially would produce bimodal volcanism simi- Models”). In this model, the Moretown terrane was a microcontinent lar to the Ammonoosuc Volcanics. In the subduction polarity reversal of Gondwanan-derived crust on the east side of Iapetus. The Moretown model, an intra-arc basin, such as the Okinawa Trough north of Tai- terrane was intruded by the Hawley boninites above an east-dipping wan, forms in an extensional regime (e.g., Teng, 1996; Shinjo et al., subduction zone when Iapetus had closed suffi ciently to allow a Lau- 1999; Teng et al., 2000; Clift et al., 2003). The Okinawa Trough is rentian source to provide detritus to the Hawley Formation. Such a characterized by bimodal volcanism (e.g., Shinjo et al., 1999) that has source could be the Rowe Schist, which lay on the west side of Iapetus subduction, rising asthenospheric, and partial-melt signatures (Shinjo (and west of the trench and suture) and represents either a Lauren- et al., 1999; Teng et al., 2000; Clift et al., 2003). A third possible ori- tian microcontinent or extended Laurentian crust. After a subduction gin is that the Ammonoosuc Volcanics are an effect of aseismic ridge polarity fl ip from eastward to westward subduction at 466 Ma, the subduction when the oceanic slab began to steepen after passage of the west-dipping subduction resulted in the Oliverian and Highlandcroft subducted ridge. For example, asthenospheric upwelling and decom- plutonic suites. pression melting related to fl at-slab subduction (Appendix B; Fig. A1) Even after the recognition of multiple arcs (Shelburne, older and are proposed to have resulted in the 25–22 Ma Tambo-Tambillo younger Bronson Hill arcs) and the tectonic models that incorporated bimodal volcanics-basalts and shoshonites in the Andes (Appendix B; a subduction polarity fl ip to westward subduction for the eastern, Fig. A2; e.g., Kay and Coira, 2009). The models of Karabinos et al. youngest arc, some researchers have maintained that eastward sub- (2017) and Macdonald et al. (2017) suggest that the Ammonoosuc Vol- duction could have resulted in all of the described arcs. Hollocher canics developed during the time of both slab breakoff and subduction et al. (2002) maintained that the Shelburne Falls arc and both parts polarity reversal. of the Bronson Hill arc were generated over an east-dipping subduc- The geochemistry of the Ammonoosuc Volcanics, such as the tion zone. They suggested that slab breakoff of an east-dipping oce- enriched large ion lithophile elements (LILEs) and depleted high fi eld anic plate resulted in the younger Bronson Hill arc rocks such as the strength elements (HFSEs), is comparable to the Okinawa Trough Oliverian Plutonic Suite. Fitz (2002) also suggested that the Chick- basalt geochemistry, although the 87Sr/86Sr ratios are much higher in wolnepy Intrusions developed over an east-dipping subduction zone some of the Ammonoosuc Volcanics (compare Shinjo et al. [1999] data during slab detachment or rhombochasm growth. The recent dates in to those from Hollocher [1993], Hollocher et al. [2002], and Dorais et the Bronson Hill arc that have a Shelburne Falls age are also compat- al. [2011]). The bimodal volcanism in the Okinawa Trough north of ible with a long-lived, east-dipping subduction zone (Valley et al., Taiwan has both a subduction signature and a rising asthenosphere sig- 2015), but slab breakoff and subduction polarity reversal could also nature (Shinjo et al., 1999; Teng et al., 2000; Clift et al., 2003). Basalts result in spatially overlapping volcanics in two phases (Karabinos closer to Taiwan display varying Sr isotope ratios and incompatible et al., 2017). Van Staal et al. (1998, 2009, 2016) proposed that the trace-element compositions, suggesting local variations in interactions 476–453 Ma Popelogan arc in Maine and New Brunswick also devel- with the extensional trough system, the colliding plates (Shinjo et al., oped over an east-dipping subduction zone, one that was affected by 1999), and sediment fl ux. signifi cant trench rollback with concomitant back-arc development. Bimodal volcanism is also a hallmark of aseismic ridge subduc- The western, youngest belt of the Popelogan arc appears to be the tion when the downgoing slab begins to steepen after fl at-slab sub- along-strike correlative of the Bronson Hill arc (van Staal et al., 2016; duction (e.g., the 25–22 Ma Tambo-Tambillo bimodal volcanics in Karabinos et al., 2017; Fig. 1A). Van Staal et al. (2016) proposed that the Andean margin; Appendix B; Fig. A2; e.g., Kay and Coira, 2009). after collision of the Popelogan arc with Laurentia at about 455 Ma, Since the slab breakoff and polarity reversal models both involve a west-dipping subduction zone (which they called the “Salinic” or asthenospheric upwelling and decompression melting, it is diffi cult to “Silurian” subduction zone) initiated at ca. 450 Ma in a former back- differentiate between these models and an aseismic ridge subduction arc of the Popelogan arc (the Tetagouche basin). This subduction zone model, which also involves asthenospheric upwelling. dipped westerly beneath the composite Laurentia and accreted ter- The Ammonoosuc Volcanics in New Hampshire where Moench ranes, including the Bronson Hill arc. and Aleinikoff (2003) worked appear to be older than the volcanics in the central and southern Bronson Hill arc (see reviews by Hollocher Tectonic Setting of the Ammonoosuc and Partridge Volcanics et al., 2002; Moench and Aleinikoff, 2003). Moench and Aleinikoff (2003) correlated Hollocher’s (1993) bimodal volcanism in the Par- The bimodal nature of the Ammonoosuc and Partridge volca- tridge Formation with their Quimby sequence. If the Ammonoosuc nics and the wide range of tectonic settings that can be inferred from Volcanics resulted from a subduction zone polarity fl ip, and if the their geochemistry (island-arc tholeiite, MORB, calc-alkaline basalt, volcanics are diachronous along strike, then the subduction polarity back-arc basin basalt) have led to multiple suggested origins. Based fl ip probably migrated along the arc, as proposed for Taiwan (Clift et on major, trace, and REE geochemistry, the Ammonoosuc Volcanics al., 2003). In contrast, if some of the bimodal volcanism represents (Hollocher, 1993; Dorais et al., 2011) and the geochemically similar bimodal volcanism typical of early post-fl at-slab subduction, and if Partridge Formation volcanics (Hollocher, 1993) were proposed to the ages of the volcanics are diachronous along strike, then the strong have formed in a backarc basin over an east-dipping subduction zone. variability in ages could refl ect either oblique subduction of an aseis- Additionally, Dorais et al. (2011) suggested that the Ammonoosuc Vol- mic ridge (Appendix B; see Kay and Coira, 2009) or subduction of canics have a peri-Gondwanan crustal signature from incorporation of multiple aseismic ridges at different times. Ganderian sediment, based on Nd and Pb isotopes. Moench and Aleini- In the recent tectonic model of Karabinos et al. (2017), the koff (2003) argued for volcanic generation in a suprasubduction-zone Partridge volcanics postdate both the reversal and the slab break- region over an east-dipping subduction zone, but their tectonic model off, as marked by a volcanic “fl areup” in the Barnard volcanics at

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NORTHERN PUNA ~ 230 S ALTIPLANO ~ 200 S

Oxaya domes Cordillera magmatic gap ignimbrites mafic flows thrusts trench trench 0 crust 0 STAGE 1 STAGE 2A crust FLAT SLAB mantle lithosphere FLAT SLAB/ mantle lithosphere 26 -17 Ma 100 STEEPENING 100 (300 S) Nazca oceanic slab SLAB oceanic slab 200 26 -17 Ma 200 km (280 S) km 400 600 400 600 0 200 800 km 0 200 800 km

small stocks Puna and Cordillera arc domes Altiplano and Cordillera and domes Oriental thrusts shoshonite Oriental thrusts 0 0 STAGE 2 STAGE 2B STEEPENING STEEPENING SLAB 100 SLAB 100 16 -11 Ma 16 -11 Ma (280 S) (270 S) 200 200 km km 400 600 400 600 0 200 800 km 0 200 800 km

near-arc and back arc Subandean Los Frailes ignimbrites Subandean thrusts ignimbrites thrusts 0 0 STAGE 3 STAGE 3 crustal flow DELAMINATION DELAMINATION 10 - 6 Ma 100 & UPLIFT 100 (270 S) 10 - 6 Ma (260 S) 200 200 km km 400 600 400 600 0 200 800 km 0 200 800 km

near-arc Subandean ignimbrites mafic flows ignimbrites Subandean thrusts thrusts 0 0 STAGE 4 STAGE 4 CONTINUED 100 CONTINUED 100 STEEPENING & STEEPENING & DELAMINATION DELAMINATION 5- 3 Ma 200 5- 3 Ma 200 (260 S) km (240 S) km 400 600 400 600 0 200 800 km 0 200 800 km

Central tes Central Volcanic Zone Volcanic flows Subandean Zone Subandean thrusts mafic ignimbri thrusts 0 0 STAGE 5 STAGE 5 FLAT SLAB FLAT SLAB 100 100 EFFECTS EFFECTS MINIMAL MINIMAL 3 - 0 Ma 200 3 - 0 Ma 200 (230 S) km (200 S) km 400 600 400 600 0 200 800 km 0 200 800 km

Figure A1. Models of the evolution of the Altiplano and the Puna regions of the Andean margin, where it is proposed that oblique subduction of the Juan Fernandez aseismic ridge occurred on the downgoing Nazca plate between 17°S and 30°S and between 26 Ma and present (from Kay and Coira, 2009). The aseismic ridge effects include fl at-slab subduction with a magmatic gap (stage 1 in the northern Puna region), slab steepening accompanied by asthenospheric upwelling and decompression melting that gave rise to mantle and crustal melts, as well as lithospheric delamination. Signifi cant ignimbrite fi elds from the lithospheric melts at long distances from the trench and mafi c fl ows are common. Retroarc thrusts including cover and basement can extend upwards of 800 km away from the trench and may be related to a weakened crust and relatively high relative convergence rates.

466.0 ± 0.14 Ma. We suggest that the 3–5 m.y. “magmatic hiatus” in intrusions into the Dunnage Mélange have also been suggested to the Partridge Formation between about 461 and 455 Ma (Moench and originate from seismic ridge (spreading center) subduction under an Aleinikoff, 2003), followed by the bimodal Quimby volcanism, is the accretionary prism (Kidd et al., 1977; van Staal et al., 1998; Zagorev- sort of linkage that might indicate aseismic ridge subduction. In the ski et al., 2012). In the present tectonic model, this subduction took recent models by Karabinos et al. (2017) and Macdonald et al. (2017), place beneath the forearc region of the Victoria arc (Zagorevski et al., the magmatic quiescence in the Partridge Formation is too young 2012). The model of Coaker porphyry resulting from seismic ridge to mark the time of subduction polarity fl ip as originally proposed subduction is based on trace-element and REE geochemical analyses, (Moench and Aleinikoff, 2003). zircon inheritance data from the porphyry, and geochemical, isotopic, and spinel characteristics of xenoliths in the Coaker porphyry. Zag- Previously Proposed Taconic Plate-Tectonic Models That Involve orevski et al. (2012) suggested that the deformation in the Dunnage Ridge Subduction in Newfoundland and Maine mélange also resulted from the seismic ridge subduction beneath the arc-trench gap. Subduction of both seismic and aseismic ridges has been incor- In New Brunswick, Maine, and northernmost New Hampshire porated into Taconic (sensu lato) plate-tectonic models for the Cana- (Fig. 1A), van Staal et al. (2016) proposed that seismic ridge sub- dian Appalachians and Maine. In Newfoundland (Fig. 1B), Jacobi and duction may have resulted in intra-arc/backarc spreading in the Wasowski (1985) and Wasowski and Jacobi (1985) proposed that the Popelogan arc, a southwestern analog of the Victoria arc. Compila- Ordovician tholeiitic to alkalic volcanics of the Summerford Group tion of existing radiometric dates and new age determinations of represent a seamount chain/oceanic plateau that had partly subducted volcanic systems exposed in three subparallel outcrop belts through northwesterly (present-day coordinates) under a peri-Laurentian arc New Brunswick and Maine suggested that the west-facing Popelo- and transferred into an accretionary prism located on the northwest- gan arc and trench migrated northwest (present coordinates) from ern margin of Iapetus. They also proposed that the seamount subduc- about 476 Ma to 460 Ma. Van Staal et al. (2016) proposed that this tion process left a trail of highly deformed units as it passed north- migration was the result of trench retreat/rollback associated with westwardly through the accretionary prism, resulting in the Dunnage eastward subduction. In each of the outcrop belts, an up-section Mélange. The seamount chain hypothesis was based on (1) trace- switch occurs from island-arc tholeiites and calc-alkaline arc-related element and limited REE abundances that indicated the volcanics were volcanics to tholeiitic/alkalic volcanics and dikes with within-plate within-plate basalts (oceanic-island basalt) or possibly E-MORB, (2) basalt affi nities. In the two southeastern, older outcrop belts, the up- associated carbonates and chert that suggested the volcanic edifi ces section evolution to within-plate basalt is ascribed to the develop- were tall enough to pass through the carbonate compensation depth, ment of backarc basins, backarc spreading within these basins, and and (3) the long-lived (Tremadocian through Caradocian) nature of associated signifi cant trench rollback/retreat to the northwest. In the volcanic complexes (Caradocian is approximately equivalent to Maine, for the Munsungan-Winterville Inlier, the northwesternmost Sandbian in modern usage). outcrop belt with the youngest volcanics (ca. 467 Ma to 453 Ma Continued fi eld work and further geochemical analyses suggested of the Balmoral phase), van Staal et al. (2016) suggested that the that the Iapetan suture (Red Indian Line) lies west of the Summer- within-plate basalt might record the passage of the seismic ridge ford Group and the Dunnage mélange (e.g., Williams et al., 1988; van beneath the arc. They proposed that eastward subduction of the Staal, 2007). In this tectonic framework, the Summerford seamounts/ seismic ridge began in New Brunswick between 459 and 455 Ma, oceanic plateau would have accreted to the peri-Gondwanan Victoria just before Iapetus closure and collision of the Popelogan arc with arc (and the stratigraphically underlying Penobscot arc), which lay east Laurentia at about 455 Ma. The seismic ridge subduction might of the Iapetan suture (Red Indian Line) above an east-dipping subduc- have resulted in the ca. 458–453 Ma within-plate alkalic volcanics tion zone (van Staal et al., 1998; Kim and Jacobi, 2002; Zagorevski observed in the youngest and most northwesterly outcrop belt of the et al., 2007). The Summerford Group may have arrived at the Victo- Popelogan arc in Maine, the Balmoral phase (Fig. 1A). Van Staal ria arc-trench region about the time of Dunnage mélange generation et al. (2016) regarded the present-day regions between the Ordo- (ca. 469 Ma; van Staal et al., 1998). However, Zagorevski et al. (2007) vician volcanic outcrop belts as parts of the intra-arc/backarc rifts speculated that arrival of the Summerford seamounts/oceanic plateau with local spreading centers. Van Staal et al. (2016) and Karabinos might have been earlier and might have precipitated the ca. 480 Ma et al. (2017) proposed that the Popelogan arc in Maine (Balmoral closing of the Penobscot backarc basin, with obduction of the oceanic phase for van Staal et al., 2017) is on strike with, and correlates crust onto Ganderia, although they could not eliminate subduction of with, the Bronson Hill arc. Consistent with this correlation, Karabi- a spreading center as a causal mechanism. Subsequently, during the nos et al. (2017) suggested that in the Exploits lithotectonic zone of collapse of Iapetus, thought to be marked by an approximately 455 Ma Newfoundland (Fig. 1B), the Popelogan/Victoria arc formed on the black shale (e.g., Zagorevski et al., 2008), the Victoria arc collided trailing (eastern) margin of the Dashwoods block, not the leading with, and subducted beneath, the peri-Laurentian Annieopsquotch (western) margin of Ganderia, as previously portrayed (e.g., Zago- accretionary tract (which includes the Red Indian Lake Group arc), revski et al., 2007, 2012). After the Popelogan-Laurentia collision, which lay on the west side of Iapetus (e.g., Zagorevski et al., 2008), westward subduction initiated in the Tetagouche basin (formerly a and the Summerford seamounts were ultimately transferred to the peri- backarc of the Popelogan arc) at about 450 Ma and underthrust the Laurentian margin (van Staal et al., 1998). composite Laurentian margin and accreted/obducted terranes. In contrast to the seamount subduction model for the Summerford In Maine, Schoonmaker and Kidd (2006) proposed that Ordovi- Group, more recent research suggested the Summerford Group volca- cian E-MORBs and within-plate basalts of the Bean Brook Gabbro nics, the Dunnage Mélange, and the Coaker porphyry intrusions into (and associated Dry Way volcanics) that intruded into continentally the Dunnage Mélange are all the result of spreading center (oceanic derived sediments and the Hurricane Mountain mélange (Fig. 1A) seismic ridge) subduction (Zagorevski et al., 2012). With additional represented spreading-center ridge subduction under an accretionary trace-element and REE analyses, Zagorevski et al. (2012) reinterpreted prism. They proposed west-dipping subduction based on the present- the within-plate to transitional arc affi nities of the Summerford Group day location of the Chain Lakes massif microcontinent (Fig. 1B) west volcanics to represent part of an extensional phase of the Victoria of the mélange and intrusions. In contrast, van Staal et al. (2016) sug- arc. Zagorevski et al. (2010, 2012) suggested that the approximately gested that although the geochemistry does indicate ridge subduction, 472 Ma to 467 Ma anomalous rift magmatism in the Victoria arc may the subduction under the composite Popelogan arc was to the east, refl ect ridge subduction beneath the arc. The 469 Ma Coaker porphyry since the arc is younger to the west.

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APPENDIX B: ELEMENTS OF ASEISMIC RIDGE its length by seamount/ridge/plateau intersections with the arc, with SUBDUCTION AND FLAT-SLAB SUBDUCTION blocks on the order of 10–35 km wide, measured along arc strike (Fisher et al., 1998). We focus here on the effects of shallow-dipping subduction of a Although the localized uplift can be signifi cant in the accretionary buoyant oceanic slab: i.e., fl at slab subduction to moderately dipping prism, the doming above subducting seamounts and the trail behind, slab subduction associated with aseismic ridges. These conditions lead appear to be transient for the relatively small seamounts, with as short to the set of features summarized in the following paragraphs. The a duration as 0.5 m.y. in the Costa Rica accretionary prism (von Huene elements discussed below are keyed to the list of effects from aseis- et al., 2000). Slumping off the structural high, deposition in the trail mic ridge subduction in the main body of the paper. These discussion behind the seamount, and tectonic accommodation relatively quickly points are based primarily on examples from the Caribbean Antilles, bring the slopes back into the regional tapers (von Huene et al., 2000). Middle America Trench, and Andean margin. In fact, Laigle et al. (2013) suggested that local accretionary prism uplift is followed by subsidence as the ridge/seamount/plateau sub- Element 1A: Uplift of the Accretionary Prism ducts obliquely beneath the accretionary prism in the Lesser Antilles. These uplifts can result in local unconformities. For example, In the Lesser Antilles island arc, seismic refl ection profi les indi- von Huene et al. (2000) noted the similar timing (late Miocene) of cate that the accretionary prism is uplifted (compared to elevations the arrival of the Cocos Ridge at the accretionary prism and an uncon- along arc-strike) by as much as 3+ km where large aseismic ridges formity observed in seismic refl ection data and in Deep Sea Drill- intersect, such as the Barracuda and Tiburon Ridges (Bouysse and ing Project cores, although others have suggested younger ages of Westercamp, 1990; Bangs et al., 2003; Laigle et al., 2013). Addition- impingement (Morell et al., 2012; Vannucchi et al., 2013). Similarly, ally, outcrop geology suggests that the Caribbean upper (overriding) an unconformity is presently developing in the Lesser Antilles where plate is tilted and uplifted by as much as 2 km where a large oceanic the uplifted (by 2 km) Caribbean plate exposes Lower Cretaceous arc plateau is proposed to have subducted (e.g., Bouysse and Wester- volcanics as a result of an oceanic ridge subduction in the Eocene– camp, 1990). Miocene (e.g., Bouysse and Westercamp, 1990). In the Middle America Trench, along the continental margin of Costa Rica, the forearc displays variations in uplift along the length of Element 1B: Deformation Results from the Aseismic Ridge the arc that correspond to subduction of aseismic oceanic plateaus, sea- Penetration into the Accretionary Prism mounts, and oceanic “rough” topography (e.g., von Huene et al., 1995; Fisher et al., 1998). The larger seamounts, plateau, and the Cocos Ridge In the Lesser Antilles, increased folding and faulting in the accre- are related to the Galapagos hotspot track on the Cocos plate (e.g., von tionary prism are observed on seismic refl ection data along the conver- Huene et al., 2000). Lücke and Arroyo (2015) warned, however, that the gent path where the Barracuda and Tiburon Ridges, and other aseismic timing of Cocos Ridge impingement, and the links between aseismic ridges, or their extensions, intersected the accretionary prism/forearc ridge subduction and observed tectonic effects, such as uplift, may be (e.g., Bangs et al., 2003; Laigle et al., 2013). In this particular case, it too simplifi ed with yet-unrecognized causal mechanisms. is hypothesized that a buoyant aseismic ridge was transferred to the Isolated seamounts appear to “tunnel through” (von Huene et al., upper (overriding) plate and began to function as the backstop (e.g., 2000) the accretionary prism, with localized doming and uplift of the Bangs et al., 2003). McCann and Sykes (1984) also proposed trench preexisting prism on the order of the seamount relief (1.5–2.5 km). The rollback as a result of oceanic platform suturing onto the overriding structural relief diminishes up-section and upslope as the seamounts plate in the Puerto Rico–Hispaniola region. penetrate the accretionary prism. In contrast, the larger oceanic pla- In the Middle America Trench of Costa Rica, the extensions of teaus appear to infl uence and build more permanent structures and two large oceanic ridges, the Cocos Ridge and the Quepos Plateau, uplift across the entire width of the accretionary prism. For example, affected both the offshore and onshore portions of the accretionary the Quepos Plateau extension affected the offshore and onshore por- prism in Costa Rica (von Huene et al., 1995, 2000), unlike the short- tions of the accretionary prism in Costa Rica (von Huene et al., 2000), term nature of the seamount effects (as discussed above). The “tunnel- and Meschede et al. (1999) suggested that the subduction extension of ing” of the seamounts and plateaus into the accretionary prism/outer the Cocos Ridge uplifted the presently onshore Osa mélange from the arc must cause localized extreme deformation of the accretionary sedi- base of the accretionary prism, which here is between 4 and 8 km deep ments, forming mélange, as proposed by Jacobi and Wasowski (1985) (von Huene et al., 2000). for the Dunnage Mélange in Newfoundland, and perhaps as evidenced The size of the impinging seamount/plateau appears to exert by the exposed Osa mélange above the Cocos Ridge extension in Costa considerable infl uence on the structural response of the trench and Rica (Meschede et al., 1999). accretionary prism/outer arc. In the Mariana Trench, Fryer and Smoot (1985) found that seamounts larger than about 100 km in diameter do not exhibit the horsts and grabens typical of the outer wall of the Element 2: Flat-Slab to Moderately Dipping Slab Subduction trench, and these large seamounts can remain complete in the accretionary prism. Similarly, von Huene et al. (2000) showed that the larger The dip of the subducting oceanic plate shallows under the oceanic plateaus, such as the 30-km-wide, 2-km-high Quepos Plateau upper plate where “buoyant” aseismic ridges on the downgoing plate (which is located along the border of the Cocos Ridge complex), also inhibit slab sinking. This shallowing is commonly called “fl at-slab do not exhibit graben and horst structures. Additionally, the trench in subduction.” Classic examples of fl at/shallow-slab subduction can the region of the crest of the ~200-km-wide, 3-km-high Cocos Ridge is be observed under the Andes (e.g., Gutscher et al., 1999; for in-depth only about 1 km deep, compared to 5+ km deep in the region of smooth reviews, see Kay and Coira, 2009; Ramos and Folguera, 2009) and east oceanic crust to the northwest. of the Middle America Trench (e.g., Lücke and Arroyo, 2015). The along-strike wavelength of the structural uplifts in the Costa Along the Andean margin, two large fl at-slab subduction seg- Rica accretionary prism is similar to that of the bathymetry of the ments are observed that are associated with magmatic gaps, the Pam- impinging “rough” topography and the Cocos Ridge (Fisher et al., pean or Chilean fl at-slab segment, which is caused by the oblique 1998; von Huene et al., 2000). This variance in along-strike uplift subduction of the Juan Fernandez Ridge (Fig. 1A), and the Peruvian indicates, along with faults transverse to the strike of the accretion- fl at-slab segment, which is related to oblique subduction of the Nazca ary prism, that the accretionary prism/arc is highly segmented along Ridge (e.g., Kay and Coira, 2009; Ramos and Folguera, 2009; Baudino

and Hermoza, 2014), and perhaps a second aseismic ridge, the pro- segment (e.g., Ramos and Folguera, 2009). Smaller Quaternary volca- posed (and presently totally subducted) Inca Plateau (Gutscher et al., nic gaps also occur, such as the Pica Gap in northern Chile, where the 1999). Additionally, a third magmatic gap, the Patagonian volcanic Iquique aseismic ridge subducted (Kay and Coira, 2009). Similarly, gap, occurs where the Chile Rise spreading center intersects the South Quaternary volcanoes are absent above the landward extension of the American margin. The scale of these fl at-slab segments and volcanic subducted Cocos Ridge (e.g., Lücke and Arroyo, 2015). gaps is immense; the Peruvian fl at-slab segment is about 1000 km Island arcs display similar volcanic gaps. For example, in the long, measured parallel to the Andean margin, and the Pampean or Lesser Antilles, the northern part of the arc experienced a 10 m.y. hia- Chilean segment is about 620 km (e.g., Ramos and Folguera, 2009). tus in volcanic activity, which may have been the result of a shallow- Yet, the Nazca Ridge is only about 260 km wide (e.g., Gutscher et al., dipping subducting slab related to a series of seamount chains that 1999), and the Juan Fernandez Ridge varies from about 16 to 88 km intersected the arc during the late Oligocene (e.g., McCann and Sykes, wide (Ramos and Folguera, 2009). The great trench-parallel length of 1984; Bouysse and Westercamp, 1990). the Peruvian fl at-slab segment can be partially explained by oblique Adakites are dispersed across the volcanic gap above the sub- subduction, with the Nazca Ridge sweeping along the margin during ducted Cocos Ridge (e.g., Drummond et al., 1995). The proposed ori- oblique subduction, but Gutscher et al. (1999) proposed a second sub- gin of these adakites is uncertain and includes (1) partial melting of ducted plateau, the Inca Plateau, to explain the long trench-parallel the downgoing slab from contact with the mantle/asthenosphere, either length of Peruvian fl at-slab segment and the signifi cant trench-parallel at a slab window (e.g., Abratis and Wörner, 2001) or a detached slab dips and high in the subducting slab depth inferred from earthquakes. (Gazel et al., 2011), or (2) melting of basalt or cumulate mafi cs near The fl at-slab lengths along the margin are impressive when compared the base of the upper plate (Bindeman et al., 2005), or (3) fl uid-induced to the scale of outcrop geology in New England. mantle melting (Hidalgo and Rooney, 2014). The fl at part of the oceanic slab under Peru extends from about Volcanic centers can develop far removed from the trench during 250 km to about 550 km away from the trench, at about 100 km depth, fl attening of the subducting plate. For example, volcanism occurred as before the slab begins sinking down to 150 km depth at 700 km away far as 850 km from the Peru-Chile Trench at 32°S, where the fl atten- from the trench (e.g., Ramos and Folguera, 2009). At the maximum ing Nazca plate with the Juan Fernandez Ridge fi nally dipped below extent of the present Chilean fl at slab, the oceanic slab reaches 100 km 200 km depth (e.g., Ramos and Folguera, 2009)! depth about 250 km from the Peru-Chile Trench and dips gently from there to 150 km depth about 650 km from the trench, where it then Element 4: Signifi cant Uplift of Parts of the Upper (Overriding) begins to dip more steeply (e.g., Cahill and Isacks, 1992; Kay and Plate (during Initial Steepening of the Downgoing Slab after Coira, 2009). Passage of the Ridge) In Costa Rica, earthquake seismic tomography (e.g., Syracuse et al., 2008; Arroyo et al., 2009) integrated with gravity data and mod- Along the Andean margin, one of the most dramatic elements eling (Lücke and Arroyo, 2015) show a relatively shallowly dipping related to fl at-slab subduction is represented by the uplifted portions of subduction zone segment below the “Seamount Province.” The oce- the western continental regions, including the Altiplano (Bolivia, Fig. anic plate (Cocos plate) dips at 58° between 80 and 200 km depth, A1), the Puna region (Chile and Argentina, Fig. A1), and the Fitzcarrald compared to a dip of 71° for the same depth to the north where smooth arch (Peru). The Altiplano is a 400-km-wide (trench-normal) region ocean crust is being subducted (Lücke and Arroyo, 2015). This shal- with an elevation of about 3750 m that is located south of the Bolivia lower subduction translates to about a 48 km offset away from the orocline and extends some 200 km to the east of the trench. The plateau trench for the 100 km depth structural contour on the top of the sub- is estimated to have uplifted between 2500 and 3500 m in ~3.5 m.y. ducting slab. The shallow-dipping portion of the subducting oceanic (10.3 Ma to 6.8 Ma; Garzione et al., 2006; Ghosh et al., 2006). A range plate is only about 75 km wide (measured parallel to the strike of the of contrasting models have been proposed to explain the Altiplano, arc), with relatively narrow boundaries (Lücke and Arroyo, 2015). from those that suggest the uplift is related to slab steepening after In the Cocos Ridge region of the Middle America Trench, sev- passage of a subducted ridge (e.g., Kay and Coira, 2009) to those that eral effects result from the increased rigidity and resistance to bend- do not call upon ridge subduction as a causal mechanism (e.g., Molnar ing caused by the extra thickness of the seamount province, aseismic and Garzione, 2007). Complicating the ability to construct a realis- plateaus, and ridges. These manifestations include reduced relief on the tic tectonic model is the likelihood that either crustal thickening or peripheral bulge, a shallower trench (by as much as 4 km), less relief on removal of dense mantle (and dense lower crust) can result in isostatic trench-parallel grabens and horsts in the seamount province, no grabens uplift (e.g., Garzione et al., 2008). Based in part on the observation and horsts related to oceanic-plate fl exing in the Cocos Ridge region, of low seismic velocities that imply no mantle presently remaining and a dip only half the usual under the near-trench part of the accretion- under the Altiplano (Myers et al., 1998), Molnar and Garzione (2007) ary prism (e.g., von Huene et al., 2000; Ranero et al., 2003; Grevemeyer and Garzione et al. (2008) proposed that rapid removal (delamina- et al., 2007). Further, the strikes of the usually trench-parallel horst and tion) of the eclogitic lower crust and lithosphere during a relatively grabens curve toward parallelism with the Cocos Ridge, some 45° away short period (3.5 m.y.) drove Altiplano uplift. In this model, the lower from the trench axis, in response to the lack of fl exing and descent of crust and deeper lithosphere removal was accomplished perhaps by the Cocos Ridge at the outer wall of the trench, compared to regions to sinking drips of dense crust/lithosphere (Garzione et al., 2008). The the north (see fi gure 2 in von Huene et al., 2000). mantle removal would allow lighter asthenospheric infl ow that in turn would result in isostatic uplift in late Miocene time (e.g., Molnar and Element 3: Volcanic Gap above the Flat-Slab Garzione, 2007; Garzione et al., 2008). Molnar and Garzione (2007) Subduction Segment proposed that the extended ~30 m.y. period of crustal shortening and thickening (and possibly even underthrusting by the Brazilian Shield; Magmatic/volcanic gaps (measured along arc strike) occur above Myers et al., 1998) did not result in signifi cant uplift before delamina- subducted aseismic ridges (Fig. A1). For example, along the Andean tion because of a dense eclogitic lower crust. margin, both the Peruvian and Chilean (Pampean) fl at-slab segments Perceived problems with this model of uplift from delamination display a distinct lack of Quaternary volcanoes above the fl at-slab seg- (see, for example, Kay and Coira, 2009) can be resolved with a model ments (e.g., Isacks and Barazangi, 1977; Ramos and Folguera, 2009), that suggests that mid- to lower-crustal (silicic) fl ow from regions on i.e., on the order of 1000 km (measured parallel to the trench) for the either side of the present Altiplano (the Cordillera Occidental and the Peruvian fl at-slab segment and 600 km for the Pampean or Chilean Cordillera Oriental, which were both elevated before the Altiplano)

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depressed the lower crust beneath the future Altiplano, causing the that postdates fl at-slab subduction developed in three bands: the “fron- lower crust of the future Altiplano to eclogitize and then delaminate, tal arc,” which is about 200 km from the trench, the “inner arc,” which resulting in Altiplano uplift (e.g., Husson and Sempere, 2003; Kay and has a maximum extent of 300 km to 500 km away from the trench, and Coira, 2009). Kay and Coira (2009), among others, suggested that slab the Tambo-Tambillo volcanics, which separate the two bands and are steepening after Juan Fernandez Ridge passage resulted in decompres- about 300–400 km behind the trench (Figs. A1 and A2; e.g., Hoke and sion melting (Fig. A1). This melting gave rise to mafi c magmas that Lamb, 2007; Kay and Coira, 2009). ponded at the base of the crust. The resistate (residuum) of the dif- The fi rst magmatic events to occur when the slab began to steepen ferentiating mafi c magmas, along with the eclogitized crust, resulted (but while the slab was still dipping relatively shallowly; Kay and Coira, in a lower crust that was denser than the mantle, which in turn caused 2009) were the 25–22 Ma (Oligocene–Miocene) Tambo-Tambillo delamination and sinking of the lower crust. This delamination and bimodal volcanics, including basalts and shoshonites (Fig. A2). The sinking would have been accompanied by a rise of asthenosphere, as source of the volcanics was spinel lherzolite mantle at <90 km depth, proposed for the Altiplano and other parts and times of the Andean based on REE modeling (Hoke and Lamb, 2007). margin, such as the Carboniferous–Permian magmatism and tectonics The inner arc, which lies east of the Tambo-Tambillo volcanics, is in the Pampean region of Argentina (e.g., Ramos and Folguera, 2009). a long-lived series (ca. 25 Ma to <1 Ma) of andesitic to dacitic domes, Along the Andean margin in Peru, north of the Bolivia orocline, calderas, shoshonite fl ows, and ignimbrites (Fig. A2; Kay and Coira, the fl at-slab subduction of the Nazca Ridge extension may have caused 2009). The dacitic ash fl ows from one complex alone, the 9 Ma to the uplift of the Fitzcarrald arch (Ramos and Folguera, 2009), about 5 Ma Los Frailes complex, cover about 2000 km3 (Kay and Coira, 550 km from the trench (measured parallel to the relative conver- 2009). A low-velocity zone below the Los Frailes complex in the con- gence orientation), as well as some 400 km inland from the coastline tinental crust at 15 km depth is thought to indicate a zone where the at Lima. In the Maranon Basin, which is located in northeastern Peru lithosphere has either been detached or signifi cantly transformed by east of the Andes (the western margin of the basin is about 500 km partial melting (e.g., Beck and Zandt, 2002; Kay and Coira, 2009). from the trench), an estimated 800 to 1200 m section is missing, based Helium isotope data also suggest that mantle melting occurred under on seismic refl ection profi les, well logs, and thermal history modeling the Los Frailes complex, as well as other complexes in the inner arc (Baudino and Hermoza, 2014). The timing of the uplift and erosion (Hoke and Lamb, 2007). The Los Frailes complex is associated with represented by the regional unconformity is consistent with the timing proposed decompression melting and continental delamination during (Pliocene, ca. 4 Ma) of the arrival of the “Inca Plateau” at the basinal slab steepening that occurred between 10 and 6 Ma (Figs. A1 and A2; margin (Baudino and Hermoza, 2014), which Gutscher et al. (1999) Kay and Coira, 2009). Ignimbrites at another complex (the 8.4 to 6.5 proposed presently lies immediately below the basin. Morococala center; Fig. A2) include two-mica rhyolitic tuffs (commonly thought of as continentally derived), and biotite-quartz latite Element 5: Post-Flat Slab Magmatism: Alkalic and Rhyodacitic Volcanism with Continental Partial-Melt Signatures in Large Calderas Far Removed from the Trench

71° W 65° W If the trajectory of the subducting aseismic ridge is oblique to the 69° W67°W

relative convergence vector, or if the aseismic ridge is discontinuous, 17° S 15 Ma ~ 9-5 Ma at some point, the subducting ridge will move away from a particular Morococala PERU BOLIVIA Complex region on the upper plate that was formerly above the aseismic ridge. The result is steepening of the downgoing slab in that area, with con- ~25 - 22 Ma sequent decompression melting and asthenospheric upﬂ ow into the Tambo ~22 - 19 Ma Tambillo Oxaya newly created wedge between the downgoing slab and the upper plate 25 Ma (Fig. A1). These effects can result in continental delamination and par- Ignimbrite tial melting of the lithosphere. The result is magmatic provinces with 19° S mantle and continental partial-melt signatures that are stepped signiﬁ - PACIFIC

cantly back from the trench (as much as 500 km to 800 km), compared 2-3 Ma to the location of the usual frontal volcanic arc (Fig. A1; Kay and Kay, OCEAN 1993; Kay and Coira, 2009; Ramos and Folguera, 2009). This is the Los Frailes <1 Ma “inner arc” that defi nes the “arc-jump” compared to location of the arc Complex in times preceding fl at-slab subduction. Kari Kari Classic examples for magmatism that postdates fl at-slab sub- 21° S CHILE duction occur in the Andes; Kay and Coira (2009) developed three transects that stretch across the Andes (southern Altiplano and farther south) in order to demonstrate the magmatic effects of fl at-slab subduction (two of which are displayed in Fig. A1). Below we review TRENCH 50 km

the southern Altiplano transect (19°S to 20°S) in order to establish the Large Miocene Arc Centers > 9 Ma Caldera Inner arc general components of the post-fl at-slab magmatism (Fig. A1). Late Miocene Arc Centers Ignimbrite field The Andean Altiplano-Puna Plateau, located just south of the Pliocene - Recent Early Miocene mafic units Bolivia orocline in Chile and Bolivia, records the effects of slab steepening after the southward passage of the aseismic Juan Fernandez Ridge. Figure A2. Map of the Altiplano magmatic centers discussed in text For the southern Altiplano region at 19°S to 20°S, volcanic inactivity (from Kay and Coira, 2009). Dashed and solid lines indicate the east- and uplift occurred from about 38 Ma to 27 Ma (e.g., James and Sacks, ern extent of volcanism at 25 Ma and 15 Ma, respectively (Hoke and 1999; Kay and Coira, 2009). James and Sacks (1999) proposed that the Lamb, 2007). Note the extreme distance of the eastern ignimbrite cen- volcanic inactivity and uplift are indicative of fl at-slab subduction as ters (“inner arc”) from the trench, and the mafi c units that separate the Juan Fernandez Ridge passed by, although Kay and Coira (2009) these centers from the western volcanic centers (“frontal arc”). Volca- questioned whether the subduction had been fl at or merely moderately nic distribution in time, space, and composition is related to subduc- dipping. In the region of the southern Altiplano transect, magmatism tion of the Juan Fernandez aseismic ridge.

tuffs (Kay and Coira, 2009). Pliocene to Pleistocene (<5 Ma) basaltic response to thrust loading on its western margin, west-directed reverse to high-K basaltic andesites also erupted in the same region. faults involving basement also occur along the eastern margin of the After the subducting aseismic ridge has moved past a particular Bermejo foreland basin (Zapata and Allmendinger, 1996; Jordan et al., region of the trench/convergent margin, the frontal arc is re- established. 2001); these thrusts are related to basin and range uplift of the Sierra The frontal arc, about 200 km from the trench, also produced early Pampeanas in response to the fl at-slab subduction. ignimbrites: the voluminous 22.7 to 19.4 Ma Oxaya rhyodacites, Thrust loading resulted in an increase in the Miocene Bermejo which cover over 3000 km3 (Fig. A2; e.g., Wörner et al., 2000; Kay and foreland basin subsidence rate, from 0.33 mm/yr in pre-fl at-slab time Coira, 2009), are centered about 200 km from the trench. Large andes- (20–9 Ma) to 0.77–0.95 mm/yr during initiation of fl at-slab subduction ite shield volcanoes were built from 20 to 9 Ma. Late Miocene and (9–6 Ma; Ramos and Folguera, 2009). Ramos and Folguera (2009) Pliocene to Holocene volcanism in the frontal arc show about a 40 km estimated that the foreland basin response in subsidence rate to thrust migration cratonward from large Miocene arc centers and the early loading is very rapid in broken continental crust, on the order of 1 m.y. ignimbrite fi elds in the Oxaya fi eld (Fig. A2; Kay and Coira, 2009). Modeling of the subsidence in the foreland basins, including the Ber- Arc migration away from the trench after the aseismic ridge passes mejo Basin, from about 29°S to 31.5°S showed that from 20 to 9 Ma, has also been proposed for several island-arc systems. For example, in the thin-skinned thrust loading did not have a signifi cant effect on the Lesser Antilles, the northern part of the arc jumped 50 km farther the Bermejo Basin (Cardozo and Jordan, 2001). Cardozo and Jordan from the trench after 10 m.y. of volcanic inactivity (e.g., Bouysse and (2001) and Jordan et al. (2001) found that the response of basins to Westercamp, 1990). loading is unsteady and spatially variable, based on the rigidity of the basin crust, including such factors as amount of faulting and terrane Element 6: Retroarc Thrusting of Both Basement and boundaries, and thickness of the crust. Sedimentary Cover A second example of retroarc thrusting along the Andean margin is the Agrio fold-and-thrust belt in the Neuquen Basin at ~37°S. Retroarc thrusting is a hallmark of fl at-slab subduction, and it can The downgoing Nazca plate presently has a moderate dip of about involve both basement and cover sequence units (Figs. 4 and A1). As 30° (e.g., Vera et al., 2015), but late Miocene rhyodacitic magmas up Ramos and Folguera (2009) summed up, as the fl at-slab (or moder- to 550 km from the trench, high gravity anomalies, and tomographic ately dipping) subduction steepens, the resulting heating and partial analyses that suggest shallow asthenosphere together indicate that this delamination of the crust/lithosphere signifi cantly weaken the con- region (the Payenia fl at-slab segment) underwent fl at-slab subduction tinental crust. Since the continent is under horizontal compression, from 15 to 5 Ma (Kay and Copeland, 2006; Kay et al., 2006a, 2006b; the weakened crust shortens and fails in a series of thrusts and folds Folguera et al., 2007; Ramos and Folguera, 2009). Fault systems that that involve basement. The retroarc thrusting loads the foreland plate, developed during continental breakup (detachments and listric faults) resulting in a rapidly responding and rapidly subsiding foreland basin were reactivated in several phases, including during Eocene and the that is yoked to the thrusts from the hinterland (Fig. 4). Additionally, fl at-slab Miocene compressional events. Basement-involved thrusts retroarc thrusting with a basement component occurs during fl at-slab accommodated shortening in the west, whereas suprastructure thrusts subduction as well, with the same results of a loaded foreland devel- are common in the Agrio fold-and-thrust belt farther east (e.g., Ramos oping a foreland basin (Fig. 4; e.g., Hilley et al., 2004; Ramos and and Folguera, 2009; Giambiagi et al., 2012). Folguera, 2009; Kay and Coira, 2009). These retroarc thrust systems Island arcs commonly display retroarc thrusts, fi rst observed over may have an additional causal factor. Maloney et al. (2013) found that 40 yr ago (e.g., Hamilton, 1979; Silver et al., 1983) in the Banda/Sunda most compressional events along the Andean margin (including the arc. However, Silver et al. (1983) suggested that the retroarc thrusting fold-and-thrust belts east of the Altiplano-Puna Plateau in the central there was the result of arc-continent collision. Researchers ten Brink Andes) correlate with trench-normal convergence rates of >4 cm/yr. et al. (2009) catalogued several retroarc thrust systems around the The retroarc region of the Andes provides an example of the globe that have various proposed origins, including incipient subduc- structural elements that develop during and after fl at-slab subduction polarity fl ips or subduction-related mantle fl ow. However, detailed tion, coupled with a relatively high trench-normal convergence rate study of faults revealed in multibeam bathymetric data of the Muertos (e.g., Kay and Coira, 2009; Ramos and Folguera, 2009; Maloney et thrust belt, which is a retro-thrust belt on the south side of Hispan- al., 2013). One example is the Aconcagua fold-and-thrust belt above iola and Puerto Rico, as well as sandbox experiments, suggested to the Pampean fl at-slab segment in the central Andes at 32.5°S (Fig. 4), ten Brink et al. (2009) that retroarc thrusting in island arcs can form which contains the highest peak in the western hemisphere. As the without incipient subduction polarity fl ip or mantle fl ow. Rather, rigid Juan Fernandez aseismic ridge arrived at the margin in this region behavior of an arc will transmit stress into the retroarc side of the arc. at about 9 Ma, thin-skinned shortening in the Principal Cordillera It appears that retroarc thrusts at an island arc are thus not defi nitive in (located immediately west of the Chile-Argentina boundary) trans- terms of ridge subduction. formed into intracontinental thrusts (“thick-skinned” thrusts that involved basement rocks) in the Frontal Cordillera east of the Chile- APPENDIX C: KEY TO BIOSTRATIGRAPHIC RANGES IN Argentina boundary about 260 to 300 km east of the trench (Fig. 4; FIGURE 7 e.g., Hilley et al., 2004; Ramos and Folguera, 2009; Giambiagi et al., 2012). The eastward migration of the thrust front increased from For Undeformed Flysch Zone: (1) Riva (1974) and Fisher ~2.5 mm/yr in pre-fl at-slab time to ~13.3 mm/yr during shallow slab (1977). (*2) Mitchell, herein, Diplacanthograptus spiniferus, Genic- subduction (Ramos and Folguera, 2009). ulograptus typicalis, and Orthograptus quadrimucronatus recovered From 5 Ma to 2 Ma, east-directed thrusting migrated farther east, from Schenectady Formation strata immediately beneath the west- forming the Precordillera thrust belt between 300 and 360 km from ern limit of the Vischer Ferry thrust zone in bluffs on north side of the trench (Fig. 4). This east-directed thrusting involved Paleozoic the Mohawk River (GPS: 42.822593°N, 73.858149°W on private sequences, minor Mesozoic sequences, and the Cenozoic sedimentary property; 42.824676°N, 73.860769°W, accessible from the Mohawk section of the western part of the Bermejo foreland basin. In some Landing Nature Preserve). interpretations, Precordilleran thrusting involved continental base- Vischer Ferry Thrust Zone (new): (*3) Mitchell, herein; (3a, b) ment as well (e.g., Hilley et al., 2004, see their fi gure 13), but in other D. spiniferus from siltstones within strongly folded, thin-bedded sand- models generally did not (Zapata and Allmendinger, 1996; Ramos and stone between mélange zones in bluffs on north side of the Mohawk Folguera, 2009). In contrast to a simple model of basin development in River east of the western limit of the Vischer Ferry thrust zone (GPS:

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42.820786°N, 73.856315°W, on private property) and along access APPENDIX D: BIOSTRATIGRAPHIC AGE CONSTRAINTS road to Lock 7, south side of Mohawk River (GPS: 42.79986307°N, ON THE TIMING OF DEFORMATION 73.84593118°W); (3c) D. spiniferus, Dicranograptus nicholsoni, Amplexograptus praetypicalis, and Cryptograptus insectiformis from Figure 7 reviews and updates the biostratigraphic control on the folded, silty black shales beneath prominent mélange zone, exposures units involved in the several mélange zones and deformed fl ysch zones adjacent to Lock 7 Powerhouse, north side of Mohawk River (GPS: west of the Taconic allochthon exposed along the Mohawk River. To 42.80174447°N, 73.84220098°W). accurately assess the biostratigraphic age of these rocks, however, it Vischer Ferry Folded and Faulted Flysch Zone: (*4) Berry is necessary that we touch briefl y upon certain details of the biostrati- (1963a) Sites S1 and S2, D. spiniferus Zone faunas. Re-collection graphic zones employed. Graptolite faunas from these rocks have been from bluffs on south side of Saratoga Lake by Mitchell also yielded D. referred to different zonal schemes: those of Berry (1960) and Riva spiniferus Zone faunas. (*5a) Berry (1963a), Orthograptus truncatus (1969, 1974), and one or another of these schemes has been used sub- intermedius Zone in silty black shales region between Round Lake and sequently by other authors (Fig. A3). However, confusion about the Saratoga Lake; Berry sites S5, S6, and S8 lie within this zone and con- meaning and scope of these zones has led to mistaken assertions about tain D. spiniferus Zone fauna. S3–S4 lie west of Saratoga fault; S1, S2, the age of the rocks. In particular, both zonal schemes include a N. S7 lie within Vischer Ferry zone (see below). (*5b) D. spiniferus Zone gracilis Zone, but these zones differ greatly in duration and concept faunas from recent road-cut exposures on Round Lake Bypass (GPS: (Figs. 7, A3). Berry (1960) employed two zones for rocks in this inter- 42.948825°N, 73.793396°W) and roadside exposure on Ruhle Road val of concern: the N. gracilis and C. bicornis zones, distinguishing North (GPS: 42.960475°N, 73.816947°W). (6) Snake Hill olistolith, the latter (younger) zone by the fi rst appearance of the eponymous from sandstones of Snake Hill Formation, Berry (1963a) and English species, among others. Riva (1974) contended that these zones had et al. (2006) reported “Trenton” trilobites and brachiopods, and Mitch- the same fauna and, relying on an assemblage zone approach, com- ell (in English et al., 2006) reported Normalograptus mohawkensis, bined them into a single, longer N. gracilis Zone. Following the British indicative of the O. ruedemanni Zone. tradition, he recognized a Diplograptus multidens Zone for the upper Western Exotic Mélange: (7) Ages poorly controlled; Ruedemann part of the interval that Berry referred to the C. bicornis Zone. This (1930) provided a joint list for a few poor outcrops but not for indi- upper C. bicornis Zone interval lacks N. gracilis and generally has a vidual sites; Plesch (1994) concluded age range is probably similar to lower-diversity fauna. More recent work in the southern and central Vischer Ferry zone. Appalachians by Finney et al. (1996), as well as work on Sandbian Halfmoon Graywacke Zone: (8) Western locality, the “old rocks elsewhere in Laurentia (e.g., Lenz and Chen, 1985; Mitchell et quarry” site of Berry (1962, 1977), Rickard and Fisher (1973), Plesch al., 2003), Australasia (Vandenberg and Cooper, 1992), China (Chen et (1994), lower Corynoides americanus Zone. (*9) Interstate-87 (Riva al., 2017), and Wales (Bettley et al., 2001), demonstrates that, contrary fi d e Kidd in Plesch, 1994), Nemagraptus gracilis Zone of Riva (1974), to Riva’s (1974) assertion, a distinct N. gracilis–bearing interval with but likely upper part, i.e., within the lower Climacograptus bicornis a unique fauna precedes the appearance of C. bicornis and the associ- Zone of Berry (1962). (10) Eastern locality, C. bicornis Zone (Ruede- ated species of the C. bicornis Zone. N. gracilis continues upward into mann, 1912). the lower half of the C. bicornis Zone, which we may recognize as an Eastern Exotic Mélange: (11) Blocks at Cohoes Falls locality, informal lower C. bicornis Zone, along with an upper subzone that is Riva fi d e Kidd in Plesch (1994), N. gracilis Zone of Riva (1974), but roughly equivalent to Riva’s D. multidens Zone, which lacks N. graci- likely as at site 9, through C. americanus Zone. (*12) Stark’s Knob lis but includes a few new species. olistolith, “Trenton”-age pillow lava (Landing et al., 2003), age based Turning to the age of the classic Normanskill graptolite faunas, on presence of Liospira? sp. (misspelled as Leiospira in Landing et the collections tabulated by Berry (1962) from the Mount Merino and al.), possibly representing a gastropod from mid-Sandbian (Turinian; Austin Glen formations are nearly all referable to the lower C. bicornis C. bicornis Zone) and younger rocks in North America and Europe Zone as defi ned above. They contain both N. gracilis and C. bicornis (Paleobiology Database, 2017). (13) Black mudstone block near con- together with a diverse assemblage of other graptolites. Nine of the tact with N. gracilis Zone fauna (Riva fi d e Kidd in Plesch 1994), but samples reviewed by Berry (1962), mostly low-diversity collections likely as at site 9. from near the base of the Mount Merino Formation, contain N. graci- Flysch Mélange Zone and Waterford Shale Zone: (14) Water- lis without C. bicornis, but nearly all of these collections contain one ford Dam, Peebles Island, and Green Island, C. americanus to O. or another of the usual C. bicornis Zone species or lack many of the ruedemanni zones (Ruedemann, 1901, 1912, 1930; Riva fi d e Kidd other species expected in an N. gracilis Zone fauna, or both. Thus, in Plesch, 1994; Mitchell in English et al., 2006). (15) “Type” Nor- they either are from very high in the N. gracilis Zone or are simply manskill Group, Mount Merino Formation, cherty argillite blocks at poor samples. Riva provided identifi cations of N. gracilis Zone col- classic exposures along Normans Kill at Glenmont and Kenwood rail lections as personal communication to several workers, which they road cut, C. bicornis Zone (Ruedemann, 1901, 1912, 1930; Berry, subsequently quoted in their published work, but none of these sources 1962; Plesch, 1994; Kidd et al., 1995). (16) Albany Rural Cemetery, provided detailed fauna lists, and so we cannot determine the precise C. americanus Zone (Ruedemann, 1908, 1930; Berry, 1963a; Riva, zonal assignment for these collections. Based on the available data, 1974; Goldman, 1995). there is no convincing evidence that any of the Mount Merino Forma- Frontal Exotic Mélange: (*17) O. truncatus intermedius Zone, tion is early Sandbian in age, and, in the absence of positive evidence Austin Glen–like sandstone blocks and shale matrix of wildfl ysch at to the contrary, we assume that all reports of N. gracilis Zone faunas Moordener Kill (Berry, 1962; Berry in Zen, 1967, cited erroneously by from Normanskill and related rocks in New York State and nearby Bradley and Kusky [1986, p. 677] as indicating a D. spiniferus Zone regions in New England actually represent the C. bicornis Zone strata age), and also from sandstone blocks “1½ miles east of Rensselaer” and are mid- to late Sandbian, or ca. 456.6–453 Ma, in age. (Berry, 1962, 1977). (18) Rysedorph Hill, wildfl ysch pebbles, Lower Macdonald et al. (2017) presented new geochronological data Cambrian–“Trenton” (Ruedemann, 1930) and limestone blocks with that demonstrate that the Indian River Formation is early Darriwilian “Snake Hill” shelly fauna (Zen, 1967), too restrictively interpreted as in age, based on dates on volcanic ashes of 466.1 ± 0.2 Ma and 464.2 ± D. spiniferus Zone by Bradley and Kusky (1986), as both units include 0.1 Ma. In their fi gures 2 and 11, they showed continuity between equivalents of C. bicornis to D. spiniferus zone faunas. (19) Black this now-very-old Indian River Formation and the overlying Mount chert-bearing, Mount Merino–like blocks at four Ruedemann (1930) Merino Formation and placed the bulk of the duration of the latter unit localities, C. bicornis Zone, summarized in Plesch (1994). within the mid- to late Darriwilian (as old as ca. 463 Ma). This old age

Ma Riva Berry Mohawk Valley Parautochthon Western Cover Taconic Allochthon G. SE NY & NJ Sequence Figure A3. Chronostratigraphic diagram il- 450 pygmaeus lustrating the revised ages of the main strati-

O. quad Schenectady D. spinif / Pen Argyl graphic units discussed in the text. Ordovi- O. rued cian time scale is from Cooper and Sadler Utica Austin Glen 452 C. amer (2012). Graptolite biozones are after Riva Martinsburg (1974) and Berry (1962). Data sources for Trenton subsidencesubsidence Walloomsac thruststh age control of the Sandbian to Katian strata 454 D rusts Austin Glen are those described in Appendix C. Darriwil- D. Black River / Pawlet multidens Balmville / subsidencesubsidence ian units of the western cover sequence and Jacksonburg Taconic allochthon follow those in Macdon- ald et al. (2017), including position of dated C. bicornis 456 ash beds in the Indian River Formation (red Knox Unconformity Mt. Merino dashed lines with MacDonald et al., 2017, N. Orwell / gracilis sample numbers), except as noted in the text Middlebury Ls 458 / West (e.g., uncertain interval of Indian River beds N. gracilis O. trunc Bridgewater below securely dated Mount Merino Forma- ? tion). Note late Darriwilian interval is omit- 462 ted from the time scale for the sake of brevity. A. Approximate timing of Taconic thrusts (from decoratus east/older to west/younger) is shown with S1456 solid arrows, and propagation of subsidence 464 L. (brown shaded units over blue shaded units— intersitus Indian River shale over carbonate, respectively) is shown Darriwilian Sandbian Katian Providence Ls S1450 with dashed arrows, i.e., arrival of thrusts in L. / Bridport region of western cover sequence occurred 466 austrodentatus at ca. 454 Ma, leading to subsidence in ad- Poultney jacent parautochthon (next column to left) at ca. 453.5 Ma, etc. D—Deicke volcanic ash (K-bentonite position [orange line] in parautochthon projected [dashed line] based on biostratigraphic and lithostratigraphic context; see Sell et al., 2015); Ls—Limestone. Biozones: L. austrodentatus—Levisograptus austrodentatus, L. intersitus—Levisograptus intersitus, A. decoratus—Archiclimacograptus decoratus, other biostratigraphic units as in Figure 7. Lithologies: gray—black slate; white triangles—chert; blue- gray—restricted intertidal to shallow subtidal wackestone and mudstone; blue—open shelf carbonates; brown—laminated black to green shale and mudstone; green—ﬂ ysch. NY—New York, NJ—New Jersey.

assigned for much of the Mount Merino confl icts with the analysis pre- zone about 10 km east of the Saratoga-McGregor fault (Figs. 2 and sented above. Rather, we suspect that the Indian River strata and dark 7). In the central part of this domain, the mélange zones are separated slates of a similar mid-Darriwilian age also known from the Taconic by regions of less-deformed, generally openly folded strata. Exotic allochthons (Ash Hill Quarry, Mount Merino, New York; Ruedemann, clasts are rare in the western exotic mélange belt (Fig. 7) and most 1904) may be equivalents of the Dauphin Formation in Pennsylvania consist of deformed remnants of the beds surrounding the mélange. (Ganis, 2005), which formed prior to initiation of the Taconic foreland In situ strata exposed at the surface in the Vischer Ferry Zone and basin and which are likewise separated from the overlying mid-Sand- farther west in the Undeformed Flysch Zone contain a D. spiniferus bian and younger foreland basin succession by an unconformity with Zone graptolite assemblage that has a D. spiniferus Zone age (sites an ~5-m.y.-long hiatus in deposition (Ganis and Wise, 2008; Fig. A3). 1–5, Fig. 7). Four new graptolite localities in the western extent of In Figure 7, we attempt to distinguish determinations that rep- the Vischer Ferry Zone establish that the sediments in this part of the resent intact strata or mélange matrix from those based on clasts. belt include rocks of D. spiniferus Zone age and that these rocks were The matrix and blocks of mélange both indicate the maximum age of thrust onto rocks of this same age (Figs. 7, A3). The type locality of the mélange formation and associated thrusting. The blocks either were Snake Hill Formation is an olistolith (roughly 350 m in width) on the plucked from the hanging wall and footwall during thrusting or were eastern shore of Saratoga Lake, within the Vischer Ferry zone, and its incorporated as the thrust overrode slumps associated with the thrust sandy tempestite-dominated and shelly fossil–bearing succession has regime (such as the slumps on the Middle America Trench wall; von produced graptolites of the Orthograptus ruedemanni Zone (Mitchell Huene et al., 2000). Soft-sediment deformation of the mélange blocks in English et al., 2006). Berry (1963a) recovered D. spiniferus Zone can provide a limit to the minimum age of the fi rst stage of mélange graptolites from black shales on the west side of the lake, and we have formation, and therefore thrusting, assuming the soft-sediment defor- obtained additional D. spiniferus Zone assemblages from new expo- mation resulted from thrusting and not earlier slumping. The signifi - sures near Round Lake (Appendix C; Fig. 7). Older Utica strata most cance of the age of the youngest blocks is diffi cult to gauge; it could likely underlie both the Vischer Ferry zone and the undeformed fl ysch represent the minimum age of thrusting, i.e., the time the thrust ramp to the west, but no strata are known to crop out in that region. breached the surface and no sediments were yet deposited that were The matrix of the Western Exotic Mélange and of the zones younger, or the age of the blocks could mean merely that the thrust had farther to the east are poorly constrained. Where the data are clearly not sampled strata of a younger age. derived from matrix, those data in all the belts east of the Vischer The Cohoes Mélange consists of a series of tectonic mélange Ferry Zone suggest C. americanus to D. spiniferus zone ages. In the zones between the frontal thrusts of the Taconic allochthon and a fault intervening Halfmoon graywacke zone, Plesch (1994) reported an

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N. gracilis Zone age based on personal communication from J. Riva to in the Taconic thrust sequence. Berry (1962, 1963b, 1977) reported C. W.S.F. Kidd in 1983, but it is unclear whether this was from an olisto- americanus Zone equivalent ages from the upper part of the Austin lith or intact formation. Parautochthonous rocks from the eastern zones Glen graywacke at a site west of Troy within the Giddings Brook (especially those in the Waterford Shale Zone, variously referred to as slice. Furthermore, the Giddings Brook succession may include gra- Normanskill Shale and Snake Hill Shale) contain mainly C. america- dational contacts between Balmville-like shelly grainstones (which nus Zone faunas, although some may be of late C. bicornis Zone age contain the conodont Phragmodus undatus, of late Sandbian or (Rickard and Fisher, 1973; Fisher and Warthin, 1976). On the other younger age) and the overlying Walloomsac black shales, which are hand, several of the classic Normanskill sites of Ruedemann (1901, also of C. bicornis Zone age (Potter, 1959; Zen and Bird, 1963; Zen, 1930) are within the Waterford Shale Zone and are blocks of Mount 1967). Finally, wildfl ysch units such as that at Whipstock Hill (Zen, Merino strata (Plesch, 1994; Kidd et al., 1995). Thus, the Halfmoon 1967) again consist of a vast range of clast lithologies, including Aus- Graywacke Zone and the mélange zones to the east of that domain tin Glen–like sandstones along with Trenton-like carbonates within (the Eastern Exotic Mélange, the Waterford Shale Zone, and the Fron- a matrix of Walloomsac lithology. Accordingly, the Frontal Exotic tal Exotic Mélange Zone; Fig. 7) potentially have older ages (up to Mélange Zone appears to form part of a continuum of deformation ~4 m.y. older), or incorporate older recycled material, compared to the that extends into the lower slice of the Taconic allochthon and frontal mélange zones to the west of the Halfmoon Graywacke Zone. thrusts of the Giddings Brook slice of the Taconic allochthon, and it The Frontal Exotic Mélange, especially the conglomeratic, appears to have arrived near its present locale by late C. bicornis to C. wildfl ysch-like units at Rysedorph Hill and Moordener Kill, have a americanus Zone time. stunning range of clast ages, from mid-Cambrian to possible D. spiniferus Zone age. 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Agle, P.A., Jacobi, R.D., and Mitchell, C.E., 2006, Fault-related fractures, Kidd, 1981). veins, and fl uid migration; Mohawk Valley, NYS: Geological Society of The matrix ages reported from the wildfl ysch rocks in the Fron- America Abstracts with Programs, v. 38, no. 2, p. 85. tal Exotic Mélange are C. americanus to D. spiniferus (Berry, 1962, Aleinikoff, J.N., and Moench, R.H., 1992, U-Pb zircon ages of the Ordovi- 1977; Berry in Zen, 1967). Note that Bradley and Kusky (1986, p. 677) cian Ammonoosuc volcanics and related plutons near Littleton and referred to the Berry in Zen information, correctly noting that Berry Milan, New Hampshire: Geological Society of America Abstracts with referred faunas from Moordener Kill wildfl ysch to his Orthograptus Programs, v. 24, no. 3, p. 2. truncatus intermedius Zone but then mistakenly equated this with Aleinikoff, J.N., Wintsch, R.P., Tollo, R.P., Unruh, D.M., Fanning, C.M., and the D. spiniferus Zone. Berry’s zone corresponds to the entire C. Schmitz, M.D., 2007, Ages and origins of rocks of the Killingworth americanus–D. spiniferus zone interval of Riva (1969, 1974). Berry dome, south-central Connecticut: Implications for the tectonic evolution of southern New England: American Journal of Science, v. 307, no. 1, cited personal communication from Elam, who considered the Moor- p. 63–118, https://doi.org/10.2475/01.2007.04. dener Kill wildfl ysch matrix to be “the same as those in the Canajo- Aleinikoff, J.N., Rankin, D.W., Moench, R.H., and Walsh, G.J., 2015, New SHRIMP harie Shale” (Berry, 1962, p. 713), which, if correct, indicates that the U-Pb zircon ages for felsic Ammonoosuc volcanics, northern NH-VT: Geo- fauna is older than that of the D. spiniferus Zone. logical Society of America Abstracts with Programs, v. 47, no. 3, p. 41. 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